A quantitative ecological investigation of the littoral zoobenthos of an oligotrophic Finnish lake

Bathymetric, monthly and intralacustrine variations in species composition of the macrozoobenthos and meiozoobenthos were investigated on soft and stony bottoms of lake Konnevesi. Regional differences were significant for many variables, although the lake was expected to be quite homogeneous. On soft bottom macrobenthos the numbers and biomasses were generally greater in spring than in late summer and the bathymetric maxima generally occurred at a depth of between 2 and 4 m. On stony bottoms, the variations were not as clear as on soft bottoms and the proportion of predator taxa was about half, as opposed to about one third on soft bottoms. The seasonal distribution in numbers of meiobenthos was opposite to that of macrobenthos, showing a maximum in late summer and not in spring. Soft bottom abundances were 3.3 times higher than stony bottom abundances for macrobenthos and 8 times higher for meiobenthos. Oligochaeta, Megaloptera and Chironomidae were more abundant on soft than on stony bottoms. Hirudinea, Trichoptera and Coleoptera were more abundant on stony than on soft bottoms. Insects formed about 3/4 of the abundances and 2/3 of the biomasses. It seems that, with regard to macrobenthos biomasses, if the investigating resources are limited, the sampling times ought to be reduced to one, rather than reducing the number of sampling depths or subareas. The amounts of the merolimnic part of the zoobenthos were greater than the amounts of hololimnic zoobenthos on both kinds of bottom, and on stony bottoms the number of merolimnic fauna species was also greater than that of hololimnic fauna. On stony bottoms the number of species of merolimnic fauna increased with depth. The diversity and number of species were higher in spring than in late summer, which is not in agreement with the assumed food diversity. The bathymetric maximum of diversity occurred at depths of between 2 and 4 m and not in deeper zones, where the stability of environment ought to increase diversity. The quantitative proportion of predators on soft bottoms was lowest at the same depths at which the diversity was highest, which is in contrast with the predation hypothesis. However, examination of the numbers of species gave different results. On stony bottoms, the proportion of predators in the biomass was exceptionally high, which may indicate that the predators favour stones for habitation but gather food from the surrounding soft areas. A correlation analysis of the zoobenthos abundances