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Life in Freshwater

Types of Lake

There are two types of lake - those that are shallow, saucer-like ones called eutrophic lakes with, usually, turbid water and deep, steep-sided ones called oligotrophic lakes with clear water. The former tends to have a great deal of vegetation around it as well as a high biodiversity of animals. The opposite occurs in the second. Much of this is down to LIGHT.

Eutrophic lake
Eutrophic lake. Shallow lake with plenty of vegetation around the edge and even in the middle light reaches the bottom.

Oligotrophic Lake
Oligotrophic Lake. A deep lake surrounded by mountains. Any nutrients drop to the bottom where there is no light. 

The significance of the depth is that light will only penetrate so far. Even if the water is crystal clear by 30 metres light will be virtually non-existent. For plant and algae to grow (including plankton) they need mineral nutrients as well as light for growth and photosynthesis. Any nutrients entering the lake will invariably sink to the bottom. If this is in a shallow lake where the light reaches the bottom then photosynthesis can occur and the nutrients (e.g. phosphate, nitrate and sulphate) are used for growth. Diatoms need silica to form their outer walls. This too will lie on the bottom. If the nutrients remain in the dark they cannot be used. This is why the oligotrophic lake has a low diversity. However, it is possible for nutrients to move up into the lit zone of a lake. This is due to what is called a seiche (see Factors for more info). The seiche can occur in the spring and autumn and may bring nutrients into the light so that plankton can suddenly bloom. An otherwise low-nutrient lake becomes green with plankton.

The oligotrophic lake
The oligotrophic lake in the foreground is undergoing a plankton bloom whilst the two smaller ones behind are not.

A Reservoir with the dam to the right
 Reservoir with the dam to the right. The flooded valley is steep sided and so has formed an oligotrophic water with a low biodiversity. It too can have plankton blooms.

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