Ciliate community structure in the zone of mixing of sea and river waters

摘要

The structure of a ciliate community was investigated in an estuary (the White Sea). The estuary was characterized by non-stable environment and a pronounced salinity gradient (0-22 per thousand). Communities in different estuary sites are dynamic combinations of marine and brackish-water species, substituting each other depending on the changes in water salinity and other environmental factors (pH, Eh, organic matter content, and granulometric composition of sediments). Where the average salinityexceeds 7-9 per thousand, marine euryhaline species dominate, at lower salinity, brackish-water species, with freshwater species occurring sporadically. Within the estuary, in the direction from the sea riverwards, the values of all integral community characteristics (biomass, abundance, species diversity, and mean size) decrease, and so does the percentage of algivorous ciliates. As compared to macrobenthos, marine ciliates penetrate deeper into the freshwater part of the estuary, the boundary betweenmarine and brackish-water ciliate communities markedly shifting towards the river.