Polygon ponds and their ostracode assemblages as bioindicators in the Indigirka Lowland (north-east Siberia)

Abstract: Freshwater ostracods (crustacea, ostracoda) are sensitive to environmental conditions, and are widely used as biological indicators for past and present environmental changes. The abundance and diversity of ostracods from permafrost areas is currently documented in scattered records with incomplete ecological characterizations. The objectives of the thesis were to determine the taxonomic and ecological range of ostracod assemblages and their habitat conditions in polygon ponds in different landscape units of the Indigirka Lowland (north-east Siberia, Russia). A monitoring approach focused seasonal meteorological and limnological variability of a selected pond site, its ostracod population dynamics, and the geochemical properties of ostracod valve calcite. Shallow, well-oxygenated, and dilute ponds with slightly acidic to circumneutral pH hosted an abundant and diverse ostracod fauna. A total of 4849 identified ostracods from eight species and three taxa represent the first record of the ostracod fauna in the Indigirka Lowland. Fabaeformiscandona krochini and Fabaeformiscandona groenlandica were documented for the first time in continental Siberia. Fabaeformiscandona sp. I and Fabaeformiscandona sp. II were newly found taxa holding a strong indicative potential for hydrochemical parameters. Repeated sampling of a typical low-center polygon pond revealed detailed insights in the population dynamics of Fabaeformiscandona pedata and its reproduction strategy. Substrate properties, physical and hydrochemical conditions in the studied ponds offered largely homogeneous habitats across different landscape units and pond types to ostracods. River flooding and differences in morphology between pond types resulted in variations in sediment, vegetation, hydrochemical and stable water isotope composition of the ponds. Ponds in the river floodplain and intrapolygon ponds hosted the most diverse ostracod fauna while species diversity was lowest in thaw lakes. Air temperature and precipitation were identified as the main external drivers of water temperatures, water levels, ion concentrations, and stable water isotope composition in small periglacial waters on diurnal and seasonal scales. Ostracod valve calcite recorded seasonal variations in stable oxygen isotopes of the ambient waters, but needs to be interpreted carefully with regard to species-specific background knowledge.