Macro- and meiozoobenthos in some small stratified lakes of Estonia; pp. 116–135

Macro- and meiozoobenthos (300 samples with a 225 cm2 Boruckij grab and 180 with a 6.38 cm2 corer) from unvegetated, soft-bottom areas in ten small stratified lakes were studied in July–August 1998–1999. The sampling sites were located just above, in the middle, and below the position of the metalimnion in each lake. Chironomidae and Oligochaeta were common in both macro- and meiozoobenthic samples but more abundant (by an order of magnitude) in the latter because the smallest individuals were included. The meiobenthic samples were dominated by planktonic crustaceans, particularly Cyclopoida, with respect to abundance while most of their biomass was formed by the macrobenthic animal groups. Eumeiobenthos sensu stricto, such as Nematoda, Ostracoda, etc., were scarce. Thirty-three variables (abundance, biomass, and number of taxa in different samples) were related to water layer (epi-, meta-, and hypolimnion), sampling depth, lake, year, and O2 saturation %. Most variables depended on lake but not on year. Abundance and biomass of the studied taxa and the number of taxa of macrozoobenthos per sample decreased significantly from the epi- to hypolimnion, except for Chaoborus flavicans, which was more abundant in deeper layers of water. From the results of ANOVA, it was concluded that the biomass and abundance of Chaoborus, biomass of Hydrachnidia, abundance of Chironomidae and other ‘large’ animals in the meiobenthic samples, as well as the number of taxa of macrozoobenthos per sample did not follow the layers but only the depth. The distribution of the biomass of macrozoobenthic Chironomidae as well as the total biomass and abundance of macrozoobenthos without Chaoborus were more tied to separate layers. For the majority of individual taxa no dependence on depth was found. Discriminant analysis revealed a rather weak separation of the water layers in summer by the benthic characteristics. An index formed of 33 variables of zoobenthos with the highest statistical significance for testing the depth effect (Depth Index) revealed the most apparent changes in environmental conditions in the upper part of the metalimnion.