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We observed changes in the composition, structure and seasonal succession patterns in the community of interstitial ciliates in the White Sea over a period of 18 years. Considerable inter-year fluctuations of species abundances as well as species richness, diversity, trophic and size structure were recorded. During the first 12 years, the changes appeared to be multi-vectorial. However, in the last 6 years the community modification became direct, with an increasing number of specific interstitial ciliate species, and Trachelocerca sagitta as the leader. These changes were caused by the increase in the silt content and the amount of dead organic matter, as well as by the decrease in the concentration of oxygen and the size of interstitial spaces in the sediments. All the changes took place on the background of the same set of ciliate species and the same niche structure. The changes in the community structure during the period equivalent to thousands of generations of ciliate populations are realized through two major mechanisms. The first one acts through inter-annual fluctuations, and manifests itself in recombinations of species composition in response to local environmental factors. The second one acts through long-term directional changes in the community composition, in accordance with the trended transformations of the littoral ecosystem. Inter-year changes (e.g., the degree of seasonal fluctuations of abundance, species diversity and community structure) were also manifested at all the stages of seasonal succession. During the last 6 years, within-year variability of abundances clearly increased, and so did stability and direction of seasonal succession. Yearly renewed seasonal successions do not lead to the increase in the community complexity and order, but are rather stages on the way of the communitys adaptation to the changing environment. Inter-year variability of the patterns of seasonal succession reflects the evolutionary potential of the communitys resilience in a fluctuating environment.

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.

During the period of 1998 2000, 123 psammophilous ciliate species were found in the White Sea estuary. Community consists of marine stenohaline species occurring in the marine part of the estuary, marine euryhaline ciliates represented both in the marine and the freshened zone and brackish water organisms dominating in the riverine part of the estuary. On the other hand, community is composed of basic structure forming species (the community "nucleus"), constantly prevailing in a certain estuarine zone, and accidental, or very rare species (cryptic diversity), sporadically occurring at different stations during different seasons. The total list of the White Sea benthic ciliate fauna contains 273 species from 94 genera, recorded in the Kandalaksha Bay. Only 3.5 5.8% of the total White Sea species richness constitute "active" diversity in a given microhabitat at a particular point in time. Relations between the number of samples taken and the number of species registered testify to species saturation at all stations. Spatial heterogeneity of the biotope causes a greater increase in species richness than temporal one.

Vertical structure of the interstitial ciliate community in the Chernaya River estuary (the White Sea) was investigated. Distinct change of the species structure of the ciliate community in the sediment column is observed at all stations in the estuary along the salinity gradient. There are three species groups, which prefer certain horizons of the sediment: 1) eurytopic species, not specific for the interstitial, occupying top horizons of the sediment; 2) basic structure-forming species represented by specific interstitial ciliates in the marine part of the estuary. They occupy subsurface layers and develop down to the border of the H2S zone; 3) microaerophilous and anaerobic organisms, occupying the deepest layers of the sediment. A decrease in biomass, abundance, species richness and diversity of ciliate community with depth is detected. A sharp vertical Ehgradient in the freshened zone of the estuary results in concentration of different ciliate species within the limits of small layers and, as a result, in sharp vertical differentiation of the community. Moreover, in the freshened zone most of abundance and biomass of ciliates (more than 70 %) is concentrated in the top 0.5 cm layer of the sediment. More gradual changes of the Eh-gradient and its higher absolute value in the marine zone determine deeper penetration of many species into the sediment and, as a consequence, a smaller differentiation of the community.