Can the morphological traits of benthic testate amoebae in a freshwater lake be indicators of depth and environmental conditions?

Testate amoebae are widely used as proxies in palaeoecological reconstructions of lacustrine environments. However, our knowledge on their morphological adaptation to depth is still limited. This study aims to determine correlations between the morphological and size structure of testate amoeba communities and depth and some other environmental variables along a depth gradient from 0 to 57 m in mesotrophic Valdayskoe Lake, Russia. The morphological structure of testate amoeba communities in the bottom surface sediments of the lake and their distribution along the depth gradient were described. Redundancy analysis indicated that sampling depth, temperature, pH, and slope angle significantly explained the total variance in the community compositions, which were classified into three morphological groups. The set of morphotraits of testate amoebae communities differed drastically above and below the thermocline and may be used as an indicator of the depths. Above and below the thermocline, the morphological structure of testaceans is also determined by other environmental variables associated with the habitats they were found. The weighted and median body size of testate amoebae generally decreases with depth, but bottom relief influences the body size structure of the community via differences in the accumulation of organic matter on slopes and flat sites. These data may contribute to a better interpretation of palaeoecological records of subfossil testate amoebae in lacustrine surface sediments and serve as a basis for the development of a transfer function for reconstructing lake depths.