BENTHOS OF LAKE MIKRI PRESPA (NORTH GREECE)

An extensive sampling of Lake Mikri Prespa was carried out in summer 1 990, aimed at evaluating benthic abundance, recording the species invo lved and detecting their response to some environmental variables. Thr ee oligochaete species are endemic to the region, Potamothrix prespaen sis, Psammoryctides ochridanus variabilis and Psammoryctides ochridanu s typica. The bathymetric curve of the benthos resembles that of eutro phic lakes. A great abundance of Chironomus plumosus and Chaoborus fla vicans occurred. Four community patterns were revealed by TWINSPAN ana lysis, division level two, explicitly discriminated by the influence o f substrate type, depth, C/N ratio and pH. Species richness and divers ity was minimal in the profundal and maximal in the shallows, whilst t he equitability index was high in all site groups. Depth and bottom ty pe were the most important variables in defining species distribution. More precisely, Chironomus plumosus, Chaoborus flavicans and Potamoth ix hammoniensis were ubiquitous, preferring mostly deep waters. Tanyta rsus sp., Einfeldia dissidens, Cladopelma lateralis, Tanypus punctipen nis and Potamothrix prespaensis favoured shallower and less productive waters with a bottom rich in decaying plant fragments, and the Psammo ryctides species showed a littoral origin and regular occurrence on gr avel beds. A few species displayed a particular distribution with chan ges of depth, C/N ratio and pH.