Field observations, laboratory experiments and a literature survey were conducted to evaluate the habitat characteristics of Eurytemora lacustris (POPPE 1887), a freshwater calanoid copepod species. Combined effects of temperature and oxygen concentration in the deep water of thermally stratifying lakes seem to be the ultimate factors governing the occurrence of the species throughout its home-territory. E. lacustris is largely restricted to relatively deep lakes (>30 m) providing a hypolimnetic refuge characterised by low temperatures (<~10°C) and oxygenated water during summer. Therefore, although the species is spread over much of Europe it was only found in a small number of lakes. Long-term records in different lakes revealed E. lacustris to be perennial with relatively high biomasses occurring from May to September. During thermal stratification on average 87% of the nauplii and 72% of the copepodite biomass was found in hypolimnetic waters colder than 10°C. Diurnal vertical migration was observed for the copepodid stages, but the migration amplitude clearly decreased from May to September. The migration amplitude was significantly related to light intensity. According to its special habitat requirements, E. lacustris might be considered a glacial relict sensitive to temperature increase and oxygen depletion. IntroductionThe distribution of a given species within its home-territory largely depends on the species' ecological requirements and the extent to which these needs are satisfied by a certain biotope. As a result of the co-evolution between environment and biota, plants, animals and micro-organisms are restricted to particular geographical regions and habitats (ILLIES, 1971). Some species might be abundant in many places while others are rare and characterised by a scattered distribution (ILLIES, 1978). With respect to lakes, chemical and physical features of the habitat such as temperature, oxygen concentration and solar radiation are as important as biological factors like food supply and predation (THIENEMANN, 1928;LINDEMAN, 1942; STRAŠKRABA, 1980;ZARET, 1980). Eurytemora lacustris (POPPE, 1887), a calanoid freshwater copepod, can be found in lakes throughout an area reaching from the Boreal highlands of northern Norway south to the Black Sea and from the Central European lowlands to the eastern parts of the Caspian reInternat. Rev. Hydrobiol. 90

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