A Beginner’s Guide to Water Management Water …A Beginner’s Guide to Water Management — Water Clarity Information Circular 103 Florida LAKEWATCH UF/IFAS Department of Fisheries

A Beginner’s Guide toWater Management — Water Clarity

Information Circular 103

Florida LAKEWATCHUF/IFAS

Department of Fisheries and Aquatic SciencesGainesville, FloridaSeptember 2001

3rd Edition

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Phone: 352/392-4817Fax: 352/392-4902E-mail: [email protected] address: http://lakewatch.ifas.ufl.edu

Copies are available for download from the LAKEWATCH Web site:http://lakewatch.ifas.ufl.edu/LWcirc.html

or from the

UF/IFAS Electronic Document Information Source (EDIS) Web site:http://edis.ifas.ufl.edu

This publication was produced by:

Florida LAKEWATCH © 2001

3rd Edition Reviewed January 2017 by Mark HoyerUF/IFASDepartment of Fisheries and Aquatic Sciences7922 NW 71st StreetGainesville, Florida 32653-3071

A Beginner’s Guide toWater Management —Water Clarity

Information Circular 103

Florida LAKEWATCHUF/IFAS

Department of Fisheries and Aquatic SciencesGainesville, FloridaSeptember 2001

3rd Edition

Lake Santa Fe Melrose, FL

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Introduction

Water clarity, the clearness or transparency of water, is one of the most noticeable attributesof a waterbody. It’s also something of great importance to many people. The publicjudges water quality by what they can see, and so their evaluation is often based on this

standard. For example, lakes with very clear water may be perceived as good, unpolluted, or pristine,while lakes with limited transparency may be described as undesirable, polluted, or degraded.

Contrary to this popular perception, crystal clear water is NOT the ruler to which all lakesshould be compared. It is not true that lakes with lower transparency are necessarily the result ofpollution or degradation. In Florida, lakes with a wide range of water clarity occur naturally, even inlocations that are unaffected by human impacts. It is also not true that clearer water is safer to swimin or to drink. On the contrary, clear water is just as likely as murky water to harbor pathogens,bacteria, or other contaminants that could be harmful to human health.

In some instances, the standard for water clarity is often influenced by regional values or ideasabout how the waterbody is to be used. For example, in Canada, the Canadian government recommendsthat water should be sufficiently clear so that a Secchi disc is visible at a minimum depth of 1.2meters (about 4 feet). This recommendation stems from the fact that swimmers want their swimmingareas to be clear enough to see underwater obstacles. The 1.2-meter water clarity standard is onereason many of the lakes in Canada, particularly those with an abundance of free-floating algae, donot meet Canadian standards for swimming and are deemed “undesirable.” However, it should beunderstood that many of these lakes have water clarity less than 1.2 meters naturally and have notbeen impacted by human activity.

Similar conditions exist in Florida, with many people believing that less clear water is undesirable.However, one’s preference for clear water is a value judgement, not a scientific measure, and shouldbe based on how people envision using the waterbody. For example, less clear waters typicallysupport abundant populations of fish, plants, birds, or other wildlife — creating opportunities forpopular outdoor activities such as fishing, hiking, and nature watching. In fact, some of Florida’sbest fishing is found in murky, algae-rich waters.

In light of such popular misconceptions surrounding water clarity, one thing is clear — allFlorida residents and visitors stand to benefit from a greater understanding of the dynamics andsignificance of water clarity in Florida lakes. This circular provides a first step by discussing a fewimportant strategies used to manage water clarity. Basic information about water clarity, with anemphasis on its relationship to algal growth in lakes, is provided in the following segments:

1 Measuring Water Clarity2 What Affects Water Clarity?3 Water Clarity and Biological Productivity4 Managing Lakes for Water Clarity

Before you begin however, we encourage you to review the definitions for commonly usedscientific terms provided in Appendix A, particularly for algae and chlorophyll. More comprehensiveinformation may also be obtained by reading A Beginner’s Guide to Water Management – TheABCs (Circular #101) and A Beginner’s Guide to Water Management – Nutrients (Circular #102).These publications can be downloaded for free from the Florida LAKEWATCH web site:http://lakewatch.ifas.ufl.edu/LWcirc.html.

Secchi discs areoften colored withalternating black

and whitequadrants,

as shown here.LAKEWATCH usesplain white Secchi

discs as seen on theopposite page.

Part 1

MeasuringWaterClarity

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There are several devices used by scientiststo measure turbidity, light extinction, andspectral analysis—all related to water

clarity. However, for the purposes of this publicationwe’ve decided to focus on use of the Secchi disc,one of the oldest, easiest, and most economicalmethods for measuring water clarity.

The LAKEWATCH program uses Secchidisc measurements because, aside from the factthat they’re easy and inexpensive to use, theyprovide us with an indirect way to measure thebiological productivity of a lake – an importantcomponent of lake management. But first thingsfirst. We’ll start with the Secchi disc.

The Secchi DiscObtaining a lake’s Secchi depth involves the

use of a plate-sized device called a Secchi disc(pronounced with several variations, but usuallySEH-key disk). Secchi discs of various sizes can beused, but customarily it is an 8-inch diameter discwith alternating black and white quadrants. However,some disks are solid white in color. A line, rope orchain is attached through the center of the Secchi discand is marked off in intervals like a ruler, usually infeet or meters. To measure a lake’s Secchi depth, the

1 On occasion, the Secchi disc can still be seen as it rests on thelake bottom, or it may disappear into thick submerged aquaticmacrophyte growth. While the depth at which this happensfurnishes some information about the water’s clarity, it is notconsidered to be a measurement of the waterbody’s Secchi depth.Also, the word “Secchi” is always capitalized because it refers tothe name of the individual who first used it.

disc is lowered into the water to find the depth atwhich it first vanishes from the observer’s sight.1

The Secchi disc was named after Pietro AngeloSecchi, a scientific advisor to the Pope and head of theRoman Observatory in the mid 1800s. CommanderCialdi, the commander of the Papal fleet, actuallydevised the Secchi disc. Secchi was asked by Cialdi toexperiment with this disc in the coastal waters of theMediterranean. The first disc was lowered from thePapal yacht l’Immacolata Concezione and used tomeasure water clarity in the Mediterranean Sea onApril 20, 1865. Since that time, Secchi discs have beenused to measure water clarity in tens of thousands ofwaterbodies around the world.

☛ For more on biological productivity, seeSection 3 Water Clarity and BiologicalProductivity on pages 8-11.

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To measure a lake’s Secchi depth, a Secchi disc is lowered into the water to find the depth atwhich it first vanishes from the observer’s sight.

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Part 2

WhatAffectsWaterClarity?

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Water clarity in Florida lakes rangesbetween 0.7 feet and 38 feet.Differences in water clarity are

primarily caused by the presence (or lack) ofdissolved substances and/or suspended particlesin the water. However, to fully understand thedynamics of how dissolved substances and/orsuspended particles affect water clarity, there area few things to consider.

Several factors can impact the abundance ofdissolved substances and/or particles in the water— consequently impacting water clarity. Forexample, the abundance of dissolved substancesand/or particles in the water can be influenced bythe presence of aquatic plants and/or the locationof the waterbody. Seasonal variations in climatecan also impact water clarity. These factors arediscussed in the following section.

Dissolved SubstancesDissolved organic substances or compounds

can come from many types of terrestrial andaquatic plants, and can color the water reddish orbrown, sometimes even to the point of appearingblack. When there is an abundance of dissolvedorganic compounds in the water, scientists oftenrefer to the water as being “colored” or sometimesthey’ll refer to the waterbody as being a “dark” lake.

There are two types of color that are measuredin waterbodies:

apparent color is the color of a water sample thathas NOT had particulates filtered out of the water; and

true color is the color of a water sample thatHAS had all particulates filtered out of the water.

The measurement of true color is the onemost commonly used by scientists. To measuretrue color, the color of a filtered water sample ismatched to one from a range of standard colors.Each of the standard colors has been assigned anumber on a scale of “platinum-cobalt units”(abbreviated as either “PCU” or “Pt-Co units”).On the PCU scale, a higher value of true colorrepresents water that is more darkly colored.Because dissolved organic compounds (i.e., color)absorb sunlight as the light passes through thewater, Secchi depth values decrease as theamount of color in the water increases. Color in

Florida lakes rangesfrom 0 to over 400 PCU.

ParticulatesParticulates include

free-floating algae, calledphytoplankton, as well asother solids suspended inthe water. These include

sand, clay, or organic particles stirred up fromthe bottom, washed in from the shoreline, washedin from the surrounding land, or brought in by thewind and rain. Because particulates absorb and

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scatter sunlight as the light passes through thewater, Secchi depth (water clarity) values decreaseas the amount of particulates in the water increases.

While all particles are known to affect waterclarity, studies throughout the world have shownthat free-floating algae are the dominant particlesinfluencing water clarity in most lakes.

Scientists often estimate the amount of free-floating algae in a lake by measuring the amount ofchlorophyll2 in a water sample, measured in unitsof micrograms per liter (μg/L). Lakes in theFlorida LAKEWATCH database analyzed prior toJanuary 2000 have average chlorophyll concentra-

tions ranging from lessthan 1 to over 400 μg/L.

The presence orabsense of aquaticmacrophytes3 in awaterbody is especiallyimportant in under-standing water clarity

and yet this relationship is sometimes overlooked.It’s also a double-edged sword. While waterclarity can affect the growth of aquatic macrophytes,the reverse is also true: the presence of large amountsof aquatic macrophytes can influence water clarity.

There are several explanations for this:One explanation is that submersed macrophytes,

or perhaps the algae attached to them, use availablenutrients in the water, depriving the phytoplankton(i.e., free-floating algae) of these same nutrients.Consequently, when there is less phytoplankton inthe waterbody, water clarity is usually increased.

Another explanation for the water clarity/aquaticmacrophyte relationship is that submersed macro-phytes anchor nutrient-rich bottom sediments inplace, buffering the action of waves, and deprivingthe free-floating algae of nutrients contained in

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bottom sediments that would otherwise be stirred up.It’s also thought that aquatic macrophytes

keep phytoplankton levels down due to the wavebuffering action of the plants. As a result, algal cellssettle and are prevented from being mixed into thewater column.

All three of these mechanisms are probably inaction simultaneously, influencing the amount offree-floating algae found in the water column. There’seven a formula of sorts that can be used to estimatethe impact that aquatic macrophytes may have onwhole lake water clarity:

Using the Florida LAKEWATCH database,it’s been observed that if aquatic macrophytecoverage is less than 30% of the bottom area of awaterbody, the presence of plants does notgreatly influence the amount of free-floatingalgae in open-water. However, lakes with aquaticmacrophyte coverage over 50% or more of thebottom area typically have reduced chlorophyllconcentrations and clearer water.

In fact, in a lake with aquatic macrophytecoverage greater than 50%, chlorophyll andnutrient concentrations may become so low and

The size of individualparticles, whether algaeor other suspendedparticles, has a stronginfluence on water clarity.To visualize this effect,consider putting a solidstick of chalk into abucket of water. Uponputting the chalk stick

into the bucket, you will still be able to seethrough the water to the bottom of the bucket.If, however, the same amount of chalk isground into fine particles and placed into thewater, the water will become so murky that thebottom of the bucket will not be visible. In thismanner, when smaller particulates such assmall algae dominate an aquatic system, thewater clarity is lower than in waterbodieswhere larger particles dominate — assuming thetotal amount of particulate matter is the same.

2 Chlorophyll is a green pigment found in all plants andabundant in nearly all algae.3 Aquatic macrophyte is the scientific term for largeaquatic plants. See page 23 of Appendix A for moreinformation.

Coontail (Ceratophyllum demersum)

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AquaticMacrophytes

the water become so clear that it could mistakenlybe described as a biologically unproductive lake.And yet, the presence of such large amounts ofmacrophytes tells us that the lake is extremelyproductive. In such an instance, the practice ofcharacterizing a lake based on its water clarity alonebecomes inaccurate.

Based on these observations, it becomesimportant for lake managers and /or residents tobe aware of the fact that removal of largeamounts of aquatic macrophytes can result inreduced water clarity.

For instance, it’s been observed that aquaticplant management efforts that reduce a lake’splant coverage from high levels (greater than 50%coverage) to low levels (less than 30%) can result inmajor increases in chlorophyll concentrations(i.e., phytoplankton) and reduced water clarity.

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Take Lake Brant in Hillsborough Countyfor example. See below for details on how theintroduction of grass carp affected chlorophyllconcentrations in the lake and quickly reduced thelake’s water clarity.

However, it may be reassuring to note thataquatic plant control efforts conducted on lakeswith less than 30% aquatic plant coverage do notproduce major increases in chlorophyll concen-trations even though they result in the removal ofsignificant amounts of aquatic plants. It shouldalso be noted that planting a fringe of aquaticplants around a lake generally does little toimprove water clarity unless the plants grow tocover a major portion of the lake bottom.

Be careful what you ask for...

ake Brant in Hillsborough Countyprovides us with an example of how theremoval of large amounts of macrophytes

can affect water clarity. The chlorophyll graphshown here tells the story: Grass carp, aherbivorous species of fish, were stocked intothe lake to remove (eat) nuisance plants fromthe lake. The fish did such a good job that withinthree months a large portion of the plants weregone and chlorophyll concentrations were on therise. Food for thought, if you’re contemplatinglarge-scale aquatic macrophyte removal.

By May 7, 1993 a total of 325 grass carphad been stocked into Lake Brant, a60-acre lake, to remove large areas ofsubmersed macrophytes. Within threemonths, chlorophyll concentrations morethan doubled, as evident in the bar graphbelow. Water clarity was reduced by half.

May 7, 1993

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Based on these observations, it becomes

important for lake managers and/or residents

to be aware of the fact that removal of

large amounts of macrophytes can result

in reduced water clarity.

☛ See Water Clarity and TrophicState in Appendix A.

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The geology and physiography of a lake’swatershed influences many characteristics of a lake,including algal levels and the true color of the water.Consequently, the location of a waterbody is stronglylinked to its water clarity. Here’s how it works:

Water flowing through a watershed to a lakepicks up substances such as nutrients (required foralgal growth) and humic acids (that color thewater). If a lake is located in an area with nutrient-poor or well drained soils, runoff or seepage waterpercolating up from underneath the lake has littleaffect on its water clarity. There are simply fewernutrients and/or dissolved substances being carriedinto the lake.

Lakes in northwestern Florida (in Washington,Bay, Calhoun, and Jackson counties) provide agood example. LAKEWATCH data collectedfrom this area show that these lakes tend to havechlorophyll concentrations below 3 μg/L, colorvalues generally below 10 PCU, and Secchidepths greater than 10 feet. This is documentedin Lake Regions of Florida4 (EPA/R-97/127).

“Lakes in the New Hope Ridge/Greenhead SlopeLake Region are clear, low in nitrogen and phosphorus,low in chlorophyll, and are among the most oligotrophiclakes in the United States (Canfield 1981).”

In contrast, lakes in the Lakeland/BoneValley Upland Lake Region in central Florida(Polk and Hillsborough counties) tend to havechlorophyll concentrations above 80 μg/L, colorvalues above 20 PCU, and Secchi depths lessthan 3 feet. This can be explained by the nutrient-rich and poorly-drained soils of the regiondocumented in Lake Regions of Florida(EPA/R-97/127):

“... the Bone Valley Uplands and the BartowEmbayment, within White’s (1970) Polk Uplandphysiographic region, tend to be more poorlydrained flatwoods areas. All of these areas arecovered by phosphatic sand or clayey sand fromthe Miocene-Pliocene Bone Valley Member of thePeace River Formation in the Hawthorn Group(Scott 1992; Scott and MacGill 1981). The regiongenerally encompasses the area of most intensivephosphate mining...”

This strong link between location and waterclarity suggests there may be natural limits onthe level of water clarity that waterbody managersand users can expect in a specific location.Consideration of the lake region in which the lakeis situated will provide a useful perspective andhelp managers and users evaluate the feasibilityof different management goals.

Walden Lake in Hillsborough County is locatedin the Lakeland/Bone Valley Upland LakeRegion in central Florida. Chlorophyllconcentrations for this lake are typically high,ranging above 80 μg/L. Water clarity istypically low, with Secchi depths of less thanthree feet. This can be explained by the highlyphosphatic sands the lake is situated upon.

4 Lake Regions are geographical areas in which lakes havesimilar geology, soils, chemistry, hydrology, and biologicalfeatures. In 1997, using Florida LAKEWATCH data andother information, the United States EnvironmentalProtection Agency designated 47 lake regions in Florida,using these similarities as their criteria. The results of thisproject were published in a report Lake Regions of Florida,Griffith, G.E. et al. 1997, U.S. Environmental ProtectionAgency (EPA/R-97-127). For a copy write: U.S. EPA, 200 SW35th Street, Corvallis, Oregon 97333. For more information,see Lake Regions in Appendix A. You may also call theLAKEWATCH office for a printout of a specific lake regiondescription (of your lake, for example) or for theLAKEWATCH information pamphlet, Florida Lake RegionsClassification System. Call 1-800-LAKEWATCH (1-800-525-3928).

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5 Brown, C.D., D.E. Canfield, Jr., R.W. Bachmann, and M.V.Hoyer. 1998. Seasonal patterns of chlorophyll, nutrientconcentrations and Secchi disc transparency in Florida lakes.Lake and Reservoir Management 14: 60-76.

Seasonal VariationsSeasonal variations in weather conditions such

as temperature, wind, and amount of rainfall are alsoclosely linked with a lake’s water clarity. Theseseasonal changes can affect water clarity by influencingboth algal levels and color levels within a lake.

Water ClarityIn Florida, it’s been observed that water

clarity in individual waterbodies varies in a patternover the course of several years. For example, theFlorida LAKEWATCH database shows that:

Water clarity is greatest in lakes from Decemberthrough February.

Water clarity is lowest in lakes from March through May.

Algal LevelsSimilarly, chlorophyll concentrations (algae) in

Florida lakes can be highly variable over time andhave a direct effect on water clarity. Using theFlorida LAKEWATCH database, a general seasonalpattern of chlorophyll can be shown for manyFlorida lakes. This pattern is described below:

For lakes with low to moderately high chlorophylllevels (oligotrophic to eutrophic), monthly chloro-phyll concentrations are typically lower than theannual mean chlorophyll concentration fromDecember to May.

During the months of August thru October,chlorophyll concentrations are typically higher thanthe annual mean.

Lakes with high chlorophyll levels (hypereutrophiclakes) tend to have highly fluctuating monthly levelsof chlorophyll for most of the year, but tend to havelower levels in December, January, and February.

Color LevelsChanges in the true color of a waterbody seem

to be strongly linked to the amount of seasonalrainfall a watershed receives and the amount ofrunoff into a waterbody. Runoff is the key factor toremember. During periods of drought, Floridawaterbodies tend to be clearer. Even though rainfallmay be heavy as the drought abates, water color inthe waterbody may not increase until there is sub-stantial runoff. When there is a lot of runoff, truewater color can increase quickly and substantially.

Studies of individual Florida lakes also show thatincreases or decreases in color can significantlyinfluence a lake’s water clarity. For example,Grasshopper Lake in Lake County had Secchi depthvalues greater than 12 feet during dry weather from1993-1994. Following heavy rains from 1995-1996,the same lake had Secchi depths of less than 3 feet.The correspondingchlorophyll concentrationsaveraged 1 μg/L during 1993-1994 and 4 μg/Lduring 1995-1996.

Although these data show an increase in chloro-phyll concentrations, the increase is not enough toaccount for such a drastic change in water clarity. Sohow to explain such a drastic change in water clarity?

The difference in water clarity was related tothe additional color that washed into GrasshopperLake during the rainy years. Color concentrations inthe lake changed from 0 PCU to an observed teacolored water (approximately 40-60 PCU). With thereturn of dry weather, water clarity increased as colorvalues fell below 2 PCU. This is why both chloro-phyll concentrations and color should be monitored ifwater clarity is a major lake management issue.

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It should be noted that although these patterns

are well documented, exceptions are common in

that maximum or minimum Secchi depths can

occur during any month. For more information

on patterns in Florida lakes, see Brown et al.5

It’s important to note that just like Secchi values,these patterns don’t always apply to all lakes;maximum or minimum chlorophyll values in Floridalakes can occur at any time during the year.

Part 3

Water ClarityandBiologicalProductivity

insects, fish, etc.). Conversely, if algae are abundantin a lake, then we can generally estimate thatthere is the potential for more wildlife. In fact,research in Florida lakes has shown that there is adirect correlation between chlorophyll concentra-tions in a lake and the number of zooplankton,fish, birds, and even alligators.

Water ClarityAs one might imagine, it’s not always

possible to sample lake water for chlorophyllconcentrations. (Not all research programs arefortunate enough to have dedicated LAKEWATCHvolunteers collecting samples.) So how can weestimate the biological productivity of a lakewithout collecting and analyzing water samples?

Thanks to historical water chemistry data,scientists noticed certain patterns when comparingchlorophyll and water clarity data. After lookingat hundreds of lakes, it became clear that, in mostlakes, as chlorophyll concentrations (phytoplankton)increase, water clarity decreases.

This led them to believe that, for the mostpart, they could begin to predict how biologicallyproductive a lake is based on its water clarity. Theyhypothesized that if lake water is not very clear, it’smore than likely due to an abundance of algae. Thepresence of large amounts of algae suggests thatthe lake is a productive system — providing anabundance of food for aquatic life.

However, if a lake has clear water, it’s more

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Floridians, like people throughout theworld, are concerned about water quality.Determining the water quality of our

aquatic resources is a major responsibility ofwater managers and scientists. One way theyapproach this task is to evaluate a waterbody’sbiological productivity.

Biological productivity is defined as theability of a waterbody to support life such asplants, fish, and wildlife. However, measuringthe ability of a waterbody to support all aquaticlife is difficult and prohibitively expensive bymost standards. For this reason, many scientiststry to estimate a lake’s ability to support life bymeasuring a few basic parameters, namelychlorophyll concentrations in water, waterclarity, nutrient concentrations in water, andaquatic plant abundance. Read on to discoverhow these four parameters serve as importantclues to a lake’s biological productivity.

Chlorophyll ConcentrationsOut of these four parameters, chlorophyll

concentrations (i.e., phytoplankton) are used mostoften to estimate biological productivity becausealgae represent the actual base product of a lake’sfood web. For example, if we know that chloro-phyll concentrations are low in a lake, then wecan generally estimate that the number of otheraquatic organisms will be low — especially thosethat rely on algae for food (i.e., zooplankton,

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likely to not be productive due to the smallamounts of algae available to the food web.

This strong relationship between chlorophyllmeasurements and water clarity is why scientistshave adopted use of the Secchi disc as an easy andinexpensive way to determine a lake’s biologicalproductivity. However, it should be noted thatthere are always exceptions; dissolved substances(color) in the water can greatly affect waterclarity, as can suspended particles such as clay.

Nutrient ConcentrationsJust like the flowers in your garden or the

grass in your lawn, algae and aquatic macrophytesare also dependent upon nutrients for growth. Twoof the more important nutrients are phosphorusand nitrogen. Both of these compounds are foundnaturally in rocks, soils, and even lake water.

While phosphorus and nitrogen concentra-tions can certainly affect a lake’s biologicalproductivity, the relationship between algae andthese nutrients can be somewhat complicated.For this reason, scientists often refer to otherparameters such as chlorophyll concentrations orSecchi depth measurements to estimate a lake’sbiological productivity.

Aquatic Plant AbundanceAquatic plants are another indicator of a

lake’s biological productivity. If there are smallamounts of aquatic macrophytes and algae, onecan generally state that the lake is unproductive.

Whereas, if a lake has clear water, due to lowchlorophyll concentrations, but has largeamounts of aquatic macrophytes, it can be statedthat the lake is a biologically productive system.

But there’s an additional twist to theserelationships when considering the more biologi-cally productive lakes. While the presence oflarge amounts of aquatic macrophytes can affectwater clarity,6 the reverse is also true; water claritycan affect aquatic macrophyte growth. Picturethis: When lake water is turbid, sunlight can’tpenetrate as far into the water, limiting themaximum depth at which aquatic macrophytescan grow.

This inverse relationship between waterclarity and aquatic macrophytes suggests that thebiological productivity of a lake can shift betweenbeing a lake dominated with phytoplankton to alake dominated by rooted aquatic macrophytes.

Similar to the time and expense associatedwith collecting chlorophyll measurements, thecollection of aquatic macrophyte data is notalways feasible. Fortunately, now that we knowhow closely linked water clarity is to aquaticmacrophyte growth, the Secchi disc can be auseful tool in predicting the potential for aquaticplant growth. Water clarity or Secchi depthmeasurements can help scientists estimate thedepth at which underwater aquatic macrophyteswill be expected to survive. A general rule ofthumb is that aquatic macrophytes can grow to adepth of about 1.5 times the Secchi depth measure-ment. For example, if a Secchi depth measurementis three feet, the depth at which aquatic macro-phytes can grow is limited to about 4.5 feet.

This inverse relationship between water clarity andaquatic macrophytes suggests that the biological

productivity of a lake can shift between being a lakedominated with phytoplankton to a lake

dominated by rooted aquatic plants.

6 For more on this, see Aquatic Plants in Part 2 on page 4.

☛ For more on nutrients and their relationshipto algal abundance, see Florida LAKEWATCHInformation Circular 102 A Beginner’s Guide toWater Management — Nutrients.

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When faced with the challenge of trying todescribe the various levels of biological productivityin a lake, scientists developed a system called theTrophic State Classification System. Usingthis approach, lakes are traditionally classified intofour groups according to their level of biologicalproductivity or “trophic state.”

The names of these four trophic states fromthe lowest productivity level to the highest areoligotrophic, mesotrophic, eutrophic, andhypereutrophic.

Using Secchi Depth toDetermine Trophic State

As discussed earlier in this section, overallbiological productivity is difficult to measure ina lake. However, based on what we know aboutthe strong relationship between water clarity(Secchi depth measurements) and chlorophyllconcentrations, aquatic scientists often choose touse Secchi depth measurements as an indirectway of assessing biological productivity and itsassociated trophic state.

To do this, professionals may use the criteriadeveloped for lakes by two Swedish scientists,Forsberg and Ryding. There are other classificationsystems available, but Florida LAKEWATCH usesthe Forsberg and Ryding classification systembecause it seems to work well for Florida lakes.Forsberg and Ryding’s trophic state classificationsystem, using Secchi depth, is as follows:7

Criteria for Determining Trophic State Basedon Secchi Depth

lakes with Secchi depths greater than 13 feetare classified as oligotrophic;

lakes with Secchi depths ranging from 8 feetto 13 feet are classified as mesotrophic;

lakes with Secchi depths ranging from 3 feetto 8 feet are classified as eutrophic; and

lakes having Secchi depths less than 3 feet aregenerally classified as hypereutrophic.

Using average Secchi depth readings from morethan 500 Florida lakes in the LAKEWATCHdatabase (analyzed prior to January 2000), Floridalakes were found to be distributed into the fourtrophic states as follows:7

approximately 7% of the lakes would beclassified as oligotrophic (those with Secchidepths greater than 13 feet);

about 22% of the lakes would be classified asmesotrophic(those with Secchi depths between 8and 13 feet);

45% of the lakes would be classified aseutrophic (those with Secchi depths between 3and 8 feet); and

26% of the lakes would be classified ashypereutrophic (those with Secchi depths lessthan 3 feet).

Using Algae to Determine Trophic StateWhile Secchi depth readings can help us

estimate a lake’s biological productivity, at somepoint, we may want to base a lake’s trophic stateclassification on algal levels (often measured aschlorophyll concentrations.)

Why?

Biological Productivity and Trophic State

Oligotrophic (oh-lig-oh-TROH-fic) waterbodieshave the lowest level of biological productivity.

Mesotrophic (mes-oh-TROH-fic) waterbodieshave a moderate level of biological productivity.

Eutrophic (you-TROH-fic) waterbodies have ahigh level of biological productivity.

Hypereutrophic (HI-per-you-TROH-fic) waterbodieshave the highest level of biological productivity.

7 This distribution of trophic state is based solely on Secchidepth values. It should be noted that trophic statedeterminations are more useful when scientists consider notonly Secchi depth but the concentrations of total nitrogen andtotal phosphorus, chlorophyll concentrations, and aquaticmacrophyte abundance.

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Algae are the base product of a lake’s foodweb and give us a direct indication of a lake’sbiological productivity. In other words, if algae areabundant, then other forms of aquatic life will beabundant. Forsberg and Ryding’s criteria forchlorophyll concentrations (algae) are as follows7:

Criteria for Determining Trophic State Basedon Chlorophyll Concentrations

lakes with chlorophyll concentrations less thanor equal to 3μg/L are classified as oligotrophic;

lakes with chlorophyll concentrations rangingfrom 4 to 7 μg/L are classified as mesotrophic;

lakes with chlorophyll concentrations rangingfrom 8 to 40 μg/L are classified as eutrophic;

lakes having chlorophyll concentrationsgreater than 40 μg/L are generally classified ashypereutrophic.

Using average chlorophyll concentrationsfrom more than 500 Florida lakes in the LAKE-WATCH database (analyzed prior to January2000), Florida lakes were found to be distributedinto the four trophic states as follows:8

approximately 12% of the lakes would beclassified as oligotrophic (those with chlorophyllconcentrations less than 3μg/L) ;

about 31% of the lakes would be classified asmesotrophic (those with chlorophyll concentrationsranging from 4 to 7 μg/L);

8 This distribution of trophic state is based solely on chlorophyllconcentrations. Trophic state determinations are more useful whenscientists consider not only chlorophyll concentrations but also theconcentrations of total nitrogen and total phosphorus, Secchi depth,and aquatic plant abundance. For more on trophic states, seeLAKEWATCH information pamphlet entitled Trophic State:A Waterbody’s Ability to Support Plants, Fish, and Wildlife.For a free copy call 1-800-LAKEWATCH (1-800-525-3928).

It’s important to know that a lake may be

classified in more than one trophic state

depending on the criteria used. For example,

a lake with a chlorophyll concentration of

2 μg/L could be classified as oligotrophic based

on the amount of phytoplankton found in the

lake. However, the same lake, with a Secchi

depth of 4 feet could be classified as eutrophic,

based on its water clarity .

This inconsistency may seem troublesome

but it is, in fact, useful information. It tells us

that the reduced Secchi depth could be related

to dissolved substances in the water (i.e., color)

or high sediment concentrations — instead of

phytoplankton abundance.

While Florida LAKEWATCH usescriteria from the Forsberg and Ryding

trophic state classification system, it’simportant to know that other professionals inthe water management arena may use a slightlydifferent set of criteria to determine trophicstate. Generally, the differences are not thatgreat, but non-professionals should be awarethat they do occur.

It’s also important to understand that itis a misuse of the trophic state classificationsystem to use trophic categories as indicatorsof water quality. Each trophic state classificationhas attributes that may be judged as having“good” qualities or “poor” qualities.

Judgements of quality depend largelyon how people want to use the waterbody.For example, an oligotrophic waterbodymay be good for swimming because it willtypically have clear water, but may not be arewarding fishing site, because it does notsupport large fish populations.

41% of the lakes would be classified aseutrophic (those with chlorophyll concentrationsfrom 8 to 40 μg/L); and

16% of the lakes would be classified ashypereutrophic (those with chlorophyll concen-trations greater than 40 μg/L).

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The practice of managing water clarity bycontrolling algal growth has sparked an intenseinterest in being able to predict how much changeis likely to occur in water clarity, based on changesin phytoplankton abundance. Water managershave a particular interest in being able to makethis type of prediction, because managementstrategies may only be considered successfulwhen water clarity is improved noticeably. Forexample, it’s been shown that even if a lake’schlorophyll concentration was reduced from 250μg/L to 50 μg/L (a five-fold reduction), Secchidepth measurements would most likely notchange noticeably. This is due to the hyperbolicrelationship between water clarity and chlorophyllconcentrations.

In this instance, the cost-effectiveness andsuccess of such a strategy may be questioned bythe citizenry. For this reason, managers and usersneed a way to predict before-hand whether theirproposed management strategy will produce signifi-cant results or be worth the cost. The followingsegment provides a mathematical approach formaking such predictions.

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Lake management, or the management ofany waterbody, should always begin withthe establishment of goals. And like

anything else, lake management goals are oftenas varied as the people who live on or use lakes.Some people are most interested in improvingfishing, while others are concerned with an over-abundance of aquatic macrophytes, reducing boattraffic, or preventing shoreline erosion. However,because water clarity is such a noticeable attribute inlakes, it could be listed as one of the top managementconcerns for most lake users or residents.

But how does one manage water clarity? Isit necessarily good to have extremely clear waterin a lake? When is there too much phytoplankton?These are questions that can only be answeredbased on our needs, activities, or expectations fora particular lake. There are times when no matterwhat our preferences are for water clarity, naturecalls the shots and determines nutrient levels orphytoplankton concentrations, and thus waterclarity.

Hypothetically SpeakingLet’s say that our goal is to increase water

clarity on a hypothetical lake called My Lake.Based on the large amount of data collected forFlorida lakes, it appears that potential strategiesfor improving water clarity on My Lake wouldinvolve changing the abundance of phytoplankton.9

But is this always the case?

Part 4

ManagingLakes forWaterClarity

9 This is generally true, except in waterbodies wherewater clarity is influenced by other factors such as coloror other non-algae particulates.

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☛ See hyperbolic relationships on pages 13-15.

Secchi Depth and ChlorophyllFigure 1 provides us with an excellent

example of how the relationship between Secchidepth and total chlorophyll concentrations forFlorida lakes is a hyperbolic relationship. Tobetter understand the Secchi depth/chlorophyllrelationship in Florida lakes, study Figure 1 tosee if you can recognize the following patterns:

Lakes with extremely low chlorophyll levelsare shown to potentially have high Secchi discreadings (greater than 24 feet).

For lakes in the lower chlorophyll range,water clarity decreases rapidly as chlorophyllconcentrations increase — so rapidly that evensmall increases in chlorophyll levels producesubstantial decreases in water clarity.

Once chlorophyll concentrations exceed 25 μg/L(the chlorophyll value at the rounded corner of thegraph), Secchi disc readings level off and changelittle — even when chlorophyll concentrationsincrease significantly.

Secchi Depth and ColorThere is a similar hyperbolic relationship

between Secchi depth and true color (from dis-solved substances) in water. See Figure 2 for anillustration of this relationship. In this case, thehyperbolic relationship has the followingattributes:

Lakes with low color levels have a highprobability of having clear water.

For lakes in the lower color range (0 - 50 PCU),water clarity decreases rapidly as color increases —so rapidly that even small increases in color producesubstantial decreases in water clarity (Secchi depth).

Once color levels exceed 50 PCU (the color valueat the rounded portion of the graph), water clarity islikely to be substantially reduced and remainrelatively constant for higher levels of color.

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In their efforts to predict how specificmanagement techniques will affect water clarityin a lake, scientists and/or lake managers oftenuse mathematical techniques or models. Two ofthe more widely used mathematical techniquesinclude the use of hyperbolic relationships and/or empirical models. Read on to learn moreabout how these techniques can be used topredict water clarity.

Using Water Chemistry Datato Predict Water Clarity

Science is often a matter of studying relationshipsamong two or more variables. By observing theway these variables relate to one another, scientistsare able to spot relationships.

For example, when Secchi depth measurementsare plotted on a graph along with other lake variables— such as phytoplankton abundance or the colorof the water — patterns often emerge.

Figure 1 on page 14 is an example of ahyperbolic relationship that emerges when Secchidepth measurements were plotted with chlorophyllconcentrations on a graph. Figure 2 shows acomparison between Secchi depth and color.Notice the plotted points form distinctive “L” shapesor curves, also known as mathematical hyperbo-las, hence the phrase “hyperboolic relationships.”

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Hyperbolic Relationships

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The relationship between Secchi depth and total chlorophyll (Figure 1) and Secchidepth and color (Figure 2) for Florida lakes are illustrated here as hyperbolicrelationships.

These relationships are considered to be “hyperbolic” because the plotted pointsform a curved “L” shape — a mathematical hyperbola. While it may be difficult toisolate individual data points on the graph, the overall image is what’s important.

Figure 1 Secchi Depth and Total Chlorophyll

Figure 2 Secchi Depth and Color

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The knowledge that water clarity* is hyperbolically related to phytoplankton abundance**

and dissolved substances*** in lake water has significant implications for anyone interested inmanaging a lake’s water clarity.

Graphing these relationships, as seen in Figures 1 and 2 (page 14) and below, providesa quick way of interpreting or predicting how a lake’s water clarity will “react” to increasesor decreases in phytoplankton abundance and/or true color. It all depends on where the waterclarity value for the lake is plotted on the graph: whether it’s above or below the roundedcorner of the hyperbolic “L” shaped curve.

If the water clarity value for a lake, measured as Secchi depth, is plotted above (to theleft of) the rounded corner of the hyperbola, it means the lake is probably more susceptible todramatic changes in water clarity if phytoplankton abundance or the color of the water shouldhappen to change. Conversely, if a water clarity value for a lake is plotted below (to the rightof) the rounded corner of the graph, then the lake is less susceptible to change. In otherwords, lakes that already have low water clarity will show negligible changes in water claritywhen phytoplankton growth or color concentrations increase.

There are exceptions to every rule, but these basic generalizations provide a good startingpoint for managing lakes for water clarity.

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* LAKEWATCH measures this as Secchi depth, in feet.** LAKEWATCH measures this as chlorophyll concentrations in micrograms per liter (μg/L).*** LAKEWATCH measures this as Platinum Cobalt Units or PCUs.

LAKEWATCH volunteers filter lake water through special filters to “trap” phytoplankton on thesurface of the filter. The samples are then frozen and later analyzed for chlorophyll concentrations.

The Relationship Between Water Clarity,Phytoplankton Abundance, and Water Color

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Amore precise way of using the same waterchemistry data is to transform it into amathematical format called an empirical

model. An empirical model is an equation or a set ofequations derived from statistical analysis of aspecific set of data — a chosen group of lakes.

Using Empirical Models for PredictingWater Clarity

The following segment introduces fourempirical models developed by Florida LAKE-WATCH staff, using the LAKEWATCH database.

The first model, the Secchi depth – chlorophyllempirical model on page 17 is used to identifyrelationships between Secchi depth (water clarity)and chlorophyll concentrations (algae) in a lake.It can be used to predict water clarity, based onchlorophyll concentrations.

The other three models, on pages 18 and 19,are chlorophyll – nutrient models. These modelsrelate chlorophyll concentrations to nutrients(phosphorus, nitrogen, or both). Similar to theSecchi depth – chlorophyll empirical model onpage 17, chlorophyll – nutrient models can be usedto predict how an increase or reduction of nutrientsmight affect chlorophyll levels and thus waterclarity. In fact, chlorophyll – nutrient empiricalmodels are now routinely used in conjunctionwith the Secchi depth – chlorophyll model todevelop lake management strategies for waterclarity. Here’s how they work:

Empirical Models

For step-by-step instructions on how to useempirical models see page 17. Once

you’ve mastered the Secchi depth – chlorophyllempirical model on page 17, try your hand atcalculating each of the three chlorophyll – nutrientempirical models on pages 18-19.

You may want to have your Florida LAKE-WATCH data packet handy so you can use yourlake’s average Secchi depth, chlorophyll, totalphosphorus and/or total nitrogen concentrationsfor the calculations. Or as mentioned earlier, youcan plug in hypothetical numbers to see how yourlake’s phytoplankton levels might be expected tochange.

If you happen to know what the averagechlorophyll concentration and/or nutrient concen-trations are for a lake over a given period oftime, it’s possible to plug those concentrationsinto the equations and after doing a few calculations,estimate what the average water clarity should be.

This can be taken one step further by pluggingin hypothetical chlorophyll and/or nutrient concen-trations — as a way of predicting what water clarityshould be. This type of exercise can be invaluable indetermining whether or not a particular algae manage-ment strategy is worth the cost of implementing. Forexample, is it worth a large expenditure of dollars todecrease phytoplankton levels through nutrientcontrol if water clarity will only be increased from0.5 foot of visibility to an estimated 1.0 foot?

Surveys of lakes throughout the world and

whole-lake experiments have shown that

chlorophyll concentrations in lakes are also

related to their nutrient concentrations,

especially phosphorus. Consequently, there has

been a major effort to develop empirical models

for chlorophyll – phosphorus relationships,

chlorophyll – nitrogen relationships, or

chlorophyll – nutrient relationships

(using both phosphorus and nitrogen).

In the empirical equations on pages 17-19,you’ll see the words “log” and “antilog.” Theterm log is an abbreviation for themathematical term logarithm. A logarithm isthe “exponent that indicates the power towhich a number is raised to produce a givennumber.” [For the equation 102 = 100, the logof 100 is 2. Using the equation 103 = 1000,the log of 1000 is 3.]

The term antilog is an abbreviation for themathematical term antilogarithm. Anantilogarithm is “the number correspondingto a given logarithm.” [For the equation102 = 100, the antilog of 2 is 100. Using theequation 103 = 1000, the antilog of 3 is 1000.]

Clues to understandingempirical models

Consider that a hypothetical lake called My Lake has an average chlorophyll concentrationof 30 μg/L and water clarity of 3.1 feet. Let’s suppose that our lake homeowner’s association isinterested in improving the water clarity by reducing the amount of algae in the lake. They decideto decrease chlorophyll to 10 μg/L. With the following empirical Secchi depth – chlorophyllmodel, developed from Florida LAKEWATCH data, we can plug in this hypothetical chlorophyllconcentration of 10 μg/L and “predict” what the water clarity is expected to be after reducing thechlorophyll.

➷

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➷

➷

How To Use An Empirical Model

Log (Secchi) = 1.171 – 0.463 Log (Chlorophyll)

Where: Log is the common logarithm (base 10), Secchi is the annual mean Secchi depth in feet, and Chlorophyll is the annual mean chlorophyll concentration in μg/L.

To make this calculation...use a calculator with a LOG button and follow these step-by-step instructions.

Step 1 Start by plugging in the hypothetical chlorophyll concentration of10 μg/L into the equation (replace the word “chlorophyll” with thenumber 10). Now find the Log of 10 on your calculator.To find the log of a number on your calculator, type in the number on the keypad (in this instance, type in the number 10), push the button marked “log,”then push the “=” button. For this exercise, you should get an answer of 1.

Example: Log (Secchi) = 1.171 – 0.463 x Log (chlorophyll)

Log (Secchi) = 1.171 – 0.463 x Log (10)

Step 2 Multiply that number (1) by 0.463 (from the equation).

Example: Log (Secchi) = 1.171 – 0.463 x 1.0

Log (Secchi) = 1.171 – 0.463

Step 3 Now subtract 0.463 from 1.171.

Example: Log (Secchi) = 1.171 – 0.463

Log (Secchi) = 0.708

Step 4 Find the antilog of your result. To find the antilog, leave the log (thenumber from the right side of the equation) on the calculator. You shouldsee the Number 0.708. While that number is on your screen, push theantilog key, which is usually represented by the symbol 10x, then push the“=” button. (If your calculator doesn’t have the 10x button, check the instruction booklet.)

You should get an answer of 5.1, which is your predicted Secchi depth in feet.

For Florida lakes, the following empiricalchlorophyll – phosphorus model has beendeveloped from the Florida LAKEWATCH database of 534 waterbodies. Using this model,you can predict phytoplankton abundance (chlorophyll concentrations) by plugging in ahypothetical total phosphorus concentration for a lake. [See the example How To Use An Empirical

Model for step-by-step instructions on how to do the calculations.]

Where: Log is the common logarithm (base 10),Chlorophyll is the annual mean chlorophyll concentration in μg/L, andTP is the annual mean total phosphorus concentration in μg/L.

Confidence Limit Statement:Data analysis shows this model has a 95% confidence interval that ranges from 30% to 325%. For moreon confidence limits, see How Much Confidence Can You Have In An Empirical Model? on page 20.

Empirical chlorophyll – nitrogen models can be derived in a manner similar to thatdescribed for the chlorophyll – phosphorus model above. For Florida lakes, the followingempirical chlorophyll – nitrogen model has been developed from the Florida LAKEWATCHdatabase of 534 waterbodies. Using this model, you can predict chlorophyll concentrations(phytoplankton levels) by plugging in a hypothetical total nitrogen concentration for a lake.[See the example How to Use An Empirical Model for step-by-step instructions. Apply the same steps to theequation below.]

Where: Log is the common logarithm,Chlorophyll is the annual mean chlorophyll concentration in μg/L, andTN is the annual mean total nitrogen concentration in μg/L.

Confidence Limit Statement:Data analysis shows this model has a 95% confidence interval ranging from 23% to 491% forpredicted chlorophyll concentrations (compared to 30% to 325% for the previous phosphorus-chlorophyll model). For more on confidence limits, see How Much Confidence Can You Have InAn Empirical Model? on page 20.

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Log (Chlorophyll) = – 0.369 + 1.053 Log (TP)

Log (Chlorophyll) = – 2.42 + 1.206 Log (TN)

An Empirical Model That Predicts Chlorophyll Concentrations(i.e., phytoplankton abundance) from Phosphorus

An Empirical Model That Predicts Chlorophyll Concentrations(i.e., phytoplankton abundance) from Nitrogen

The most reliable model is an empirical chlorophyll – nutrient model that factors in bothphosphorus and nitrogen concentrations to predict chlorophyll levels. Using this model,you can predict phytoplankton levels (chlorophyll concentrations) by plugging in hypotheticaltotal phosphorus and total nitrogen concentrations for a lake. For Florida lakes, the followingempirical nutrient-chlorophyll model has been developed from the Florida LAKEWATCHdatabase of 534 waterbodies. [See the example How to Use an Empirical Model for step-by-step instruc-tions. Apply the same steps to the equation below.]

Where: Log is the common logarithm (base 10),Chlorophyll is the annual mean chlorophyll concentration in μg/L,TP is the annual mean total phosphorus concentration in μg/L, andTN is the annual mean total nitrogen concentration in μg/L.

Confidence Limit Statement:Data analysis shows that this model is the best available model for Florida lakes. It has a 95%confidence interval ranging from 33% to 312% for predicted chlorophyll concentrations. This isthe smallest confidence range for any published empirical chlorophyll – nutrient model that has beentested for Florida lakes. The confidence interval is also smaller than those established for the simpleempirical phosphorus-chlorophyll (30% to 325%) or nitrogen-chlorophyll (23% to 491%) models.For more on confidence limits, see How Much Confidence Can You Have In An EmpiricalModel? on page 20.

Log (Chlorophyll) = – 1.10 + 0.91 Log (TP) + 0.321 Log (TN)

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An Empirical Model That Predicts Chlorophyll Concentrations(i.e., phytoplankton abundance) From Phosphorus and Nitrogen

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Scientists often choose to answer thisquestion by calculating confidence limits fortheir predictions. By doing a mathematicalanalysis from the same database used to createthe empirical models, scientists can calculate theseconfidence limits.

A 95% confidence interval gives the rangeof chlorophyll values that a measured chlorophyllshould fall into 95% of the time. Confidenceintervals can be smaller when the degree ofcertainty does not need to be as stringent (e.g., 90%confidence, 85% confidence, etc. ). However,water managers usually prefer to be more confident.Use of a 95% confidence interval reflects thedesire of professionals to have their predictionscorrect 95% of the time.

To further explain this concept, let’s use anexample of a lake with total phosphorus concentra-tions of 20 μg/L. If we plug this lake’s totalphosphorus concentration of 20 μg/L into thechlorophyll – phosphorus empirical model (seepage 18), we find that the lake is predicted tohave a total chlorophyll concentration of approxi-mately 10 μg/L.

How much confidence can we have in thisprediction?

According to our calculations, the 95%confidence limits for that particular chlorophyll –phosphorus empirical model ranges from 30% to325%. In other words, there is a 95% confidencethat the actual chlorophyll concentration will fallsomewhere between 3 μg/L and 33 μg/L. SeeCalculate this yourself (top right) for an explana-tion of how these percentages (30% - 325%) weretranslated into whole numbers (3 μg/L - 33 μg/L).

Empirical Models and TheirLimitations

While the confidence interval for this empiricalmodel may seem large (30% to 325% is a ratherexpansive range), it’s not unusual. The confidencelimits of even the most reliable empirical modelcan yield a broad range of chlorophyll values.

The confidence limits provided with thethree nutrient empirical models in this circularare based on Florida LAKEWATCH lakes andtruly reflect the variability of chlorophyll con-centrations found in waterbodies in this state.(Look for confidence interval statements at thebottom of each of the nutrient empirical modelson pages 18 and 19.) Such variability makespredictions from all empirical chlorophyll – nutrientmodels somewhat uncertain, particularly whenonly small changes occur in nutrient concentrations.

Also, keep in mind that when dealing withreal waterbodies, as opposed to hypotheticalones, there is a broad range of possible chlorophyllconcentrations that can occur based on anyspecific amount of nutrients in the system.It’s difficult to predict precise quantities whendealing with real-world waterbodies and the

How much confidence can youhave in an empirical model?

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Calculate this yourselfUsing the chlorophyll – phosphorus

empirical model example on page 18, weknow that a chlorophyll concentration of 10 μg/Lwas predicted. We can use this predictedchlorophyll concentration of 10 μg/L along withthe 95% confidence limits of 30% (0.30) to325% (3.25), to do the following calculations:

0.30 X 10 μg/L = 3 μg/L

30% of 10 μg/L is 3 μg/L

and

325% of 10 μg/L approx. = 33 μg/L

3.25 X 10 μg/L approx. = 33 μg/L

In other words, the actual chlorophyllvalue for this sample lake should be some-where between 3 μg/L and 33 μg/L, 95% ofthe time.

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multitude of factors that can come into play.Because other environmental factors such as

local climate, geology, and aquatic macrophytescan also influence phytoplankton levels, managersmay make their predictions more accurate bydeveloping empirical models using data fromwaterbodies within the same local geographicregion. When developing these empirical mod-els, a basic understanding of how waterbodiesfunction in that area should be combined withthe best available data.

Of course, there are instances when anindividual lake may fall outside the predictionsfound while using any empirical model. Whenthis happens, it’s important for that lake to bestudied independently of others in its region tofind out what is “driving” the phytoplanktonproductivity of the lake.

Probably the most importantlesson to be learned fromempirical models is that,in Florida lakes, it’s been foundthat small changes in nutrientconcentrations will notproduce noticeable changes inwater clarity, except perhaps inlakes with generally lowproductivity.

In other words,if you want to decreasechlorophyll concentrations(meaning algal levels) to thepoint where people actuallysee a change in water clarity,you will have to dramaticallydecrease nutrientconcentrations.

While there are several empirical modelscurrently being used throughout Florida, westrongly suggest that lake managers and citizensconsider using the Secchi depth – chlorophyllmodel (page 17) as well the three chlorophyll –nutrient empirical models provided in thiscircular (pages 18 – 19). These models are basedon a large number of Florida lakes and offer agood starting point for determining the mostappropriate management options for your lake orwaterbody.

Lastly, remember that empirical models merelyprovide a framework for evaluating how changingnutrient concentrations could affect phytoplanktonlevels in a lake, and thus water clarity. Thesemodels provide a guide, not absolute answers.

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Appendix ADescriptions of TermsExcerpts from Florida LAKEWATCHInformation Circular 101

Algaeare a wide variety of tiny, often microscopic, plants(or plant-like organisms) that live both in water andon land. The word “algae” is plural (pronouncedAL-jee), and “alga” is the singular form (pronouncedAL-gah).

One common way to classify water-dwellingalgae is based on where they live. Using this system,three types of algae are commonly defined asfollows:

phytoplankton float freely in the water;

periphyton are attached to aquatic vegetationor other structures;

benthic algae grow on the bottom.

Algae may further be described as beingsingle-celled, colonial (grouped together incolonies), or filamentous (hair-like strands). Themost common forms of algae are also describedby their colors: green, blue-green, red, andyellow. All these classifications may be usedtogether. For example, to describe blue-green, hair-like algae that are attached to an underwater plant,you could refer to them as “blue-green filamen-tous periphyton.”

In addition to describing types of algae, it isuseful to measure their quantity. The amount ofalgae in a waterbody is often called algalbiomass. Scientists commonly make estimatesof algal biomass based on two types of measurements:

Because most algae contain chlorophyll (thegreen pigment found in plants), the concentrationof chlorophyll in a water sample can be used toindicate the amount of algae present. This methodhowever, does not include all types of algae,

only the phytoplankton. Chlorophyll concentra-tions are measured in units of micrograms perLiter (abbreviated μg/L) or in milligrams per cubicmeter (abbreviated mg/m3).

In certain cases, scientists prefer to count andmeasure individual algal cells in a sample and usetheir count to calculate the volume of the algae.

Most people consider algae to be unsightly,particularly when it is abundant. For instance, aphytoplankton bloom can make water appear sogreen that it’s described as “pea soup.”

In Florida, when chlorophyll concentrationsreach a level over 40 μg/L some scientists willcall it an “algae bloom” or “algal bloom.” Thepublic, however, usually has a less scientificapproach. They often define an algal bloom aswhenever more algae can be seen in the waterthan they are accustomed to seeing (even thoughthis may be a low concentration in some cases).

Algal blooms may be caused by humanactivities, or they may be naturally occurring.Sometimes, what seems to be an algal bloom ismerely the result of wind blowing the algae into acove or onto a downwind shore, concentrating it ina relatively small area. This is called “windrowing.”

The Role of Algae in Waterbodies:Algae are essential to aquatic systems. As a

vital part of the food web, algae provide the foodnecessary to support all aquatic animal life.

Filamentous algal blooms and benthic algalblooms have the potential to interfere withrecreational uses like boating and fishing.

An algal bloom can trigger a fish kill. InFlorida, this is most likely to occur after severaldays of hot weather with overcast skies.

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Aquatic Macrophytesare aquatic plants that are large enough to beapparent to the naked eye. In other words, they arelarger than microscopic algae. The general phrase“aquatic plants” usually refers to aquaticmacrophytes, but most scientists use it to meanaquatic macrophytes and algae.

Aquatic macrophytes characteristicallygrow in water or in wet areas and are quite adiverse group. For example, some are rooted inthe bottom sediments, while others float on thewater’s surface and are not rooted to the bottom.Aquatic plants may be native to an area, or theymay have been imported (referred to as “exotic”).

Most aquatic macrophytes are vascularplants, meaning they contain a system of fluid-conducting tubes, much like human bloodvessels. Cattails, waterlilies, and hydrilla areexamples. Large algae such as Nitella, Lyngbya,and Chara are often included in the category ofaquatic macrophytes.

Even though they are quite diverse, aquaticmacrophytes have been grouped into four generalcategories:

emergent aquatic plants are rooted in thebottom sediments and protrude up above thewater’s surface;

submersed aquatic plants primarily growcompletely below the water’s surface; and

floating aquatic plants float on the water withroots suspended down into the water;

floating-leaved aquatic plants can be rooted tobottom sediments and have leaves that float onthe water’s surface.

Aquatic macrophytes are a natural part ofwaterbodies, although in some circumstancesthey can be troublesome. The same plant may bea “desirable aquatic plant” in one location and a“nuisance weed” in another. When exotic aquaticplants have no natural enemies in their adoptedarea, they can grow unchecked and may becomeoverly abundant.

In Florida for example, millions of dollarsare spent each year to control two particularlyaggressive and fast-growing aquatic macrophytes— water hyacinth, an exotic floating aquatic

plant that is thought to be from Central and SouthAmerica, and hydrilla, an exotic submersedaquatic plant that is thought to be from Asia.However, the term “weed” is not reserved forexotic aquatic plants only. In some circumstances,native aquatic plants such as cattails or Potamogeton(i.e., pondweed) can cause serious problems.

When assessing the abundance of aquaticplants in a waterbody, scientists may choose tomeasure or calculate one or more of the following:

PVI (Percent Volume Infested or PercentVolume Inhabited) is a measure of the percentageof a waterbody’s volume that contains aquaticplants;

PAC (Percent Area Covered) is a measure ofthe percentage of a waterbody’s bottom area thathas aquatic plants growing on or over it;

frequency of occurrence is an estimate of theabundance of a specific aquatic plant; and

average plant biomass is the average weightof several samples of fresh, live aquatic plantsgrowing in a given amount of a lake’s area.

The Role of Aquatic Macrophytes in Waterbodies:Aquatic macrophytes perform several

functions in waterbodies, often quite complexones. A few are briefly described below.

Aquatic macrophytes provide habitat for fish,wildlife, and other aquatic animals.

Aquatic macrophytes provide habitat and foodfor organisms that fish and wildlife feed on.

Aquatic macrophytes along a shoreline can protectthe land from erosion caused by waves and wind.

Aquatic macrophytes can stabilize bottomsediments by dampening wave action.

The mixing of air into the water that takesplace at the water’s surface can be obstructed bythe presence of floating plants and floating-leaved plants. In this way, they can cause loweroxygen levels in the water.

Floating plants and floating-leaved plants createshaded areas that can cause submersed plantsbeneath them to grow slower and even die.

When submersed aquatic plants become abun-

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dant, these plants can cause water to become clear.Conversely, the removal or decline of largeamounts of submersed aquatic plants can causewater to become less clear.

When aquatic macrophytes die, the underwaterdecay process uses oxygen from the water. Ifmassive amounts of plants die simultaneously, afish kill can result due to low oxygen.

Decayed plant debris (dead leaves, etc.) contrib-utes to the buildup of sediments on the bottom.

Biological Productivityis defined conceptually as the ability of a waterbodyto support life (such as plants, fish, and wildlife).Biological productivity is defined scientificallyas the rate at which organic matter is produced.Measuring this rate directly for an entire waterbodyis difficult and prohibitively expensive.

For this reason, many scientists base esti-mates of biological productivity on one or morequantities that are more readily measured. Theseinclude measurements of concentrations ofnutrients in water, concentrations of chlorophyllin the water, aquatic plant abundance, and/orwater clarity. The level of biological productivityin a waterbody is used to determine its trophicstate classification.

Chlorophyllis the green pigment found in plants and in nearlyall algae. Chlorophyll allows plants and algae touse sunlight in the process of photosynthesis forgrowth. Thanks to chlorophyll, plants are able toprovide food and oxygen for the majority ofanimal life on earth.

Scientists may refer to chlorophyll a, whichis one type of chlorophyll, as are chlorophyll band chlorophyll c. Measurements of total chloro-phyll include all types. Chlorophyll can beabbreviated CHL, and total chlorophyll can beabbreviated TCHL.

The Role of Chlorophyll in Waterbodies:Measurements of the chlorophyll concentra-

tions in water samples are useful to scientists. Forexample, they are often used to estimate algalbiomass in a waterbody and to assess awaterbody’s biological productivity.

In Florida:Waterbodies in the Florida LAKEWATCH

database analyzed prior to January 2000, hadaverage chlorophyll concentrations which rangedfrom less than 1 to over 400 μg/L. Using theseaverage chlorophyll concentrations from thissame database, Florida lakes were found to bedistributed into the four trophic states as follows:

12% of the lakes would be classified as olig-otrophic (those with chlorophyll values less thanor equal to 3 μg/L);

about 31% of the lakes would be classified asmesotrophic (those with chlorophyll valuesgreater than 3 and less than 7 μg/L);

41% of these lakes would be classified aseutrophic (those with chlorophyll values greaterthan 7 and less than or equal to 40 μg/L); and

nearly 16% of the lakes would be classified ashypereutrophic(those with chlorophyll valuesgreater than 40 μg/L).

In Florida, characteristics of a lake’sgeographic region can provide insight into howmuch chlorophyll may be expected for lakes inthat area. For example, water entering the water-bodies by stream flow or underground flowagethrough fertile soils can pick up nutrients thatcan then fertilize the growth of algae and aquaticplants. In this way, the geology and physiographyof a watershed can significantly influence awaterbody’s biological productivity.

Health Concerns:Chlorophyll poses no known direct threat to

human health. There are some rare cases wherealgae can produce toxins in high enough abundanceto cause concern. However, toxic algae aregenerally not a problem.

Eutrophicis an adjective used to describe the level ofbiological productivity of a waterbody. FloridaLAKEWATCH and many professionals classifylevels of biological productivity using four trophicstate categories (oligotrophic, mesotrophic,eutrophic, and hypereutrophic). Of the fourtrophic state categories, the eutrophic state isdefined as having a high level of biological

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productivity, second only to the hypereutrophiccategory. The prefix “eu” means good, well, orsufficient.

A eutrophic waterbody is capable of pro-ducing and supporting an abundance of livingorganisms (plants, fish, and wildlife). Eutrophicwaterbodies generally have the characteristicsdescribed below:

Eutrophic lakes are more biologically productivethan oligotrophic and mesotrophic lakes and areoften some of Florida’s best fishing lakes. Theyusually support large populations of fish, includingsportfish such as largemouth bass, speckledperch (black crappie), and bream (bluegill).

Typically, eutrophic waters are characterizedas having sufficient nutrient concentrations tosupport the abundant growth of algae and/oraquatic plants.

When algae dominate a eutrophic waterbody,its water will have high chlorophyll concentrations(i.e., greater than 7 μg/L). The water will be lessclear, causing Secchi depth readings to be low.In contrast, when instead of algae, aquatic plantsdominate a eutrophic waterbody, its water willhave lower chlorophyll concentrations and oftenlower nutrient concentrations and clearer water.The resulting water clarity will be reflected inSecchi depth readings that are greater than ineutrophic waterbodies that have few aquaticplants.

Despite being classified as eutrophic, theseplant-dominated waterbodies display the clearwater, low chlorophyll concentrations, and lownutrient concentrations that are more characteristicof mesotrophic or oligotrophic waterbodies.

Regardless of whether eutrophic waterbodiesare plant-dominated or algae-dominated, theygenerally have a layer of sediment on the bottomresulting from the long-term accumulation ofplant debris. In some eutrophic lakes, however,the action of wind and waves can create beachesor sand-bottom areas in localized places.

Eutrophic waterbodies can have occasionalalgal blooms and fish kills. However, fish killsgenerally occur in hypereutrophic lakes whenchlorophyll concentrations exceed 100 μg/L.

Geologic Regionis an area that has similar soils and underlyingbedrock features. The characteristics of thegeologic region in which a waterbody is locatedmay be responsible for the water’s chemicalcharacteristics and trophic state. Geology can alsohave a significant influence on the shape of awaterbody’s basin, a factor that affects many offeatures of a waterbody.

Hypereutrophicis an adjective used to describe the level ofbiological productivity of a waterbody. FloridaLAKEWATCH and many professionals classifylevels of biological productivity using four trophicstate categories — oligotrophic, mesotrophic,eutrophic, and hypereutrophic.

Of the four trophic state categories, thehypereutrophic state is defined as having thehighest level of biological productivity. The prefix“hyper” means over abundant. Hypereutrophicwaterbodies are among the most biologicallyproductive in the world. Hypereutrophicwaterbodies generally have the characteristicsdescribed below.

Hypereutrophic waterbodies have extremelyhigh nutrient concentrations.

While hypereutrophic waterbodies can bedominated by non-sportfish species (gizzardshad or threadfin shad), they can also supportlarge numbers and large sizes of sportfish includ-ing largemouth bass, speckled perch (blackcrappie) and bream (bluegill).

A hypereutrophic waterbody has either anabundant population of algae or an abundantpopulation of aquatic macrophytes — andsometimes it will support both.

Hypereutrophic waterbodies that are dominatedby algae are characterized by having high chlo-rophyll concentrations (greater than 40 μg/L).These waterbodies will have reduced waterclarity, causing Secchi depth readings to be lessthan 1 meter (about 3.3 feet). In contrast, whenaquatic macrophytes instead of algae dominate ahypereutrophic waterbody, its water can havelower chlorophyll concentrations. The resulting

water clarity will be reflected in higher Secchidepth readings (clearer water), mimicking those ofless biologically productive waterbodies.

Regardless of whether a waterbody is plant-dominated or algae-dominated, typically it willhave organic bottom sediments as the decayingplant and/or algal debris accumulates.

Hypereutrophic waterbodies may experiencefrequent algal blooms.

Oxygen depletion may also be a commoncause of fish kills in these waterbodies.

Lake regionis a geographic area in which lakes have similargeology, soils, chemistry, hydrology, and biologicalfeatures. In 1997, using Florida LAKEWATCHdata and other information, the United StatesEnvironmental Protection Agency divided Floridainto 47 lake regions using these similarities as theircriteria.

Lakes in an individual lake region exhibitremarkable similarities. However, lakes in onelake region may differ significantly from those ina different lake region. For example, most lakesin the New Hope Ridge/Greenhead Slope lakeregion in northwestern Florida (in Washington,Bay, Calhoun, and Jackson counties) tend to havelower total nitrogen, lower total phosphorus, lowerchlorophyll concentrations, and greater Secchidepths when compared to other Florida lakes.

While lakes in the Lakeland/Bone ValleyUpland lake region in central Florida (in Polkand Hillsborough counties) tend to have highertotal nitrogen, higher total phosphorus, higherchlorophyll concentrations, and reduced Secchidepths when similarly compared.

Using descriptions of lake regions, water-body managers can establish reasonable, attain-able water management goals for individual lakes.Lake region characteristics can also be used tohelp choose management strategies that arelikely to be effective in achieving managementgoals. In addition, lakes with water chemistrythat differs markedly from that of other lakes inthe same lake region can be identified andinvestigated to determine the cause of their beingatypical.

The lake regions are mapped and describedin Lake Regions of Florida (EPA/R–97/127).The Florida LAKEWATCH Program can provideyou with a free handout describing (1) how andwhy the lake regions project was developed; (2)how to compare your lake with others in its LakeRegion; and (3) how the Lake Region Classifica-tion System can be useful to you.

Limnologyis the scientific study of the physical, chemical,and biological characteristics of inland (non-marine) aquatic systems. A limnologist is ascientist who studies inland aquatic systems.

MacrophytesSee Aquatic Macrophytes.

Mean Depthis another way of saying “average water depth.”The mean water depth is measured in either feet ormeters and is designated in scientific publicationsby the letter “z.”

Mean depth can be estimated by measuringthe water depth in many locations and averagingthose values. Individual depth measurementsmay be taken by using a depth finder (fathometer)or by lowering a weight, at the end of a stringor rope, into the water and measuring how far itsinks below the surface until it rests on the bottom.

If more accuracy is needed, mean depthshould be calculated by dividing a waterbody’svolume by its surface area. This method willoften result in a different value than if measureddepths are averaged.

Mesotrophicis an adjective used to describe the level ofbiological productivity of a waterbody. FloridaLAKEWATCH and many professionals classifylevels of biological productivity using four trophicstate categories — oligotrophic, mesotrophic,eutrophic, and hypereutrophic.

Of the four trophic state categories, themesotrophic state is defined as having a moderatelevel of biological productivity. The prefix “meso”means mid-range. A mesotrophic water-body iscapable of producing and supporting moderate

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populations of living organisms (plants, fish, andwildlife). Mesotrophic waterbodies generally have:

moderate nutrient concentrations;

moderate growth of algae, aquatic plants or both;

water that is clear enough (visibility between8 and 13 feet) that most swimmers are notrepelled by its appearance and can generally seeany potential underwater hazards.

Nitrogenis an element that, in its different forms, stimulates thegrowth of aquatic plants and algae.

Nutrientsare chemicals that algae and aquatic plants need fortheir growth. Nitrogen and phosphorus are thetwo most influential nutrients in Florida waterbodies.Nutrients can come from a variety of sources.

In most cases, nutrients are carried into awaterbody primarily when water drains throughthe surrounding rocks and soils, picking upnitrogen and phosphorus compounds along theway. For this reason, knowledge of the geologyand physiography of the area can provide insightinto how much nutrient enrichment can bereasonably expected in an individual waterbodyfrom this natural source.

For example, lakes in the New Hope Ridge/Greenhead Slope lake region in northwesternFlorida (in Washington, Bay, Calhoun, andJackson counties) can be expected to have lownutrient levels, because they are in a nutrient-poor geographic region. While lakes in the Lake-land/Bone Valley Upland lake region in centralFlorida (in Polk and Hillsborough counties) can beexpected to have high nutrient levels, becausethe land surrounding the lakes is naturallynutrient-rich.

There are many other sources of nutrientsthat are generally not as substantial as nutrientcontributions from surrounding rocks and soils.Some occur naturally, and some are the results ofhuman activity. For example nutrients are conveyedin rainfall, stormwater runoff, seepage fromseptic systems, bird and animal feces, and the airitself. Most nutrients can move easily through theenvironment. They may come from nearby woods,

farms, yards, and streets — anywhere in thewatershed.

Oligotrophicis an adjective used to describe the level ofbiological productivity of a waterbody. FloridaLAKEWATCH and many professionals classifylevels of biological productivity using four trophicstate categories — oligotrophic, mesotrophic,eutrophic, and hypereutrophic.

Of the four trophic state categories, theoligotrophic state is defined as having the lowestlevel of biological productivity. The prefix“oligo” means scant or lacking.

An oligotrophic waterbody is capable ofproducing and supporting relatively small populationsof living organisms (plants, fish, and wildlife).The low level of productivity in oligotrophicwaterbodies may be caused by there being a lowlevel of a limiting nutrient in the water, particularlynitrogen or phosphorus, or by limiting environ-mental factors other than nutrients.

Oligotrophic waterbodies generally havethe following characteristics:

Because nutrients are typically in short supply,aquatic plants and algae in oligotrophic water-bodies are in low abundance.

An oligotrophic waterbody typically will havelittle plant debris accumulated on the bottomsince aquatic plants and algae are in low abundance.

Oligotrophic waterbodies will often tend tohave clear water, because the clarity is notdiminished by the presence of free-floating algaein the water. The clarity may be decreased,however, by the presence of color, stirred-upbottom sediments, or washed-in particulate matter.

Fish and wildlife populations will generally besmall, because food and habitat are often scarce.Oligotrophic waterbodies usually do not supportabundant populations of sportfish such as large-mouth bass and bream, and it usually takes longerfor individual fish to grow in size. Fishing may begood initially if the number of anglers is small, butcan deteriorate rapidly when fishing pressureincreases and fish are removed from the waterbody.

A waterbody may have oligotrophic charac-

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teristics even though it has high nutrient levels.This can occur when a factor other than nutrientsis limiting the growth of aquatic plants and algae.For example, where a significant amount ofsuspended sediments (stirred-up sediments orparticles washed in from the watershed) or darklycolored water is retarding plant growth byblocking sunlight.

PACis an abbreviation for percent area covered andis a measure of the percentage of a waterbody’sbottom area that has aquatic plants growing onor over it. Scientists use PAC to assess theabundance and importance of aquatic plants in awaterbody.

Waterbodies in the Florida LAKEWATCHdatabase analyzed prior to January 2000, hadPAC values that ranged from 0 to 100%. PACvalues are linked with the biological productivity(trophic state) of waterbodies:

In the least productive (oligotrophic) water-bodies, PAC values are usually low. In rare caseswhere PAC values are high (occasionally reach-ing 100%), it is usually due to a thin layer ofsmall plants growing along the bottom.

In moderately productive (mesotrophic) andhighly productive (eutrophic) waterbodies, PACvalues are generally greater than those measuredin oligotrophic waterbodies, and the averageplant biomass is also greater.

In extremely productive (hypereutrophic)waterbodies that are dominated by algae, PACvalues are often less than 25%. In Florida how-ever, many hypereutrophic waterbodies containmostly aquatic plants, not algae. In these cases,PAC values often tend to be greater than 75%.

Particulatesare any substances in the form of small particlesthat are found in waterbodies, often suspended inthe water column. Substances in water are eitherin particulate form or in dissolved form. Passingwater through a filter will separate these twoforms. The filter will trap most of the particulates,allowing the dissolved substances to pass through.

Phosphorusis an element that, in its different forms, stimulates thegrowth of aquatic plants and algae in waterbodies.

Physiographic regionis a geographic area whose boundaries encloseterritory that has similar physical geology (i.e., soiltypes, land formations, etc.).

Phytoplanktonare small, free-floating aquatic plants that aresuspended in the water column. They aresometimes called “planktonic algae” or just“algae.” Though small, phytoplankton performimportant functions in waterbodies. For example,phytoplankton abundance often determines howbiologically productive waterbodies can be —how much fish and wildlife waterbodies cansupport. Also, the public is concerned about theabundance of phytoplankton, because itsignificantly affects water clarity.

Aquatic scientists assess phytoplanktonrelative abundance by estimating its biomass.Two common methods are used: (1) viewingphytoplankton through a microscope and countingthem, and (2) measuring the chlorophyll concentra-tions in water samples. Florida LAKEWATCHuses the chlorophyll method because it’s fasterand less costly.

Planktonic AlgaeSee Phytoplankton.

PVIis a measure of the percentage of a waterbody’svolume that contains aquatic plants. Historically,PVI represented the percent volume infested withaquatic plants. Recently, it has become anabbreviation for the more neutral phrase percentvolume inhabited. Regardless of the terminology,PVI is used to assess the abundance of aquaticplants in a waterbody.

In Florida:Numerous plant surveys performed on

Florida LAKEWATCH lakes have shown thatprior to January 2000, PVI values ranged from 0to 100%. In Florida, PVI values are strongly

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linked with the biological productivity (trophicstate) of waterbodies as described below:

In the least biologically productive waterbodies,(oligotrophic) PVI values are generally low.

In moderately biologically productivewaterbodies (mesotrophic) and highly productivewaterbodies (eutrophic) dominated by aquaticplants, PVI values are higher than those measuredin oligotrophic waterbodies.

The most highly biologically productive(hypereutrophic) waterbodies that are dominatedby algae usually have low PVI values. However,hypereutrophic waterbodies dominated byaquatic plants usually have high PVI values.

Secchi depthis a measurement that indicates water clarity.Traditionally, the transparency or water clarity of awaterbody has been measured using an 8-inchdiameter disc called a Secchi disc, that wasnamed in honor of its inventor. A Secchi disc isusually painted in alternating quadrants of blackand white, although it can be solid white. Thereis a line (a rope or chain) attached through theSecchi disc’s center that is marked off inintervals, usually in feet or meters.

To use the Secchi disc to measure water clarity,it’s lowered into the water to find the depth atwhich it first vanishes from the observer’s sight.

Note that if the disc can still be seen as it restson the lake bottom or if it disappears into plantgrowth, the depth at which this happens is not ameasurement of the waterbody’s Secchi depth.

Surface Wateris water found on the earth’s surface. It isdistinguished from “groundwater” which is foundunderground. Surface waters include many typesof waterbodies such as estuaries, lakes, marshes,ponds, reservoirs, rivers, streams and swamps.

Total Chlorophyllis a measure of all types of chlorophyll. TheFlorida LAKEWATCH abbreviation for totalchlorophyll is CHL.

Total Nitrogenis a measure of all the various forms of nitrogenthat are found in a water sample. Nitrogen is anecessary nutrient for the growth of aquaticmacrophytes and algae. Not all forms of nitrogencan be readily used by aquatic macrophytes andalgae, especially nitrogen that is bound withdissolved or particulate organic matter. Thechemical symbol for the element nitrogen is N,and the symbol for total nitrogen is TN.

Total nitrogen consists of inorganic andorganic forms. Inorganic forms include nitrate(NO3

-), nitrite (NO2-), unionized ammonia (NH3),

ionized ammonia (NH4+), and nitrogen gas (N2).

Amino acids and proteins are naturally-occurringorganic forms of nitrogen. All forms of nitrogenare harmless to aquatic organisms except union-ized ammonia and nitrite, which can be toxic tofish. Nitrite is usually not a problem inwaterbodies because nitrite is readily converted tonitrate.

The Role of Nitrogenin Waterbodies:

Like phosphorus, nitrogen is an essentialnutrient for all plants, including aquatic macrophytesand algae. In some cases, the inadequate supplyof TN in waterbodies has been found to limit thegrowth of free-floating algae (i.e., phytoplankton).This is called “nitrogen limitation,” and occursmost commonly when the ratio of total nitrogento total phosphorus is less than 10 (in otherwords, the TN concentration divided by the TPconcentration is less than 10: TN/TP < 10). TNin waterbodies comes from both natural andman-made sources, including:

the air (some algae can “fix” nitrogen; that is,the algae can pull it out of the air in its gaseousform and convert it to a form they can use);

stormwater run-off (even “natural” run-offfrom areas where there is no human impact,because nitrogen is a naturally-occurring nutrientfound in soils and organic matter);

fertilizers; and

animal and human wastes (sewage, dairies,feedlots, etc.).

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In Florida:Waterbodies in the Florida LAKEWATCH

database analyzed prior to January 2000, hadtotal nitrogen concentrations which ranged fromless than 50 to over 6000 μg/L. Using theseaverage concentrations of total nitrogen fromthis same database, Florida lakes were found tobe distributed into four trophic states as follows.

approximately 14% of the lakes would beclassified as oligotrophic (those with TN valuesless than 400 μg/L);

about 25% of the lakes would be classified asmesotrophic (those with TN values between 401and 600 μg/L);

50% of the lakes would be classified aseutrophic (those with TN values between 601and 1500 μg/L); and

nearly 11% of the lakes would be classified ashypereutrophic (those with TN values greaterthan 1500 μg/L).

The location of a waterbody has a stronginfluence on its total nitrogen concentration. Forexample, lakes in the New Hope Ridge/GreenheadSlope lake region in northwestern Florida (inWashington, Bay, Calhoun, and Jackson counties)tend to have total nitrogen values below 220 μg/L.While lakes in the Lakeland/Bone Valley Uplandlake region in central Florida (in Polk andHillsborough counties) tend to have values above1700 μg/L.

Health Concerns:The concentration of total nitrogen in water

is not a known direct threat to human health. It isthe individual forms of nitrogen that contributeto the total nitrogen measurement and the use ofthe water that need to be considered.

For example, nitrate in drinking water is aconcern. Drinking water with nitrate concentra-tions above 45 mg/L has been implicated incausing blue-baby syndrome in infants. Themaximum allowable level of nitrate, a componentof the total nitrogen measurement, is 10 mg/L indrinking water. Concentrations of nitrate greaterthan 10 mg/L generally do not occur in waterbodies,because nitrate is readily taken up by plants andused as a nutrient.

Total Phosphorusis a measure of all the various forms of phosphorusthat are found in a water sample. Phosphorus is anelement that, in its different forms, stimulates thegrowth of aquatic macrophytes and algae inwaterbodies. The chemical symbol for the elementphosphorus is “P,” and the symbol for totalphosphorus is “TP.” Some phosphorus compoundsare necessary nutrients for the growth of aquaticmacrophytes and algae. Phosphorus compounds arefound naturally in many types of rocks. Mines inFlorida and throughout the world provide phosphorusfor many agricultural and industrial uses.

The Role of Phosphorus in Waterbodies: Like nitrogen, phosphorus is an essential

nutrient for the growth of all plants, includingaquatic macrophytes and algae. Phosphorus inwaterbodies takes several forms, and the way itchanges from one form to another, also calledcycling, is complex. Because phosphoruschanges form so rapidly, many aquatic scientistsgenerally assess its availability by measuring theconcentration of total phosphorus rather than theconcentration of any single form. In some water-bodies, phosphorus may be at low levels thatlimit further growth of aquatic macrphytes and/or algae. In this case, scientists say phosphorus isthe “limiting nutrient.”

For example, waterbodies having TP con-centrations less than 10 μg/L will be nutrientpoor and will not support large quantities ofalgae and aquatic macrophytes. There are manyways in which phosphorus compounds enterwater. The more common ones are described below:

Some areas of Florida and other parts of theworld have extensive phosphate deposits. In theseareas, rivers and water seeping or flowing under-ground can become phosphorus enriched and maycarry significant amounts of phosphorus intowaterbodies.

Sometimes phosphorus is added intentionallyto waterbodies to increase fish production byfertilizing aquatic macrophytes and algal growth.

Phosphorus can enter waterbodies inadvert-ently as a result of human activities like landscapefertilization, crop fertilization, wastewater

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disposal, and stormwater run-off from residentialdevelopments, roads, and commercial areas.

In Florida:Waterbodies in the Florida LAKEWATCH

database analyzed prior to January 2000, hadtotal phosphorus concentrations which rangedfrom less than 1 to over 1000 μg/L. Using theseaverage concentrations of total phosphorus fromthis same database, Florida lakes were distributedinto the four trophic states as follows:

approximately 42% of the lakes would beclassified as oligotrophic (those with TP valuesless than 15 μg/L);

about 20% of the lakes would be classified asmesotrophic (those with TP values between 15and 25 μg/L);

30% of the lakes would be classified aseutrophic (those with TP values between 25 and100 μg/L); and

nearly 8% of the lakes would be classified ashypereutrophic (those with TP values greater than100 μg/L) .

The location of a waterbody has a stronginfluence on its total phosphorus concentration.For example, lakes in the New Hope Ridge/Greenhead Slope lake region in northwesternFlorida (in Washington, Bay, Calhoun, andJackson Counties) tend to have total phosphorusvalues below 5 μg/L. While lakes in the Lake-land/Bone Valley Upland lake region in centralFlorida (in Polk and Hillsborough Counties) tendto have values above 120 μg/L.

Health Concerns:There is no known level of total phosphorus

in water that poses a direct threat to human health.

TransparencySee Water Clarity.

Trophic Stateis defined as “the degree of biological productivityof a waterbody.” Scientists debate exactly what ismeant by biological productivity, but it generallyrelates to the amount of algae, aquatic macrophytes, fishand wildlife a waterbody can produce and sustain.

Waterbodies are traditionally classified into

four groups according to their level of biologicalproductivity. The adjectives denoting each ofthese trophic states, from the lowest productivitylevel to the highest, are oligotrophic, mesotrophic,eutrophic, and hypereutrophic. Aquatic scientistsassess trophic state by using measurements ofone or more of the following:

total phosphorus concentrations in the water; total nitrogen concentrations in the water; total chlorophyll concentrations — a measure

of free-floating algae (phytoplankton), in thewater column;

water clarity, measured using a Secchi disc; aquatic macrophyte abundance.

The Florida LAKEWATCH professionalsbase trophic state classifications primarily on theamount of chlorophyll in water samples. Chlorophyllconcentrations have been selected by LAKE-WATCH as the most direct indicators of biologicalproductivity, since the amount of algae actuallybeing produced in a waterbody is reflected in theamount of chlorophyll present. In addition,Florida LAKEWATCH professionals may modifytheir chlorophyll-based classifications by takingthe aquatic plant abundance into account.

Water Clarityis the transparency or clearness of water. Whilemany people tend to equate water clarity withwater quality, it’s a misconception to do so.Contrary to popular perceptions, crystal clearwater may contain pathogens or bacteria thatwould make it harmful to drink or to swim in,while pea-soup green water may be harmless.

Water clarity in a waterbody is commonlymeasured by using an 8-inch diameter Secchidisc, attached to a string/rope. The disc is loweredinto the water, and the depth at which it vanishesfrom sight is measured. Measured in this way,water clarity is primarily affected by threecomponents in the water:

free-floating algae called phytoplankton,

dissolved organic compounds that color thewater reddish, brown, or black, and

sediments suspended in the water, either stirredup from the bottom or washed in from the shore.

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Water clarity is important to individualswho want the water in their swimming areas tobe clear enough so that they can see where theyare going. In Canada, the government recommendsthat water should be sufficiently clear so that aSecchi disc is visible at a minimum depth of 1.2meters (about 4 feet). This recommendation is onereason that many eutrophic and hypereutrophiclakes that have abundant growths of free-floatingalgae do not meet Canadian standards for swimmingand are deemed “undesirable.” It should be notedthat these lakes are not necessarily “undesirable” forfishing nor are they necessarily polluted in thesense of being contaminated by toxic substances.

The Role of Water Clarity in Waterbodies:Water clarity will have a direct influence on

the amount of biological production in a water-body. When water is not clear, sunlight cannotpenetrate far and the growth of aquatic plantswill be limited. Consequently aquatic scientistsoften use Secchi depth measurements (alongwith total phosphorus, total nitrogen, and totalchlorophyll concentrations) to determine awaterbody’s trophic state.

Because plants must have sunlight in orderto grow, water clarity is also directly related tohow deep underwater aquatic macrophytes willbe able to live. This can be estimated usingSecchi depth readings. A rule of thumb is thataquatic macrophytes can grow to a depth ofabout 1.5 times the Secchi depth measurement.For example, for a Secchi depth measurement of3 feet, the depth at which aquatic macrophytescan grow is limited to about 4.5 feet.

Water clarity affects plant growth butconversely, the abundance of aquatic plants canaffect water clarity.

Generally, increasing the abundance ofsubmersed aquatic macrophytes to cover 50% ormore of a waterbody’s bottom may have the effectof increasing the water clarity.

One explanation is that either the sub-mersed macrophytes, or perhaps the algaeattached to the aquatic macrophytes, use theavailable nutrients in the water, depriving the free-floating algae of them. Submersed macrophytes alsoanchor the nutrient-rich bottom sediments in place

— buffering the action of wind, waves, andhuman effects — depriving the free-floating algaeof nutrients contained in the bottom sedimentsthat would otherwise be stirred up.

In Florida:Waterbodies in the Florida LAKEWATCH

database analyzed prior to January 2000, hadSecchi depths ranging from less than 0.2 to over11.6 meters (from about 0.7 and 38 feet).

The trophic state of a waterbody can bestrongly related to the water clarity. Using theseaverage Secchi depth readings, Florida lakeswere found to be distributed into four trophicstates as follows:

approximately 7% of the lakes would beclassified as oligotrophic (those with Secchidepths greater than 3.9 meters— about 13 feet) ;

about 22% of the lakes would be classified asmesotrophic (those with Secchi depths between2.4 and 3.9 meters — between about 8 and 13feet);

45% of the lakes would be classified aseutrophic (those with Secchi depths between 0.9and 2.4 meters — between about 3 and 8 feet); and

26% of the lakes (those with Secchi depthsless than 0.9 meters —about 3 feet) would beclassified as hypereutrophic.

The location of a waterbody has a stronginfluence on its water clarity. For example, lakesin the New Hope Ridge/Greenhead Slope lakeregion (in Washington, Bay, Calhoun, andJackson counties) tend to have Secchi depthsgreater than 9 feet (3 meters). While lakes in theLakeland/Bone Valley Upland lake region (inPolk and Hillsborough counties) tend to haveSecchi depths less than 3 feet (0.9 meters).

Health Concerns:Water clarity is not known to be directly

related to human health.

Water Depthis the measurement of the depth of a waterbodyfrom the surface to the bottom sediments. Waterdepth can vary substantially within a waterbodybased on its morphology (shape).

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Florida LAKEWATCH volunteers measurewater depth using a weighted Secchi disk attachedto a string or cord that is marked in one-footincrements. The weighted Secchi disk is droppeddown until it hits bottom and then the distance isdetermined by measuring the length of ropebetween the bottom and the surface of the water.These measurements are then recorded for futurereference.

Water depth can also be measured using adevice called a fathometer by bouncing sonicpulses off the bottom and electronically calculatingthe depth. Several fathometer readings takencontinuously along a number of transects(shore-to-shore trips across the waterbody) areused to calculate an average lake depth. Thistechnique can be used instead of the traditionalmethod of dividing the lake’s volume by itssurface area to obtain a “mean depth.”

Water Qualityis a subjective, judgmental term used to describethe condition of a waterbody in relation tohuman needs or values. The terms “good waterquality” or “poor water quality” are often relatedto whether the waterbody is meetingexpectations about how it can be used and whatthe attitudes of the waterbody users are.

Water quality is not an absolute. One personmay judge a waterbody as having good waterquality, while someone with a different set ofvalues may judge the same waterbody as havingpoor water quality. For example, a lake with anabundance of aquatic macrophytes and algae inthe water may not be inviting for swimmers butmay look like a good fishing spot to anglers.

Water quality guidelines for freshwatershave been developed by various regulatory andgovernmental agencies. For example, the Cana-dian Council of Resource and EnvironmentalMinisters (CCREM) provides basic scientificinformation about the effects of water qualityparameters in several categories, including rawwater for drinking water supply, recreationalwater quality and aesthetics, support of freshwateraquatic life, agricultural uses, and industrialwater supply.

Water quality guidelines developed by theFlorida Department of Environmental Protection(FDEP) provide standards for the amounts ofcertain substances that can be discharged intoFlorida waterbodies (Florida AdministrativeCode 62.302.530). The FDEP guidelines providedifferent standards for waterbodies in each offive classes that are defined by their assigneddesignated use as follows:

Class I waters are for POTABLE WATERSUPPLIES;

Class II waters are for SHELLFISHPROPAGATION OR HARVESTING;

Class III waters are for RECREATION,PROPAGATION AND MAINTENANCE OF AHEALTHY, WELL-BALANCED POPULATION OFFISH AND WILDLIFE;

Class IV waters are for AGRICULTURAL WATERSUPPLIES; and

Class V waters are for NAVIGATION, UTILITYAND INDUSTRIAL USE.

All Florida waterbodies are designated asClass III unless they have been specificallyclassified otherwise; refer to Chapter 62.302.400,Florida Administrative Code for a list ofwaterbodies that are not Class III.

Watershedis the area from which water flows into awaterbody. Drawing a line that connects thehighest points around a waterbody is one way todelineate a watershed’s boundary. A moreaccurate delineation would also include areasthat are drained into a waterbody throughunderground pathways.

In Florida, these might include drainagepipes or other man-made systems, seepage fromhigh water tables, and flow from springs. Activi-ties in a watershed, regardless of whether theyare natural or man-made, can affect the charac-teristics of a waterbody.

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