Aquatic Ecosystems • Water has the ability to hold a large amount of energy. Due to this characteristic large bodies of water will take longer to heat up and cool down. This provides unique characteristics for aquatic ecosystems.
Lake Structure • Littoral Zone: This is the area of a lake that is closest to the shore. It is a shallow zone and light can penetrate all the way to the bottom. There is a large amount of photosynthesis that can occur here and aquatic plants will live here.
Limnetic Zone • This is the area of a lake that is out in the middle of the lake away from the shore. It too can have a high photosynthesis rate, but aquatic plants will not be found. In other words the open water.
Phytoplankton • The main type of organism which does photosynthesis is called phytoplankton. Plankton are tiny organisms (often microscopic) which cannot completely control there movement. They are so small that waves and current move them through the lake.
Profundal Zone • This is the deep dark bottom of the lake, murky depths and muddy ooze of the bottom.Photosynthesis is low or if the lake is deep enough does not occur. Cell Respiration and decomposition are the dominant processes.
Lake Stratification • Small lakes will have the same temperature throughout the lake for the most part. If a lake is large enough, a process called stratification happens. This just means different layers of water form in the lake.
Epilimnion • This is the top layer. It is well-mixed due to waves and currents. Light penetrates through this layer and it heats up first in the spring and summer and cools down first in the fall and winter. It does not mix with the bottom of the lake. Photosynthesis is dominant.
Metalimnion: aka the Thermocline • This is the area of a lake that has a dramatic temperature change. Light barely penetrates to this area. It does not mix well with the upper or bottom layer.
Hypolimnion • The bottom layer. Cold and dark, does not mix with the upper layers. Decomposition and cell respiration dominant.
How these layers form • In summer what happens is the top layer becomes warm and the bottom layer becomes cold, with a radical temp. change in the middle. Freshwater is most dense at 4C so this is the layer at the bottom and warmer water is less dense so it floats above the cooler water. In the summer the temperature between the bottom and top is so different the lake does not mix.
Fall and Spring Turnover • Mixes oxygen and nutrients throughout the lake. If this does not happen nutrients stay at the bottom and oxygen will stay at the top.
Temperatures • In fall as the temperatures get cooler, the epilimnion cools down as it approaches the temperature of the hypolimnion, the lake will mix. This is called fall turnover and it brings up nutrients from the bottom and mixes O2 throughout the lake. It will also turnover during the spring, mixing nutrients and oxygen throughout the lake.
The Lake Erie Dead Zone • Lake Erie is a lake that stratifies, and because of its unique structure, it creates what is called a dead zone in the middle of the lake. It not really a dead zone, but is called an anoxic zone. Meaning there is no oxygen in the hypolimnion. It happens in the late summer in the central basin. Here is how it happens:
Beginning of Summer • At the start of summer there is oxygen at the bottom of the lake due to turnover in the spring and fall. As it gets hotter the epilimnion warms up and the lake becomes stratified, remember because of this the top and bottom do not mix.
No more oxygen added to bottom There is now a limited amount of oxygen in the bottom of the lake. (hypolimnion) Photosynthesis does not occur here only respiration. Since no oxygen is added, and oxygen is being used up by respiration, mainly because of bacteria doing decomposition, the oxygen level gradually becomes lower and lower.
No oxygen and small hypolimnion= Dead Zone • The more decomposition, the lower the oxygen levels. In Lake Erie there can be a large amount of decomposition occurring in the hypolimnion. • The hypolimnion in Lake Erie is not very big only 10-20ft at the bottom of the central basin. So the oxygen content is not very great and thus during a long hot summer, oxygen is depleted from the bottom of the lake.