Composition and depth distribution of oligochaete fauna in a Mediterranean karstic lake

The depth distribution of oligochaetes was studied in Visovac lake (Croatia) along a transect from 1 to a maximum of 24 metres. In the material collected, 21 oligochaete species belonging to 4 families were identified, all of them more or less common in European lakes. The greatest diversity of oligochaetes was found at a depth of 4-6 m (18 taxa). On silty-sands covered by reed decay at a depth of 1 m oligochaetes comprised only 2% of the total benthic biomass. Oligochaeta densities increased with depth and at a depth of 22-24 m they represented 59% of the total biomass. Depth and bottom type were the important variables in defining species distribution. Only three oligochaete species were found at all of the investigated depths Potamothrix hammoniensis, Psammoryetides barbatus and Limnodrilus hoffmeisteri. Aulodrilus pluriseta was distributed only in the area of submerged macrophytes. The remaining species occurred less frequently, and mainly with restricted distribution.