ABSTRACT
Fundamental aspects of test systems used to study the fate and biological effects of chemicals in still freshwater are discussed. The test systems used worldwide are categorized according to the size of the test ponds, as the structure and size of a pond mainly influences the type and number of species present, i.e., its diversity and, even more important, its ecological representativeness. While phyto- and zooplankton biocoenoses of a test pond of a few cubic meters volume may represent natural aquatic ecosystems quite well, fish and macrophytes cannot be investigated satisfactorily in some cases. If field studies of the biological effects of a chemical on macrophytes and fish biology are intended, the use of larger ponds may be considered. As macrophytes increase the variability of plankton biocoenosis drastically, it is proposed that the abundance of macrophytes should generally be kept at a low level.
The pond sediment plays a major role for the fate of many test chemicals in aquatic ecosystems, often being the main compartment of abiotic and biotic degradation or the sink of the chemical. For fate studies and also for investigations in which the fate of a chemical is of major importance to the biotic effects, the presence of natural sediment within the test ponds is recommended.
The advantages and disadvantages of the most commonly used systems are discussed: small scale indoor and outdoor ecosystem studies with volumes from a few L up to about one cubic meter; microcosms and enclosures of lake water without sediment with volumes of about 1 to 15 cubic meters; lake shore enclosures; microcosms of different shape and size including sediment; large scale mesocosm ponds of about 15 up to several hundred cubic meters volume, and; natural ponds of different sizes.
