Lake Ecology Nature of Lakes Lakes are enclosed

Lake Ecology

Nature of Lakes • Lakes are enclosed basins which can trap standing water • Water retention time of lakes (the time an average water molecule stays in the lake) varies from a few days to hundreds of years • Water retention time depends on the size of the lake and the rate of inflow/outflow

Lake Basins • The lake basin is the “bowl” or depression that contains the water • Lake basins are formed by numerous processes, the principal being: – – – Glacial activity Crustal movement Rivers Solution processes Human activity • These processes often occur in restricted areas giving rise to “lake districts”, areas in which there a lot of lakes (eg. The Adirondacks, Minnesota, African rift valley)

Lakes formed by Glacial Processes • Glacial activity has resulted in the greatest number of lakes and some of the largest lakes in area • The lakes of Minnesota (“Land of 10, 000 Lakes”) and the Adirondacks in New York are attributable to glacial activity • The Great Lakes are also glacial in origin

Lakes formed by Glacial Processes • Glacial lakes are found in areas of steep terrain where scour has been the mechanism

Lakes formed by Glacial Processes • They are also found in flat terrain where damming by moraines or ice blocks left behind in glacial drift is the mechanism

Origin of Lakes – Crustal Movement • Tectonic Activity (crustal instability and movement) – Graben = faulttrough = rift lake – Formed between two faults

Lakes formed by Crustal Movement • The deepest and oldest lakes in the world are those formed by crustal movement • The deepest and oldest lake in the world is Lake Baikal in Siberia

Lakes formed by Crustal Movement • Earthquake Lakes – Reelfoot Lake, TNKY – Major earthquake (8 on Richter scale) – Caused surface to uplift in some areas and subside in others – Mississippi R was diverted into a subsidence region for several days forming Reelfoot Lake

Lakes formed by Crustal Movement • Landslide Lakes – Mountain Lake, VA • One of two natural lakes in Virginia • Formed when landslide dammed a mountain valley • The lake is estimated to be about 6, 000 years old and geologists believe it must have been formed by rock slides and damming

Lakes formed by Crustal Movement • Crater/caldera Lakes – Lake occupies a caldera or collapsed volcanic crater/cone – If cone blows out the side like Mt. St. Helens, no basin left – Ex. Crater Lake, OR

Rivers Formed Lakes • Alluvial rivers leave behind bends that become oxbow lakes • Oxbow lakes are localized to areas in alluvial floodplains, like the lower Mississippi valley

Solution Lakes • Lake basins can be formed when subsurface mineral deposits (like halite or limestone) dissolve leaving a void which collapses resulting in a basin • The lakes of central Florida form a solution basin lake district

Origin of Lakes – Solution Lakes • Salt collapse basins – Underground seepage dissolves salt lenses, ground collapses and basin fills – Montezuma Well, AZ

Lakes formed by Human Activity • These may be intentional, as in the case of reservoirs created for recreation, flood control, irrigation, navigation, hydropower • Or they may be incidental, as in the case of flooded peat digs or rock quarries

Light in Lakes • Sun is virtually the only source of energy in natural aquatic habitat: photosynthesis and heat • Solar constant – Rate at which radiation arrives at edge of Earth’s atmosphere – ≈ 2 cal/cm 2/min – More than half of this is lost coming through the atmosphere

Solar Radiation Reaching Lake Surface • Absorption by different chemicals in atmosphere • Water and ozone (O 3) are especially important • Ozone is the most important in the UV range

Solar Radiation Entering Lakes • Solar radiation enters lakes and is absorbed at a constant rate • Absorption rate varies with wavelength • There is more light available near the surface and this decreases exponentially with depth • This light energy affects – The temperature of the water in a lake – The growth of primary producers in a lake

Lake Stratification and Mixing • Due to the changes in density with temperature, lakes generally stratify in summer with warmer, lighter water overlaying colder, heavier water • This creates a stable layering of water which can last well into the fall • As temperatures drop in the fall, the surface water cools and gradually reaches the temperature of the bottom water • When this occurs, we have “turnover” in which water mixes throughout all lake depths

Dimictic Lakes – Annual Cycle Seasonal heating and cooling Wind creating turbulence

Polymictic Lakes • Shallow temperate zone lakes can also be polymictic including the GMU Pond • Note the daily stratification and mixing pattern

Lake Layers during Stratification • The upper layer of the lake is called the epilimnion • And the lower layer is the hypolimnion

Stratification Affects Lake Chemistry • During stratification, the hypoliminion is cut off from the oxygen in the air • If the lake is productive, there will be organic matter from the epilimnion settling into the hypolimnion • This organic matter will be broken down by microbial respiration resulting in a decrease in dissolved oxygen • This may leave the hypolimnion critically deficient in dissolved oxygen so that it cannot support many animals like fish

Lake Chemistry - Oxygen • Vertical Distribution – Varies with lake type – Very productive lakes lose oxygen during stratification

Lake Chemistry - Phosphorus • P limits biological production in lakes • P cycle in lakes • P accumulates in the sediments

Zonation of Biota • Biological zonation is strongly influenced by light availability • The littoral zone is the portion of the lake which has sufficient light for photosynthesis to the bottom • The limnetic zone is the open water area in which sufficient light for primary producers is only available in the top of the water column • The dark portion of the open water is sometime called the profundal zone

Types of Lake Organisms • Macrophytes: large leafy plants with attached microscopic periphyton • Plankton: Suspended small organisms controlled by currents • Benthos: Bottom dwellers • Nekton: Larger, mobile organisms • Note which zone each is found in

Typical Macrophytes • submersed Floating leaved • • Emergent

Typical Phytoplankton flagellate diatoms cyanobacterium desmid

Typical Zooplankton ---------- 0. 5 mm Copepod: grazer on phytoplankton Rotifer: grazer on phytoplankton Water flea: grazer on phytoplankton

Typical Benthos • Midge larvae Dragonfly nymph • bivalves

Typical Nekton Bass: a piscivore (fish eater) Bluegill: a planktivore and benthivore Catfish: a detritivore (scavenger)

Lake Food Web • Nutrients like N and P together with CO 2 and light stimulate phytoplankton • They are fed upon by zooplankton which in turn provide food for juvenile fish

Lake Food Web • The larger fish generally eat other fish (piscivorous) and provide the top of the food web • There is sometimes even a second tier of even larger fish

Overview of the Lake Food Web

Lake Trophic Status • Oligotrophic – Low productivity, clear water, life more sparse • Somewhat Eutrophic – High productivity, murkier water, but more life

Excess Nutrients – N&P Natural Eutrophication • Productivity of lakes are determined by a number of factors: – Geology and soils of watershed – Water residence time – Lake morphometry – Water mixing regime • Over thousands of years these factors gradually change resulting in lakes becoming more productive

Cultural Eutrophication • Human activities can alter the balance of these factors, esp. when excess nutrients (P in freshwater) are introduced • Untreated sewage for example has a TP conc of 5 -15 mg/L • Even conventionally treated sewage has about ½ that. • Compare that with inlake concentrations of 0. 03 mg/L that can cause eutrophic conditions • So, even small amounts of sewage can cause problems

Cultural Eutrophication • Problems associated with cultural eutrophication include – Anoxic hypolimnion • Part of lake removed as habitat • Some fish species eliminated • Chemical release from sediments – Toxic and undesirable phytoplankton • Blooms of toxic cyanobacteria • Phytoplankton dominated by cyanobacteria and other algae that are poor food for consumers – Fewer macrophytes • Elimination of habitat for invertebrates and fish – Esthetics

Cultural Eutrophication – Case Studies • Lake Washington – Following WWII, pop’n increases in the Seattle area resulted in increases in sewage discharge (sec trted) to Lake Washington – Secchi depth decreased from about 4 m to 1 -2 m as algae bloomed from sewage P – Diversion system was built and effluent was diverted to Puget Sound in mid 1960’s – Algae subsided and water clarity increase – Daphnia reestablished itself and further clarified the lake

Cultural Eutrophication – Case Studies • Norfolk Broads, England • Shallow systems where macrophytes dominated • Increased runoff of nutrients, first from sewage and then from farming stimulated algae • First periphyton bloomed and caused a shift from bottom macrophytes to canopy formers • Then phytoplankton bloomed and cut off even the canopy macrophytes and their periphyton

Case Study: Gunston Cove on the Potomac River Households in the Gunston Cove watershed have grown dramatically since the mid-1970’s. Since the study began in 1984 the number of households has grown by about 50%. All other things equal, an increase in households should produce an increase in nonpoint contributions. The point source P load declined dramatically in the late 1970’s and early 1980’s. Formal study initiated in 1983.

Gunston Cove Recovery • Improvements in water clarity related to Plimitation and decline of phytoplankton were correlated with an increase in submersed macrophyte coverage in Gunston Cove • Since 1 m colonization depth was achieved (2004), macrophyte coverage has increased strongly

References • • • http: //waterontheweb. org/under/lakeecology/index. html http: //pearl. spatial. maine. edu/default. htm http: //www. co. cayuga. ny. us/wqma/weedswatchout/biology. htm