Freshwater Organisms in Biotopes Influenced by Sea-water

SUMMARY Freshwater ponds on the Frisian islands are frequently subject to seawater-flooding. Since the underground of the islands contains almost pure seasand, the heavy water of high salt-contents usually sinks into deeper underground-layers instantly, and fresh water can be restored by rainfall. The result is a region of brackish waters which change their salt-contents permanently, following an irregular scheme of time. These ponds are inhabited mainly by Cladocera, a group of small Crustacea. Population cycles occur in Cladocera, insofar as parthenogenetic reproduction alternates with the production of gamogenetic resting-eggs. The salt-resistance of active animals suffices for them to bear salt contents up to ß-oligohaline values or less, but the resistance of resting-eggs helps the species to survive in pure sea-water. The beginning of a population cycle is marked by the development of females from resting-eggs. This happens when suitable life conditions, including suitable salt-contents, are present. Since there is no inherent scheme of time in Cladocera, population cycles can follow the irregular scheme of desalinations. So Cladocera can actively coordinate their periods of parthenogenetic reproduction with periods of low salt contents in different ponds. Resting-eggs can be transported by birds and cattle. A possible extermination of a species in one pond or another caused by the seawater-flooding of an active population need not be the end of the species on the island, since a resettlement from neighbouring ponds may occur which were not reached by the flood or which contained resting-eggs at the time of inundation. The consideration of geographical connections, the chemistry, temperature and other conditions of the environment, together with the changings of resistance in different physiological stages of the organisms give us some impression of the complex of methods, which help freshwater-organisms to settle in the extreme biotopes of brackish regions