CARBON AND NITROGEN ISOTOPE VARIATIONS IN SEDIMENTING ORGANIC-MATTER IN LAKE-LUGANO

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 organ ic carbon and nitrogen fluxes increased by 10 and 20% with depth, resp ectively, whereas particulate phosphorus fluxes showed an increase of 114% with depth. The delta(13)C and delta(15)N of organic matter showe d large seasonal changes ranging between -40 and -22 parts per thousan d for C and 4 and 16 parts per thousand for N. The variations in delta (13)C can be attributed to variations in primary productivity level, c hanges in the carbonate chemistry, and isotope discrimination during p hotosynthesis. Very heavy nitrogen isotope compositions of organic mat ter in winter may indicate an external source of organic N. Comparison of the C and N isotope composition of organic matter in the top sedim ent with the sediment traps indicated that the observed flux increases with depth were due to a combination of lateral organic matter transp ort,sediment reworking, and possibly a contribution of allochthonous o rganic matter.