Aquatic Ecosystems

1
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.

2
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.

3
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.

4
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.

5
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.

6
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.

7
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.

8
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.

9
Hypolimnion

• The bottom layer. Cold and dark, does not mix
  with the upper layers. Decomposition and cell
  respiration dominant.

10
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.

11
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.

12
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.

13
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

14
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.

15
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.

16
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.