Carbon and nitrogen isotope variations in sedimenting organic matter in Lake Lugano

Abstract

We investigated the annual changes in sediment fluxes at two depths in Lake Lugano, Switzerland, and the associated variations in carbon and nitrogen isotope composition of sedimenting organic matter. The organic carbon and nitrogen fluxes increased by 10 and 20% with depth, respectively, whereas particulate phosphorus fluxes showed an increase of 114% with depth. The δ13C and δ15N of organic matter showed large seasonal changes ranging between −40 and −22‰ for C and 4 and 16‰ for N. The variations in δ13C can be attributed to variations in primary productivity level, changes in the carbonate chemistry, and isotope discrimination during photosynthesis. Very heavy nitrogen isotope compositions of organic matter in winter may indicate an external source of organic N. Comparison of the C and N isotope composition of organic matter in the top sediment with the sediment traps indicated that the observed flux increases with depth were due to a combination of lateral organic matter transport, sediment reworking, and possibly a contribution of allochthonous organic matter.