environmental biology (tka3104) lecture notes -7 lakes

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TKA 3104 Environmental Biology

LAKES

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• Limnology is the study of the ecology ofinland waters.

• The word limnology comes from the Greek

root limne meaning “pool” or “marsh”.

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Stratification and Turnover in Deep

Lakes

• Nearly all deep lakes in temperate climatesbecome stratified during the summer andoverturn in the fall due to changes in the

water temperature that result from theannual cycle of air temperature changes.

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• During the summer, the surface water of a lake isheated both indirectly by contact with warm air anddirectly by sunlight.

• Warm water, being less dense than cool water,

remains near the surface until mixed downward byturbulence from wind, waves, boats, and otherforces.

• Because this turbulence extends only a limiteddistance below the water’s surface, the result is anupper layer of well-mixed, warm water (epilimnion)floating on the lower water (hypolimnion), which ispoorly mixed and cool, as shown in Figure 1.

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Figure 1: Lake Stratification

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• The intermediary layer between theepilimnion and hypolimnion is calledmetalimnion.

• The thermocline may be defined as theregion having a change in temperature withdepth that is greater than 1°C/m.

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• As shown in Figure 2, in the fall, astemperature drop, the epilimnion cools untilit is denser than the hypolimnion.

• The surface water than sinks, causingoverturning.

• The water of the hypolimnion rises to thesurface, where it cools and again sinks.

• The lakes thus becomes completely mixed.

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Figure 2: Overturn in stratified lakes

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• If the lakes is in a cold climate, this processstops when the temperature reaches 4°Cbecause at this temperature water is thedensest.

• Further cooling or freezing of the surfacewater results in winter stratification, asshown in Figure 2.

• As the water warms in the spring, it again

overturns and becomes completely mixed.• Thus, temperate climate lakes have at leastone, if not two, cycles of stratification andturnover every year.

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Biological Zones

• The most important biological zone, shownin Figure 3, are the euphotic, limnetic,littoral, and benthic zones.

• Limnetic Zone. The limnetic zone is a layerof open water where photosynthesis canoccur. Life in limnetic zone is dominated byfloating organisms and actively swimming

organisms.

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Figure 3: Biological zones in a temperate lake

PROFUNDALZONE

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• Euphotic Zone. The upper layer of waterthrough which sunlight can penetrate iscalled euphotic zone. Below the euphotic

zone lies the profundal zone. The transitionbetween the two zones is called the lightcompensation point, which correspondsroughly to the depth at which the amount of

carbon dioxide being converted to sugars byphotosynthesis is equal to that beingreleased during respiration.

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• Littoral Zone. The shallow water near theshore in which rooted water plants can growis called the littoral zone.

• Benthic Zone. The bottom sedimentsconstitute the benthic zone.

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Lake Productivity

• Table 1 shows how lakes are classifiedbased upon productivity.

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• As the lake shrinks and productivityincreases, the lake may become anoxic oranaerobic, resulting in a significant change

in the lake ecology.• This process of succession can continue

until the lake becomes a marsh, then a bog,and finally a forest or grassland.

• The process is illustrated in Figure 4.

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Figure 4: Succession oflake or pond

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• Oligotrophic Lakes. Oligotrophic lakeshave a low level of productivity due to aseverely limited supply of nutrients tosupport algal growth.

• Eutrophic Lakes. Eutrophic lakes have ahigh productivity because of an abundantsupply of nutrients.

• Mesotrophic Lakes. Lakes intermediatebetween oligotrophic and eutrophic arecalled mesotrophic.

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• Dystrophic Lakes. Dystrophic lakesreceive a large quantity of organic materialfrom outside the lake and how productivitydue to low nutrient concentrations.

• Hypereutrophic Lakes. Hypereutrophiclakes are extremely eutrophic, with a highalgal productivity level and intense algalbloom. These lakes are often subject to“winter kill” and even “summer kill” duringwhich the depletion of oxygen results in anextensive kill of fish and sometimes otherorganisms.

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• Senescent Lakes. Very old, shallow lakesin advanced stages of eutrophication arecalled senescent lakes.