Interannual changes in δ15N baseline in a lake-swamp-river food web: effects of summer precipitation in West Siberian forest steppe

Frequency and intensity of precipitation events can alter hydrological conditions of lotic systems and material inflow from the catchment. In this study, interannual changes in food web structure were investigated in a lake-swamp-river system in the semi-arid West Siberian forest-steppe region using carbon and nitrogen stable isotope ratios (delta C-13 and delta N-15). The delta N-15 signatures of aquatic producers and consumers including zooplankton, benthic invertebrates, and fish sharply increased (up to 7.2 parts per thousand in range) in 2009 under high summer precipitation. This would reflect the inflow of(15)N-rich nutrients accumulated in the catchment through natural and anthropogenic activities, e.g., sewage, livestock farming, evaporation, and/or microbial denitrification. In 2010, benthic producers including epiphytes and macrophytes were more(13)C-enriched, which is likely due to limited supply of dissolved inorganic carbon (DIC) under water stagnation, leading to a large isotopic fractionation during photosynthesis. However,C-13-enrichments in the benthic producers were not reflected to those of aquatic consumers in the habitat. Interannual changes in delta C-13 of particulate organic matter (POM; a proxy of phytoplankton) were reflected to those of zooplankton, benthic invertebrates, and fish fry, suggesting the trophic importance of autochthonous phytoplankton in the system. These findings suggest that occasional precipitation events in semi-arid forest-steppe regions can modify nutrient flux from the catchment, leading to changes in delta N-15 baseline in the lotic food web.