Patterns of psammophilous ciliate community structure along the salinity gradient in the Chernaya river estuary (the Kandalaksha Gulf, the White Sea)

Species composition patterns and mechanisms of formation of ciliate community structure in the Chernaya river estuary (the Kandalaksha Gulf, the White Sea) were studied in 1996 2000 at stations distributed along the salinity gradient. From the marine part of the estuary riverwards, salinity decreases and oxidative reductive properties of the sediments become more reduced due to organic accumulation. The greatest variability of salinity, acidity and redox potential is observed in the brackish water part of the estuary. All other factors have a more complex distribution in the intertidal zone, being both gradient and mosaic. During the period of the investigation altogether 123 species of psammophilous ciliates were found. Psammophilous ciliate community in the estuary may be considered as rather unified, continual and twopolar (with marine and brackish water pool of species). A relative and fuzzy border between the two poles is situated in brackish water part of the estuary, where the greatest variability of salinity and redox potential as well as critical salinity level (3 8‰) are observed. In the estuary riverwards the complexity of ciliate community in terms of ciliate density, biomass and species richness decreases significantly. Decrease in species richness results from congeners. It probably reflects a simplification of niche structure. Seasonal changes of ciliate community structure in marine zone may be characterized as rather trended and stable. Changes in community structure in the brackish water zone of the estuary, on the contrary, are more variable and stochastic. Community in the intermediate zone of the estuary is both trended and highly heterogeneous. Community biomass, abundance and species richness are positively connected with average salinity and redox potential levels. Variability of integral community characteristics is related predominately to sediment parameters (amount of organic matter and relative ability of large size fraction of sand grains). Variability of saline regime affects that of the total abundance and biomass of the community but not the variability of species richness and diversity. Analysis of the role of salinity in ciliate community formation reveals trended changes of species composition and non linear modifications of integral community characteristics. All community variants are divided into two groups with equal complexity but different species composition: marine and brackish water. Ability of complex marine community to sustain its own structure under the influence of lower salinity is the first mechanism of community formation in brackish waters. It predominates at a relatively higher salinity level as well as during terminal succession stage. Ability of oligohaline ciliates to immigrate from marine to freshwater estuarine zone is the second mechanism. It determines the community structure in the initial stages of succession at lower salinity level.