Interannual variation of the isotopic composition of sedimenting organic carbon and nitrogen in Lake Lugano : a long-term sediment trap study

We examined the evolution of 13C and 15N of sedimenting particulate organic matter (SPOM) in response to changes in primar y productivity, nutrient loading, and plankton dynamics in Lake Lugano between 1985 and 1998. A decreasing 13C reects the decline of phosphorus loading in Lake Lugano, suggesting that the 13C of SPOM is a good indicator of P availability in the lake, with phosphate being the major control on the CO2 draw-down in the photic zone. However, annual primar y production and 13C do not correlate. The expansion of the trophogenic layer in response to lowered plankton biomass concentrations balanced the effect of reduced phosphate concentration on total primar y production (per unit area) and, thus, biased the correlation of 13C and primar y productivity. Whereas a substantial decrease of total phytoplankton biomass in the late 1980s clearly affected the 13C of SPOM, it did not seem to affect 15N. Marked changes in the plankton population composition did not have a major effect on 15N. If at all, the effects of primar y productivity, nitrate utilization, and changes in the intensity of denitrication are superimposed on a signal dominantly derived from changes in the amount and isotopic composition of dissolved inorganic nitrogen from external sources. A marked positive shift (3‰) in the 15N obser ved in 1994/1995 corresponded to the introduction of a denitrication stage in wastewater treatment. This study demonstrates that the 13C of exported organic matter is a useful indicator of the trophic state of lacustrine environments. The 15N of sedimentar y organic matter is unlikely to reect nitrate utilization or denitrication if the input of external N sources dominates the N-isotope dynamics within the lake. However, in those systems, organic matter 15N can provide information on the development of N pollution and subsequent effects of lake restoration measures.