DIVERSITY OF MEIOFAUNA IN THE LACUSTRINE PROFUNDAL ZONE - BATHYMETRICDIFFERENCES AND INFLUENCE OF ENVIRONMENTAL-FACTORS

The diversity and species richness of total meiofauna, Harpacticoida a nd Oligochaeta in relation to depth and other environmental variables were studied at nine stations and in two profundal depth zones in Lake Paijanne which includes both oligotrophic and eutrophicated or organi cally loaded areas. Diversity was on average higher in the upper part of the profundal zone than at maximum depths. This may show that the g reater temporal stability in temperature at greater depths does not su rpass the effect of the lower temporal stability in oxygen content. Th e productivity factor may be a more important reason for the bathymetr ic difference in diversity than the stability factor. Diversity in the lower profundal zone shows a negative correlation with variables indi cating pollution while in the upper profundal zone a positive one with those variables, thus emphasizing the importance of the production fa ctor in determining diversity in the upper profundal. Diversity was hi gher within the oligochaetes than within the harpacticoids, which may imply that oligochaetes have a longer evolutionary history as part of the lacustrine benthic system. Of the environmental variables, phytopl ankton biomass had the highest influence on total diversity in the low er profundal zone, oxygen being next in importance. The sedimentation of organic matter was most important in the upper part of the profunda l zone, where oxygen did not have any significant effect on diversity. Some explaining factors determining bathymetric diversity differences are discussed in relation to the theoretical expectations.