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

Abstract

We examined the evolution of δ13C and δ15N of sedimenting particulate organic matter (SPOM) in response to changes in primary productivity, nutrient loading, and plankton dynamics in Lake Lugano between 1985 and 1998. A decreasing δ13C reflects the decline of phosphorus loading in Lake Lugano, suggesting that the δ13C 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 primary production and δ13C 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 primary production (per unit area) and, thus, biased the correlation of δ13C and primary productivity. Whereas a substantial decrease of total phytoplankton biomass in the late 1980s clearly affected the δ13C of SPOM, it did not seem to affect δ15N. Marked changes in the plankton population composition did not have a major effect on δ15N. If at all, the effects of primary productivity, nitrate utilization, and changes in the intensity of denitrification 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 δ15N observed in 1994/1995 corresponded to the introduction of a denitrification stage in wastewater treatment. This study demonstrates that the δ13C of exported organic matter is a useful indicator of the trophic state of lacustrine environments. The δ15N of sedimentary organic matter is unlikely to reflect nitrate utilization or denitrification if the input of external N sources dominates the N‐isotope dynamics within the lake. However, in those systems, organic matter d15N can provide information on the development of N pollution and subsequent effects of lake restoration measures.