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 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 8°C and 615N of organic matter showed large seasonal changes ranging between -40 and -22%0 for C and 4 and 16%0 for N. The variations in SIC 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. Organic matter (OM) is an important component of settling particles and sediments in lakes. It influences a variety of biogeochemical processes and is the most important factor controlling redox conditions, the oxygen budget of bottom waters, and the cycling of phosphorus, other nutrients, and trace metals. The quantification of fluxes and accumulation rates of OM are therefore important parameters for any model of lake restoration (Bloesch and Uehlinger 1986). Secondary processes such as resuspension from the bottom sediments, lateral transport within the water column (sediment focusing), or transport at depth of detrital matter through river input (Hilton et al. 1986), however, often impair the precise determination of sediment accumulation rates. By characterizing the C and N isotope composition of settling particles during an annual cycle, it may be possible to distinguish between these processes if the seasonal variability of SIC and 61sN in primary OM is large enough and contrasts with the isotopic composition of the bottom sediment and the allochthonous input from OM derived from the catchment area. The amount of OM stored in sediments and its chemical and isotopic composition are also valuable tools for reconstructing past changes in productivity, in C and N cycling, and biological community structure. Carbon isotope composition of bulk lacustrine OM has been widely used to reconstruct paleoenvironmental conditions (e.g. Hollander and McKenzie 199 1; Schelske and Hodell 1995). However, during sedimentation and deposition, OM is microbially transformed and decomposed, and many questions remain open