Lecture 6: Plankton. Plankton: Definitions Plankton: organisms living in the water column, too small to be able to swim counter to typical ocean currents.

Lecture 6: Plankton

Plankton: Definitions

• Plankton: organisms living in the water column, too small to be able to swim counter to typical ocean currents

– Holoplankton – spend entire life in water column

– Meroplankton – spend part of life in water column, are benthic for remainder of life

Plankton: Definitions

• Phytoplankton – photosynthetic protists and bacteria. Single celled or chains of cells.

• Zooplankton – nonphotosynthetic protists and animals. Range from single celled to small vertebrates.

• Mixoplankton (or mixotrophic) - can be classified at several different trophic levels

Plankton Size Classes

Position in Water Column

• Phytoplankton must be near a source of sunlight– 50-100 m in open ocean– Shallower depths in inshore waters and estuaries

• Zooplankton usually feed on phytoplankton, or organisms that feed on phytoplankton

Vertical Position in Water Column

• Ways to avoid sinking (neutral buoyancy):– Regulate bulk density (the mass of an organism

divided by its total volume) by varying chemical composition

– Gas secretion– Body shape– Swim

Vertical Position of Plankton

• Smaller organisms denser than seawater sink with a constant velocity, proportional to organismal mass (Stokes’s Law)

• Heavier organisms will sink faster than lighter organisms

• Irregularly shaped plankton sink slower than predicted by Stokes’s Law

Phytoplankton

• Numerous groups, including many flagellated types

• High diversity• Different groups have

different nutrient needs (e.g., Fe, Si, Ca, P, N)

• Different groups have different properties such as bulk density, ability to swim

Phytoplankton

• Plantlike Single-celled Protists– Diatoms– Dinoflagellates– Coccolithophores– Silicoflagellates– Green algae– Cryptomonad flagellates

• Cyanobacteria

Zooplankton

• Crustacean Zooplankton– Copepods– Krill– Cladocera– Others

• Gelatinous Zooplankton– Cnidarians– Ctenophores– Salps– Larvacea

• Other– Arrow Worms– Pteropods– Planktonic polychaetes

• Animal-like Protists– Ciliates– Foraminifera– Radiolaria

Zooplankton

Critical Factors in Plankton Abundance

Major Physical Factors Affecting Primary Production

• Temperature• Light• Hydrodynamics• Nutrients

Patchiness of the Plankton & Its Causes

• Spatial changes in physical conditions - behavioral responses and population growth/mortality responses

• Water turbulence and current transport• Spatially discontinuous levels of grazing• Localized reproduction• Social behavior

Wind and Turbulence

• Wind can affect patchiness at a wide range of spatial scales– Langmuir circulation –

wind driven water movement creates small vortices which result in small divergences and convergences of water• Result in linear

convergences at surface

Directional Flow and Obstructions• Directional water flow can cause persistent

spatial patterns in circulation• Flow patterns can be altered at obstructions

(islands, mouths of estuaries, passes, etc. )

Depth and Plankton Layers

• Phytoplankton and small zooplankton can be concentrated in layers at different water depths

Patchiness of the Plankton

• Concentrated patch of phytoplankton must eventually disperse due to the transfer of wind and current energy into kinetic energy

Phytoplankton Patchiness• Population density

determined by interaction between turbulence and population growth

• Blooms probably caused when you have a rapid increase in phytoplankton growth in an area with restricted circulation

Spring Phytoplankton Bloom

• Predictable seasonal pattern of phytoplankton abundance in the temperate and boreal waters of depths of ~10-100m

• Spring diatom increase = phytoplankton increase dramatically and are dominated by a few diatom species

Phytoplankton, Zooplankton, Nutrients, and Light Throughout the Year in Temperate-Boreal

Inshore Waters

Latitudinal Variation in Cycle

Geographical Comparisons of Primary ProductionPolar Seas Temperate Seas Tropical Seas

Light Well lit in summer Light varies seasonally

Well lit throughout year

Stratification No stratification Seasonal stratification

Occurs throughout year

Nutrients Unlimited Mixing replenishes nutrients

Low nutrient content in surface waters

Primary Production Only occurs in ice-free summer but can be quite substantial

Major peak in spring with minor peak in fall

Low but constant year-round

Successional Patterns No real succession because production only occurs in summer

Spring: small, rapidly growing diatoms;Summer: larger diatoms; Late Summer/Fall: dinoflagellates; Winter: Small diatoms

Dinoflagellates dominate year-round

Light and Phytoplankton

• Light irradiance decreases exponentially with increasing depth

• Light becomes limiting factor to photosynthesis

Compensation Depth• Compensation depth – the depth at which the

amount of oxygen produced in photosynthesis equals the oxygen consumed in respiration

COMPENSATION DEPTH

Net increase of oxygen over time

Net decrease of oxygen over time

DEPTH

Compensation Depth

• Is controlled by season, latitude, and transparency of water column– Longer photoperiod in temperate-boreal waters– Arctic winter has a zero photoperiod– Suspended matter in coastal waters intercepts

light

Photosynthesis and Light Intensity

Before the Spring Phytoplankton Increase

• In winter:– Water density is similar at all depths– Wind mixing homogenizes water column– No bloom because any potential profit in

photosynthesis would be lost to mixing

Seasonal Changes in Mixing and Light

Water column stability is essential to the development of the spring bloom

Key Processes Leading to Spring Phytoplankton Increase

Key processes: • Development of thermocline• Trapping of nutrients• Retaining of phytoplankton

Spring Bloom in the Gulf of Maine

Decline of the Spring Phytoplankton Bloom

• Nutrients are being removed from stable water column

• No replenishment of nutrients from deeper water

• Zooplankton grazing has some effect but is often secondary to sinking

Rejuvenation of Conditions for the Spring Phytoplankton Increase

• In fall and winter: water cools, water column becomes isothermal with depth, wind mixing restores nutrients to surface waters until conditions are right next spring

Water Column Exchange in Shallow Waters and Estuaries

• Importance of water column stability varies with basin depth and season

• Benthic-pelagic coupling – nutrient exchange between the bottom and the water column – Fuels more phytoplankton growth

Water Column Exchange in Shallow Waters and Estuaries

Benthic-Pelagic Coupling and a Beach Bloom

Water Column Exchange in Shallow Waters and Estuaries

• High primary production in estuaries • Nutrient regime is determined by the

combination of the spring freshet with mixing and net water flow to the sea

Important Factors in Water Column Exchange in Shallow Waters and Estuaries

• Residence time - time water remains in estuary before entering ocean

• Rate of nutrient input from watershed• Nutrients may be released to coastal zone

Nutrients

• Nutrients are dissolved or particulate substances required by plants and photosynthetic protists;

• Can be limiting resources

Nutrients in Marine vs. Terrestrial Environments

Terrestrial• Agricultural soil = 0.5%N• Allows for greater primary

production per m3

• Long-lived plants

Marine• Ocean waters = 0.00005%N• Allows for much less

primary production per m3

• Short-lived plants• Nutrients are often limiting

Nitrogen – New vs. Regenerated Production

• New production:– Nutrients for primary production may derive from

input of nutrients from outside the photic zone

• Regenerated production:– Nutrients derive from recycling in surface waters

from excretion

Phosphorous (P)

• P is rapidly recycled between water and phytoplankton

• Sediments accumulate P from phytoplankton detritus

• Diffusion of P from bottom due to benthic decomposition

• Winter mixing returns P to surface waters

N and P as Limiting Nutrients

• N and P are depleted by phytoplankton growth

• Phytoplankton more enriched in N than P, suggesting that N is limiting to primary production on the scale of the entire ocean

• P ultimately comes from weathering of minerals

Silicon

• Important limiting element for diatoms• Sinking of diatoms from surface waters

removes silicon• Silica (Silicon dioxide) delivered to ocean by

wind and river transport

Fe, Si often enter the ocean by wind-borne particles

Iron as a Limiting Nutrient and in Climate Change

• Is commonly in short supply and is thus limiting to phytoplankton

• May be crucial in parts of the ocean where nitrogen appears not to be limiting factor (HNLP zones)

• Phytoplankton sequester large amounts of CO2 during photosynthesis

• Dr. John Martin – Idea was that if you increase phytoplankton production, you could slow global warming

• Evidence – Eruption of Mount Pinatubo in 1991

IronEx Studies

• IronEx I (1993) – First open ocean iron fertilization experiment– Single iron addition to a 100 km2 patch of water near

Galapagos Islands– Results not very dramatic– Proved that iron can limit primary production in some of the

world’s oceans• IronEx II (1995) - Sequential additions of solubilized iron

to water patch in Equatorial Pacific – Produced enormous phytoplankton bloom

• Have been 13 iron fertilization experiments since 1993

Intense and Harmful Algal Blooms

• Conditions:1. A stable water column2. Input of nutrients3. Sometimes an initial

input of resting stages • Principally some dinoflagellates

and cyanobacteria• Population crashes may reduce

oxygen in water

Red Tide off Florida Coast

Phytoplankton Succession

• Seasonal change in dominance by different phytoplankton species

• General properties correspond to the seasonal trend in nutrient availability

Phytoplankton SuccessionMechanisms poorly understood:• Shift in advantage of nutrient uptake, later species in

season may depend upon substances that are not in the water column in early spring

• Stratification • Chromatic adaptation • Allelopathy

Paradox of the Plankton - Hutchinison

• Coexistence of many photosynthetic and heterotrophic groups under nutrient limitation

• Would expect an equilibrium would be reached and one species would dominate

• Remains to be solved, but could be due to:– Spatial patchiness of nutrients– Reproductive capacity of phytoplankton

3 Major Pathways for Flux of Organic Matter

• Grazing food chain• Microbial loop• Sinking flux

The Microbial Loop1. Bacteria take up large amounts of nutrients and organic matter from the water column

2. Bacteria are consumed by ciliates and otherheterotrophs

3. These heterotrophs are consumed by othersmaller zooplankton

The Microbial Loop

DOC=dissolved organic carbonPOC=particulate organic carbonDIOC=dissolved inorganic carbon

The Microbial Loop and Deepwater Horizon

• Bacteria in microbial loop feed on oil droplets (is a source of C) and associated contaminants

• May alter microbial loop and its functioning• Allows contaminants to enter planktonic food

web and reach higher order consumers

Marine Snow• Fragile organic

aggregate made up of dissolved organic molecules or degraded gelatinous substances

• Usually enriched with microorganisms

• Found in relatively quiet water

Zooplankton Grazing

• Zooplankton growth depends on phytoplankton growth

• Zooplankton abundance usually increases after the peak of phytoplankton abundance

• Grazing effect: Difference between grazing rate and phytoplankton growth rate

• Grazing quite variable

North Sea: grazing results in alternating patches of phytoplankton and zooplankton -

cycles of abundance

Zooplankton Grazing

Copepod feeding response to diatom density

Zooplankton Feeding/Grazing

• Zooplankton can select phytoplankton particles by size

• Could influence species composition of phytoplankton

Diurnal Vertical Migration of Zooplankton• Rise to shallow water at night, sink to deeper water

during the day

Planktonic shrimp, Sergia lucens

Causes of Diurnal Vertical Migration of Zooplankton

• Strong light hypothesis – plankton are adversely affected by UV radiation and strong light, so they migrate away from surface waters during the day

• Problem?

Causes of Diurnal Vertical Migration of Zooplankton

• Phytoplankton recovery hypothesis - zooplankton migrate downward to allow phytoplankton to photosynthesize and recover during the day

• Problem?

Causes of Diurnal Vertical Migration of Zooplankton

• Predation hypothesis - zooplankton migrate downward to avoid visual predation during day

• Problems?

Causes of Diurnal Vertical Migration of Zooplankton

• Energy conservation hypothesis - zooplankton migrate downward to avoid higher surface temperatures during the day, which saves energy (metabolic rate and energy needs are lower in cooler waters)

• Problem?

Defenses Against Predation

• Body spines• Being nearly transparent• Bioluminescence– Counterillumination– Deceptive signals– Camouflage– Lures

• Toxic substances