Overview - Biology
The habitat of lakes can be divided into free water zones (pelagial) and bed zones (benthic). Benthic is in turn subdivided into the bank zone (littoral) and the deep zones (profundal). The deep zone is the layer of water through which almost no light penetrates.
In the flat bank area of the litoral, plants known as emerse plants grow on shallow underwater banks. These plants include reeds, reed mace and rushes. These are followed by the floating leaf plant zone (e.g. water lilies) and then the band of submerged water plants, i.e. they live completely below the water line (e.g. pondweed). The whole area of the bank is also the habitat for various species of fauna such as insect larva, mussels, snails, amphibians, fish and waterfowl.
The characteristic symbiosis of the free water zone is formed by plankton with its diversity of species into single and multi-celled phytoplankton (algae), zooplankton (e.g. water flea) and fish.
In the deep zone there are organisms which are described as destruents, but fish and other consumers live here. The composition of species in the deep zones depends primarily on the characteristics of the sediment, its nutritional richness and the oxygen conditions.
Habitats in lakes
The organisms living in a lake are subdivided according to their position in the food chain into:
Producers (e.g. algae, water plants) convert anorganic materials with the help of chlorophyll and sunlight into organic materials. These are then available to the consumers. Oxygen is produced as a by-product.
Consumers feed on organic materials which are made by the producers, either directly – plant eaters (e.g. water flea, crabs) or indirectly - carnivores (non-predatory and predatory fish).
Destruents are organisms (e.g. worms, bacteria) which feed on the remains of consumers and producers (dead organic substances). In this way, they convert organic materials into anorganic materials which are then once again available for the producers.,
Simple representation of the freshwater food chain
In natural conditions, there is a biological balance in a food cycle which is based on the mutual dependence of the organisms living within it. It has been shown that in lakes the relationship between prey and predators has a huge influence on vegetation. As a result, too great a number of non-predators can decimate the zooplankton. As this in turn uses plants and algae as its source of nutrition, these can spread unhindered. It is therefore necessary to keep the proportion of non-predators in natural conditions.
