The benthic community in the profundal of Lake Brunnsee: seasonal and spatial patterns

The seasonal distribution of bacteria, flagellates, ciliates, and selected metazoan organisms was investigated in the profundal sediment (18 m depth) of oligotrophic Lake Brunnsee (Germany) over a period of one year (April 1997 to April 1998). Most of the organisms displayed seasonal fluctuation with high abundance in summer and lower spring and autumn densities. Bacteria reached densities up to 2.1 x 10(10) cells/ml, with highest production values in July. Flagellates and ciliates had densities up to 151000 and 4100 cells/ml, respectively. Metazoan organisms were dominated by nematodes with a relative proportion of 76.4%, followed by rotifers with 6.5%. Other benthic taxa contributed less than 5% to overall abundance. Biotic and abiotic variables were tested for their ability to explain the variance in abundance of benthic organisms by means of multiple regression analyses. Vertical distribution of the metazoan organisms was recorded to a sediment depth of 4 cm. Most organisms preferred the uppermost centimetre. Rotifers and gastrotrichs have rarely been found (<15%) below 1 cm depth, while copepods and mites penetrate deeper (more than 60% have been found between 1 and 4 cm sediment depth). Spatial variation in abundance was recorded at a single sampling occasion by taking 16 sediment cores from the profundal of the lake. The spatial variation (differences between the cores) was highest for ciliates and bacteria and low for organic carbon, bacterial production, and bacterial extracellular activity.