Svensson, J.E., R. Lagergren & J.A.E. Stenson. (2002). Lake acidification and the distribution of males and females of a planktonic copepod. Verhandlungen der Internationale Vereinigung für Theoretische und Angewandte Limnologie. 28(2):992-995.

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Svensson, J.E., R. Lagergren & J.A.E. Stenson

2002

28(2):992-995.

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Ecological srudies on biota in anthropogenically acidified lakes have focused on numerical changes of populations and on changes at the community and ecosystem level. Most responses can be accounted for by a combination of abiotic factors such as high toxiciry or reduced nutrient levels, and biotic factors such as changed predation and competition pressures (STEINBERG & WRIGHT 1994, SVENSSON et al. 1995). Fish are usually lacking in severely acidified lakes in Scandinavia and release from fish predation is responsible for several fauna changes (APPELBERG er al. 1993, STENSON er al. 1993). Effects o n populations that show weak numerical responses to acidification have received only little interest, however. Selective fish predation is known to significancly affect many aspects of behaviour and life history in woplankton prey populations (e.g. Guwrcz & PIJANOWSKA 1989). The loss of fish in acidified lakes therefore implies changed selective pressure on 'acidtolerant' prey. The present srudy focuses on horiwntal distribution patterns in rhe herbivorous copepod Eudiaptomus gracilis Sars. This is usually the dominating calanoid in acidified as well as in neutrallakes in southern Sweden, also showing limited density changes after Jake Jiming (STENSON & SVENSSON 1994, SVENSSON et al. 1995). In a previous srudy the habitat distribution of males and females of E. gracilis in a neutrallake with fish has been described in detail (SVENSSON 1997a). Skewed distributions, with females being more bound to deep water in the middle of rhe lake, occurred in spring. Fish predation may be one factor responsible for this (SVENSSON 1996). Predation is usually most intense in near-shore areas and females may be vulnerable when they produce and carry easily derectable eggs. The aim of this srudy was to compare the sex distribution in neutral lakes with fish and acidified lakes without fish. If fish predation affected disrribution of sexes, a less skewed distriburion was predicted in the acidified lakes.

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