BENTHIC COMMUNITIES OF COASTAL RESERVOIRS AT DIFFERENT STAGES OF ISOLATION FROM THE SEA IN THE VICINITY OF THE WHITE SEA BIOLOGICAL STATION OF THE MOSCOW STATE UNIVERSITY, KANDALAKSHA BAY, WHITE SEA

Macrozoobenthos communities of the White Sea coastal reservoirs at different stages of separation from the sea are investigated. Postglacial isostatic land uplift determines the succession from a normal or depleted marine community to a completely freshwater one. Marine lagoons regularly flooded with tides are populated by the Macoma balthica - Pontonema vulgare community widespread across the marine littoral. More isolated lagoon-type lakes support a similar set of species, Mytilus edulis and Mya arenaria contribute to the biomass in addition to M. balthica. The gastropod, Hydrobia ulvae is abundant. At this stage, the fauna contains more littoral eurybionts (H. ulvae, M. balthica, M. arenaria, Mytilus edulis, Arenicola marina, Tubificoides benedeni, chironomids, etc.) and fewer sublittoral stenohaline forms. In the next stage, H. ulvae becomes dominant in biomass, the midge fly Ch. gr. salinarius and the oligochaete T. benedeni are subdominants. Shallow areas are inhabited mostly by insects and their larvae. In meromictic lakes, saline or brackish, the monimolimnion is divided to two sublayers: upper aerobic and lower anaerobic with hydrogen sulphide, relict euryhaline forms are retained in the salt aerobic sublayer from the marine fauna, ex. Gammarus duebeni and Ch. gr. salinarius. In the completely isolated lakes, as the anaerobic layer starts directly below the pycnocline, a marine fauna is lacking, while benthos is represented exclusively by freshwater forms (mainly insects and mollusks). Species diversity decreases with the separation of water bodies from the sea: the maximum number of species is found in the marine lagoons near Sonostrov Island (23-34 species). 10-15 species are registered in Lake Kislo-Sladkoye at every sampling. In the meromictic Lake Bolshie Khruslomeny, as the aerobic part of the mono-molimnion is characterized by critical salinity, the diversity is minimal (6-10 species). Further evolution of a separated lake results in the appearance of a permanent fresh layer, while the number of species is again increased. The quantitative characteristics of the macrobenthos follow a similar trend. The vertical distribution of benthos follows hydrological zonation. In the meromictic lakes with a freshwater mixolimnion, the abundance and biomass are maximal in the upper 0.5 m, whereas in the marine lagoons the maxima are located at the depth ranging from 1-2.5 m where the content of dissolved oxygen is the highest, up to supersaturation.