General Remarks with Regard to Biological Indicators Used in Water Pollution Studies

The quality of larger bodies of water such as lakes and rivers is of special importance. These aquatic biotopes differ greatly from the terrestrial ones. Accordingly, also the biological characteristics of the inhabitants differ from those of terrestrial organisms. The same holds for the functioning of plankton as compared to that of terrestrial communities. The pelagic zone is much more homogeneous than the mosaic of the various parts on land. Spatial differentiation to habitats is hardly possible in the aquatic environment. Consequently, the phytoplankton and also the Zooplankton species have more or less similar needs and an extensive niche overlap may be expected. These properties seem to be the ideal setting for a classical experiment to demonstrate competitive exclusion. Since nothing of the kind occurs in nature, but, on the contrary, a reasonable number of algae and Zooplankton species live together, Hutchinson (1961) felt justified to speak of the Paradox of the Plankton. The solution to this paradox is a set of abiotic factors always slightly varying with time in combination with specifically adapted species with a high potential to increase populations as soon as these factors have the right values. Of course, a stability of the kind observed on land is never equalled. This ever changing composition of the community renders these tiny organisms less suitable as living detectors for occasionally occurring pollutants. Their position at the beginning of the food chain and a high turnover rate render an accumulation of toxicants such as it is known for toppredators unlikely as well. At low concentrations of toxicants no lethal dose level will be reached in due time.