THE FAUNA OF FLOATING CYANOBACTERIAL MATS IN THE OLIGOHALINE EULITTORAL ZONE OFF HIDDENSEE (SOUTH-WEST COAST OF THE BALTIC-SEA)

The fauna - including macrofauna, meiofauna, and large ciliates - of f loating cyanobacterial mats in a brackish shallow-water area was studi ed by analysing six 20 cm(2) pieces of mat. Although these microbial a ggregations were scarcely 1 cm thick, their total meiofauna abundance was about five times as high as in the uppermost 4 cm of the adjacent sediment. The mat fauna was dominated by harpacticoids, although large ciliates, rotifers, nematodes, and oligochaetes were also markedly mo re abundant than in the sediment. All species occurring in the mats we re members of the surrounding sediment fauna. Out of the 47 species fo und, only a few, among them predominantly the harpacticoid, Mesochra l illjeborgi BOECK, 1864, and the nematode Daptonema setosum (BUTSCHLI, 1874), accounted for the majority of the individual abundance in the m ats. Both are regularly found in sulphidic biotopes near Hiddensee. As SEM micrographs revealed, the oligochaete Paranais litoralis (O. F. M ULLER, 1788) and the harpacticoid Cletocamptus confluens (SCHMEIL, 189 4) were apparently optimal substrates for dense lawns of cyanobacteria . This indicates possible close interactions between the meiofauna and these microbiota. The frequent predation of histophagous ciliates on nematodes and harpacticoids, which were probably weakened by oxygen de ficiency and/or high sulphide concentrations, were not only a sign of a generally neglected pathway in the food chain, but also impressively emphasized the huge variety of interactions taking place between meio - and microfauna within this specific benthic microcosm. Because of th eir floating character, the mats can play an important role in the dis persal of benthic fauna.