The role of avalanches in modifying the carbon and nitrogen pool in alpine lake derived from stable isotope analyses of sediments and Daphnia ephippia

Abstract This study presents the results of an analysis of sediments from a mountain lake located in an area with a high incidence of avalanches and their impacts on sediment accumulation and the influx of organic and minerogenic matter from the catchment area. A short sediment core obtained from Czarny Staw pod Rysami Lake (Tatra Mountains) was analyzed to determine the sedimentation rates and chronology, organic matter (OM) contents and carbon and nitrogen stable isotope compositions (δ 13 C and δ 15 N) of both the bulk sediment and the ephippia of two Daphnia morphotypes (pale and dark). The primary goal was to assess the impact of avalanche-induced sedimentation on the stable isotope signatures of OM and their subsequent incorporation into the lake-food web expressed by isotopic signatures of Daphnia . These findings underscore the crucial impact of avalanches in shaping sediment deposition, influencing biogeochemical processes, and affecting the number of ephippia; this is evidenced by the presence of distinct sedimentary layers that are characterized by higher densities, higher C:N ratios, and lower levels of total organic carbon and total nitrogen, which correspond to major avalanche events. The most pronounced event recorded in the core occurred in January 2003. Then, a four-centimeter thick layer was deposited, which is well defined in the bulk dry density, C:N ratio and stable isotope record of Daphnia ephippia. In addition, the trend of the ephippia signature generally mirrors the changes in the stable isotope compositions of bulk sediment OM. It is hypothesized that pale Daphnia utilize a broader spectrum of OM sources, whereas dark Daphnia colormorphs more effectively exploit the suspended OM that is associated with avalanche events. This difference likely results from their distinct life strategies. Moreover, the isotope signature of ephippia allows us to assume that avalanches provide an extra portion of nitrogen along with terrestrial plant and soil-derived matter to the system. Since the 1970s, there has been a clear decline in the δ 15 N values, interpreted as an influx of anthropogenic nitrogen, primarily from the combustion of fossil fuels. Graphical abstract