Abiotic Processes in Lakes

8/6/2019 Abiotic Processes in Lakes

1/10

Part OneAbiotic Processes in Lakes


2/10

Chapter 1.1Hydrology of LakesESKO KUUSISTO N D V EL I H Y V A R I N E N1 . 1 . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.1.2 What is a lake? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.1 .3 Lake classifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.4 Thermal conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1.5 Ice conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.1.6 Climate change and European lakes . . . . . . . . . . . . . . . . . . . . . . . 71.1.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

~ ~~ ~

Hydrological and Limnological Aspects of Lake MonitoringEdited by Pertti Heinonen. G iuliano Ziglio and An dre Van der Beken0 2 0 0 0 John Wiley & Sons. Ltd. ISBN 0 471 89988 7


3/10

4 Hydrology of Lakes1.1.1 INTRODUCTIONIt has been estimated that there are 12 million lakes on the Earth; their totalarea is 2.7 million km2, while the total volume is 166000km3. The 10 largest inarea account for 33% and the 10 largest in volume for as much as 90% of thecorresponding world totals.

There are almost one and a half million lakes in Europe, if small waterbodies with an area down to 0.001 km2 are included. Of these, at least 500000natural lakes are larger than 0.01km2 (Kristensen and Hansen, 1994). Many ofthem appeared 100O(r15000 years ago, having being formed or reshaped bythe last glaciation period, the Weichsel. Most of them are located in northernEurope, the Nordic Countries and the Karelo-Kola part of the RussianFederation, where lakes cover 5-10% of the surface area. Lakes are alsocommon in Iceland, Ireland and in north-western parts of the UK. In centralEurope, there are, in addition to high-altitude small lakes, some larger lakes onthe margin of the Alps (e.g. Lake Geneva, Lake Garda, Lake Maggiore andLake Constance), in the Dinarian Alps (the ancient Lake Ohrid and LakePrespan) and on the Hungarian plain (Lake Balaton and Lake Neusiedler). Incountries little affected by the glaciation period, such as Portugal, Spain,France, Belgium, southern England, central Germany, the Czech Republic, theSlovak Republic and the central European part of the Russian Federation,natural lakes are few.

The total area of European lakes and reservoirs is about 300000km2, ofwhich reservoirs make up almost one third. More than 10000 major reservoirshave been constructed in Europe. The Volga basin alone has a reservoir area of38 000km2 (Mordukhai-Boltovskoi, 1979). The total volume of European lakesis 3300 km3, and that of reservoirs is 800 km3; these figures obviously excludethe Caspian Sea.

1.1.2 WHAT IS A LAKE?A unique definition of a lake does not exist. Most textbooks on lakes startwithout actually attempting to define the object that they are going to dealwith. From a geological point of view, any workable definition should take intoaccount two distinct parts; namely the basin and the water body. From ahydrological point of view, a lake should be distinguished from a wide riversection, i.e. how much must a river widen before one calls the place a lake?

The question of the narrowness is very typical, e.g. in some parts of Canadaand in the Finnish Lake District. A lake may consist of several basins,separated by short straits. The mean water level in the downstream basins is a


4/10

Lake Classijications 5few centimetres lower than in the upstream basins. Do we have in this case asingle lake or several lakes?

The minimum size of a water body which can be called a lake should also beconsidered. However, let this be a matter to be discussed eternally. In any case,a water body smaller than a lake should be called a pond. Welch (1952) defineda pond as 'a very small, very shallow body of standing water, in which quietwater and extensive occupancy by higher aquatic plants are commoncharacteristics'. It sounds obvious that eutrophication can change a lake intoa pond.As a summary, a water body should fill the following requirements in orderto be regarded as a lake:(a) it should fill or partially fill a basin or several connected basins;(b) it should have essentially the same water level in all parts, with theexception of relatively short occasions caused by wind, thick ice cover,

large inflows, etc.;(c) even if the water body may be located in the immediate vicinity of the sea

coast, it does not have a regular intrusion of sea water;(d) the water body should have so small an inflow-to-volume ratio that a

considerable portion of suspended sediment is captured;(e) the area of the water body should exceed a specified value, e.g. 1 ha, at

mean water level.

1.1.3 LAKE CLASSIFICATIONSClassifications of lakes have been suggested in order to improve thepossibilities of estimating quantitatively the dynamic, thermal and biologicalprocesses taking place in such bodies. A detailed genetic classification hasalready been presented by Hutchinson (1957). Morphometric classificationsinclude a variety of parameters and the subdivision of the lake basins accordingto their shape. The earliest thermal classification may be that made by Fore1( 1 90 I ) , who distinguished between polar, sub-polar, temperate and tropicallakes. A more detailed thermal classification was presented by Keller (1974).A water-balance classification of lakes can be based on three simple criteria(Szesztay, 1974):(a) the inflow factor, which can be calculated by using the following

(1 )expression:

IF = "(IN +P)where IN = inflow into the lake and P = lake precipitation


5/10

6 Hydrology of Lakes(b) The outflow facto r, which can be calculated by using the followingexpression:

OF = O U / ( O U+ E ) (2)where O U = outflow from the lake and E = lake evaporation(c) The magnitude of the mean annual flux of incoming o r outgoing waters.

A closed lake obviously has an outflow factor of 0%, while the outflowfactor of a throughflow lake is almost 100%. Of the large lakes in the world,Lake Victoria might have the lowest inflow factor, i.e. below 20% . In the largeFinnish lakes, inflow factors a re typically 7O-9O0h, with outflow factors beingslightly higher than this.

1.1.4 THERMAL CONDITIONSThe temperature of water bodies is essentially determined by the radiationbalance, i.e. the flux of latent heat and convective heat supply. Higher watertemperatures tend to increase evaporation and outgoing radiation, both ofwhich, in turn, have a cooling effect. Moreover, changed runoff dynamics,together with changes in the transport of suspended sediment, can alsoinfluence the w ater temperature.The annual thermal cycle of a moderately deep, temperate lake ischaracterized by the following important dates:(a) the break-up date, i.e. when a rapid increase of temperature begins as aresult of mixing of the water mass due to wind;(b) the date of the maximum mean temperature of the water mass, i.e. whenthe amount of heat energy stored in the water mass reaches its maximum;(c) the da te of the beginning of the autu mn homothermy;(d) the date of the water-density maximum, i.e. when the whole water mass hasa temperature of 4.0"C.(e) the freezing date, i.e. when the effect of wind m ixing ceases and winterconditions begin;(f) the date of the minimum mean temperature of the water mass, i.e. when theamount of heat energy reaches its minimum.In large Finnish lakes, the date of the maximum mean temperature usuallyoccurs in August, although the deepest layers continue to warm up until theend of September. The autumn hornothermy starts in early October and lasts3-6 weeks; thereafter, it usually takes 2-3 weeks before the lake freezes. Themean temperature of the water m ass by this freezing time is around 1.O"C.


6/10

Climate Change and European Lakes 71.1.5 ICE CONDITIONSSome morphometric parameters have a close connection with the iceconditions of lakes. For lakes of equal area, an early freezing-up is associatedwith the following characteristics:0 small mean and m aximum depths;0 small effective fetch;0 large sh ore development;0 large insulosity;0 direction of major axis perpendicular to the direction of prevailing winds.Of the fifty largest lakes in the world, about half are completely or partiallycovered by ice in winter. Num erous lakes in high latitudes o r at high altitudeshave an ice-cover season longer than the open-water period. Som e lakes in highmountains, in the Arctic islands and in the A ntarctic a re always covered by ice.There ar e 50-100 lakes in the world w ith freezing an d bre aku p d at a serieswhich are over a century long. In Finland, the longest series is from LakeKallavesi, where the observations first started in the au tum n of 1833. Since tha ttime, both freezing and breakup dates have shifted by about two weekstowards a milder climate.

1.1.6 CLIMATE CHANGE AND EUROPEAN LAKESAquatic ecosystems are excellent integrators of changes in climate andcatchment conditions. The results of the complex interplay between climate,vegetation a nd soil will be summ arized in stream s an d lakes, and furthe r o n inmarine ecosystems. In lakes with minor catchment areas, the direct effects ofchanged climate a nd atmospheric deposition con trol the fu ture development.

Water quality changes are largely determined by changes in leaching. Inacid-sensitive lakes, the increased nitrogen leaching m ight dis turb the positivedevelopment started as a result of reductions in sulfur emission.In agriculturally loaded lakes, an increase in the primary production ofphytoplankton is likely due to a longer growing season, higher nutrient loadsand higher C 0 2 concentrations. Changes in p opulation an d communitystructure of the aquatic biota are also likely to occur.In areas w ith high lake densities, the position within the catchme nt area canstrongly influence the response of individual lakes to climate shifts, thusleading to divergence in biogeochemical patterns of change across a region(Webster et af., 1996).The response of European lakes to climate change can be discussed bydividing the lakes into four categories.


7/10

8 Hydrology of Lakes1.1.6.1 Deep, temperate lakesTypical representatives of this group are, e.g. Lakes Maggiore (Italy), Geneva(Switzerland/France), Ness (Scotland) and Constance (Germany/Sw itzerland/Austria) with mean depths of 177,153 , 132 and 90 m, respectively. Due to thesegreat depths and relatively mild winters, there is usually no ice cover.Most of these lakes are warm monomictic (one turnover), while the deepestones are warm oligomictic (irregular and seldom). Convective overturn occursin winter o r early spring. In some lakes, there is a correlation between ann ualair temperature and the temperature of the hypo limnion; in this case the lakecan even be used as a filtered indicator of climatic change or variations(Livingstone, 1993).The future climate change may suppress the turnover in deep m onomicticlakes, thus giving them the classification of oligomictic. This implies theenhancement of anoxic bottom conditions and an increased risk ofeutrophication. The oxygen conditions can also be expected to deterioratedue to increased bacterial activity in deep waters an d surficial bottom sediment.Although many of these lakes have a long residence time, the role ofincreased evaporation in concentrating nutrients in the w ater mass is relativelysmall. Changes in catchment conditions will be more im portant.The maritime lakes in this group (particularly in the U K and Ireland) arestrongly influenced by cyclones coming from the A tlantic. The increased powerof these weather phenomena could affect the stratification, and consequentlythe biological conditions in these lakes. Moreover, a correlation between theaverage summer biomass of zooplankton a nd the position of the Gulf Streamhas been found in U K lakes (George and T aylor, 1995).

1.1.6.2 Shallow, temperate lakesLake Balaton (596km2, 3m ) in H ungary, Lake Shkodra (368km2, 8 m ) inAlbania and Lake M iiritz (114km2, m ) in Germany are typical examples ofthis group.High water temperatures will result in intensified primary production andbacterial decom position. The probability of harmful extreme events, e.g. m assproduction of algae, will increase. The impacts may extend to fish life; withchanges in species composition and reduced fish catches being an ticipated. Theuse of the expression thermal pollution is well justified for these lakes.In lakes with relatively long retention times, increased evaporation causesconservative solutes to concentrate. This effect may be enhanced by decreasedannual inflows in southern Europe and east European lowlands.For Lake Balaton, Szilagyi and Somlyody (1991) estimated that theincreased dissociation of inorganic carbon will probably be a more importantcontributor to acidification and to ionic composition than acid deposition.


8/10

Climate Change and European Lakes 9Doubling the atmospheric C 0 2 conc entration w ould result in a decrease of thepH value by 0.2, and w ould cause a significant increase in the salt co nte nt andhardness of the lake water.1.1.6.3 Boreal lakesLake Ladoga (Russia) (17 670km 2, 51 m), Lake Onega (Russia) (9670 km2 ,30 m) and Lake Vanern (Sweden) (5670 km2, 27 m) ar e the largest lakes in thisgroup, with these also being the three largest lakes in Europe. This groupincludes abou t 120 lakes with a n area exceeding 100km2.Most lakes of the boreal zone are dimictic with two overturns in a year.Sho rtening of the ice cover period is the most obv ious consequence o f climatechange in these lakes (Huttula et al., 1996). This could improve the oxygenconditions in winter and spring. A longer ice-free period might also result inincreased turbidity due to erosion from exposed land surfaces. A longer andstronger summer stratification might have harmful effects on water quality inthe hypolimnion. This will obviously depend on future wind speeds, whichnobody is yet willing to predict.A simulation of Lake Ladoga by Meyer et al. (1994) in a 2 x C02-cl imatescenario resulted in the following changes:(a) N o ice cover was formed. In this century, a com plete ice cover h as occurred

in Lake Ladoga in about 80% of winters. The lack of ice cover maystimulate fog form ation. T he lakes biota, presently including a numb er o funique species, might also be affected by the absence of ice cover.(b ) Th e lake will remain dimictic. Intensive cooling during w inter will lead t oinverse stratification in March and April.(c) Sum mer stratified cond itions seem t o be preserved as a t present.Convective mixing can potentially have more significant consequencesdu e to the longer duratio n of overturn periods.

1.1.6.4 Mountain and arctic lakesThese are mainly small water bodies in central European and Scandinavianmountains and in the Arctic.The lakes in this gro up are generally considered to be particularly sensitive toenvironmental changes. They usually have small catchments with limitedchemical and biological erosion, and their simple and labile ecosystems reactquickly to environmental stress and changes. Many lakes are ultra-oligotrophic; the ice cover season is long.A sm all mountain lake can be biologically very isolated. It closely reflects thestatement of one of the fathers of limnology, S . T. Forbes, from th e year 1887(Hutchinson, 1957):


9/10

10 Hydrology of LakesIt forms a little world within itself - a microcosm within which all the elementalforces are at work and the play of life goes o n in full, but on so small a scale as tobring it easily within the mental grasp.

Even if mountain lakes are connected by channels, physical and ecologicalconstraints limit species migration between them. In a warming climate, there isno escape route; the only possibility for survival is adaptation.In the Arctic, melting permafrost may fatally threaten lake ecosystems. Insome cases, it may threaten the whole existence of the lake, i.e. ground thawtogether with enhanced evaporation may cause the lake to disappear.One risk in mountain lakes may be the future enhancement of UV-Bradiation. As a function of altitude, this component increases by 20% per1000m (Blumthaler and Rehw ald, 1992). Transp arent water tends t o m aximizethe UV-B dose to aqu atic alpine organisms.1.1.7 CONCLUSIONSKnowledge of the hydrology of lakes is essential for their proper use andconservation. W ater quality is closely linked to the w ater an d energy budgets,mixing, stratification and other physical aspects of lakes. Without amorphometric description of a lake, the quantitative analysis of the thermaland biological processes is impossible. If a lake has an ice cover in winter, acompletely different approach to the analysis of heat budget and dynamicprocesses is needed.

On the geological time scale, lakes are transitory features on the Earthssurface. Their life expectancies may vary from a sh or t spell between two floodsto millions of years. The variation of water levels o r the rate of sedim entationare indicators of the future development of a lake. Particularly interesting areclosed lakes - hese are laboratories for the study of basic climatic,hydrological and geological phenomena.Considerable progress has been made in the modelling of lake hydro-dynamics in recent decades. However, the complexity of w ater m otions in lakesstill hinders a detailed analysis, and simplified strategies have to be applied.Long hydrological data series of lakes have gained in importance withincreasing evidence of climate change, with the latter having essen tial effects onlakes in many regions of the world. In high latitudes, the shortening of the ice-cover period will be the most obvious consequence. In addition, increasedwater temperatures may result in intensified primary production and harmfulchanges in water quality.REFERENCESBlumthaler, M . and Rehwald, W., 1992. Solar UV-A a nd UV-B radiation fluxes at twoalpine stations at different altitudes, Theor. Appl. Climntnl., 46, 9-44.


10/10

References 11Forel. F. A., 1901. Handbook der Seenkunde, Verlag von J. Engelhorn, Stuttgar t .George, D. and Taylor, A ,, 1995. UK lake plankton and the Gulf Stream, NatureHutchinson, G. E. , 1957. A Treatise on Limnology, Vol. I , Wiley, New York.Huttula, T., Peltonen, A. and Kaipainen, H ., 1996. Effects of Climatic C han ges o n IceCond itions and Temperature Regime in Finnish Lakes, Th e Final Rep ort of S I L M U ,4/96, Helsinki, Finland, Academy of Finland, 167-172.Keller, R., 1974. Physico-geography - object and problem of research, IA HS Bull ., 19,Livingstone, D. , 1993. Tem pora l structu re in the deep-wat r temperature of fou r Swisslakes: A sho rt-term climatic change ind icator?, Verh. In9 Verein. Limnol., 25, 75-8 I .Kristensen, P. and Hansen, H. 0. Eds), 1994. European Rivers and Lakes: Assessmentof their Environmental State, EEA Environmental Monographs 1 , EuropeanEnvironment Agency, Silkeborg, Denmark.Meyer, G., Masliev, I. an d Somlyo dy, L., 1994. Imp act of Climate Change on Global

Sensitivity of Lake Stratification, IIASA Report, WP-94-28.Mo rdukhai-B oltovskoi, D., 1979. Th e River Volga an d its Life, Dr W. Junk, TheHague.Szesztay, K . , 1974. Water balance an d w ater level fluctuations of lakes, IA HS Bull . 19,Szilagyi, F. and Somlyody, L., 1991. Potential impact of climatic changes on waterquality in lakes, IAHS Publ., 206, 79-86.Webster, K . , Kratz, T., Bowser, C. and Mag nuson, J., 1996. Th e influence of landscapeposition on lake chemical responses to drought in northern Wisconsin, Limnol.Oceanogr. 41, 977-984.

(London , 378. 139.

63-7 I .

73-84.

Welch, P. S . , 1952. Limnology, McGraw-Hill , New York.

Abiotic Processes in Lakes

Documents