The oxygen concentration in near-bottom water shapes the structure of macrozoobenthos in the bradymictic Lake Urowiec

This research aimed to study the horizontal changes of macrozoobenthos structure and several abiotic parameters of water and bottom sediments along with increasing depth in the bradymictic Lake Urowiec. The samples were taken on six dates from April to October 2001, almost at monthly intervals, at the following depths: 2.5m, 5m, 7m, 9m, 10m, 11m, 15m, 20m and 30m. The high number of taxa and diversity (Shannon index) of bottom fauna were found only at depths of 2.5m (34 and 1.98, respectively) and 5m (27 and 2.18, respectively). From a depth of 9m, there were almost exclusively Chaoborus sp. larvae. The zoobenthos density decreased with depth from 2247 ind. m-2 at a depth of 2.5m to 200 ind. m-2 at 30m depth. The biomass of bottom fauna ranged from 6.51 g m-2 at a 2.5m depth to 0.43 g m-2 at a depth of 30m. A significant decrease in the diversity and, to a lesser extent, in the abundance of bottom fauna was most likely the consequence of the rapid decline in the water near bottom oxygenation with depth. Starting from a depth of 7 meters and deeper, the average oxygen concentration in the over-bottom water was very low - below 4 mg dm-3.