Lecture 17: Some Lake Notes

Lakes are formed in many ways. The most common way a lake is formed is from glaciers. This lake type is called glacial.
Other types of lakes are tectonic, pothole or kettle, landslide, volcanic-caldera, dissolution lake, and oxbow or floodplain.
Pothole lakes are generally shallow. Tectonic lakes can be deep. The Laurentian Great Lakes are glacial and some are deep.

Light, photosynthesis, and heating go from the top down in lakes. Photosynthesis is greatest and water temperatures are warmest near the surface.

Water is the most dense~4C. As water passes this threshold towards the freezing point it becomes less dense and rises to the surface. The variability in densities of water leads to stratification of lakes. A typical pattern in the summer is three layers, Epilimnion (top), Metalimnion (thermocline) (middle), and Hypolimnion (bottom).

The depth of the thermocline is determined by lake depth, latitude, wind, and clarity.

In northen lakes with four seasons lakes turnover or mix twice a year, the spring and fall. This is caused by temperature changes and winds. When this mixing happens nutrients are brought up from sediments and oxygen is driven to lower depths. In the summer thermal stratification develops and prevents mixing. The lake conditions change as the summer progresses and nutrients get used up. This lake has the conditions of what is called a Dimictic lake, meaning it mixes twice a year. Other types are Monomictic- mixes once a year; Amictic- never mixes; Polymictic- mixes several times a year.

Primary production controls productivity in most lakes and is the main source of productivity. This primary production by algal photosynthesis is dependent on light, temperature, nutrients, water movements, and other chemical factors.

Key nutrients in a lake system are N and P. Inorganic N is mainly ammonium and nitrate. Inorganic P is available mainly as phosphate. Through the uptake by algae and microbes N and P move into the biota. They return to inorganic form after extretion or death and then decomposition.

Leibig's Law of the Minimum- growth is controlled by the scarcest resource not the total nutrients. The scarcest recource is called the limiting factor.

The nutrient enrichment of an aquatic ecosystem is called eutrophication. Natural eutrophication is a process that occurs over hundreds of thousands of years as a lake ages. Cultural eutrophication is process when humans release nutrients that greatly accelerate the process of eutrophication.

Major sources of excess nutrients are inorganic fertilizers and manure, human waste, N atmosphere deposition, and crop N fixation.

Three types of Plankton: Bacterioplankton, Phytoplankton, and Zooplankton.
Bacterioplankton are small, heterotrophic and autotrophic
Phytoplankton are algae including greens and bluegreens
Zooplankton are animal plankton including rotifers, cladocerans, copepods and more

A top-down trophic cascade is when predators control herbivores and increase biomass of producers.

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