Life in our lakes

 

Basically we differ two large and completely separated habitats in our lakes. On the one hand, the biocoenosis of free water bodies, the pelagic zone, and on the other hand, the biocoenosis of the lake bottom, the benthic.

The majority of organisms in our lakes are plants and animalcules. Aquatic plants like water lilies, water milfoil and pondweed or animals such as bivalves, freshwater gastropods or fish play only a minor role in their biological diversity. However, the plankton community, which is composed of very small organisms that float in the open water body, is represented in a large number of species and individuals. Among these organisms, it is the phytoplankton, which is represented up to several million individual cells per litre of water, by only a size of a few thousandths millimetres. These floating algae live in the light-flooded surface layer, where they, like all plants, build organic, autologous substances with the help of solar energy from inorganic nutrients such as phosphorus and nitrogen compounds. In this process, nutrients are removed from the water and incorporated into the algal body.
Thus, depending on the nutrient load of the water, there exist lakes with low levels in nutrients (oligotrophic) and nutrient rich (eutrophic) lakes, whereas transitional types are called mesotrophic lakes. Because of these different amounts of nutrient concentrations, the development of the suspended algae is variable. A nutrient-rich lake has a high algae stock, similar to a well-fertilized field that produces a good yield. But while a good production in agriculture is highly desirable, a high production of algae in the lake is not appreciated because the floating algae make the water murky and unattractive. After the death of the algae the decomposition process starts and leads to deep water oxygen depletion. Thus, in nutrient-rich and algae-filled lakes these processes are very intense, whereby the oxygen content in the depth can decline heavily. This is why a high nutrient content in the lake is understandably not desirable and all the lakes pollution control measures aim to keep the nutrient levels as low as possible.

In addition to this rich variety of algae there exist animal organism in the open water bodies of our lakes, which are also microscopic in size (maximum size: 1 to 2 mm). This zooplankton community includes protozoa, rotifers and crustaceans, well-known are the water fleas and copepods. Zooplankton is the nutrition base for fish which represent the final link in the food chain in a lake. 

To get an idea of the small size and the enormous population density of these tiny living creatures, it should be mentioned that in one litre of water might live up to several millions of protozoa. In case of filamentous forms, such as Planktothrix rubescens ("blue-green algae"), dozens of metres of thread length can be found per litre, if you string together the small, only a few millimetres long and about 5/1000 mm thick individual filaments.

Quote from SAMPL, H.: Kärnten – ein Land der Seen: 63-100. In: MILDNER, P. & H. ZWANDER (1999): Kärnten Natur. Die Vielfalt eines Landes im Süden Österreichs. - Naturwissenschaftlicher Verein für Kärnten, Klagenfurt.