Planktonic and zoobenthic communities in an oligotrophic lake inhabited by an endemic and endangered seal population

In order to assess the present biological state and to give some guidelines for the planned monitoring of Pihlajavesi, one of the most important habitats for the threatened endemic relict Saimaa ringed seal (Phoca hispida saimensis Nordq.), the main communities of phytoplankton, zooplankton and benthic invertebrates were described using multivariate statistical analyses. The composition of phytoplankton assemblages was mainly determined by the nutrient concentration and the colour of the water so that the phytoplankton community of a clear-water basin differed in this lake from those of the other basins. In contrast there were no such differences in zooplankton assemblages between the basins, but the distinct difference between the pelagic and littoral communities was evident. The water depth and related parameters, such as bottom quality, were the most important factors affecting on zoobenthic assemblages. Species composition changed gradually with increasing depth, the rate of the change being greatest in the littoral area. The following soft bottom communities were distinguished from the lake: vegetated littoral (1–3 m), sublittoral (3–10 m), upper profundal (10–20 m) and deep profundal communities. The deep profundal assemblage in the effluent-loaded basin differed from that in other basins to such an extent that it could be considered to form a distinct community of its own. Special features of the communities resulting from the high transparency of water are discussed.