Seasonal variation of the δC and δN of particulate and dissolved carbon and nitrogen in Lake Lugano: Constraints on biogeochemical cycling in a eutrophic lake

Abstract

We determined the Δ13C and Δ15N of water‐column particulate organic matter (POM), dissolved inorganic carbon, and nitrate, together with water chemistry and phytoplankton biomass and species composition every month in eutrophic Lake Lugano. As primary productivity increased during spring, the Δ13C of photic‐zone POM increased from −34‰ to −24‰. This 13C enrichment reflects decreasing C‐isotope fractionation between organic and inorganic carbon pools in response to decreasing surface water [CO2(aq)]. Variations in the Δ15N of surface‐water POM (+2‰ to +8‰) collected during the productive period were attributed to isotope effects associated with nitrate uptake, nitrogen fixation, and mixing of different organic matter sources. The apparent N‐isotope enrichment (ɛ) associated with nitrate assimilation varied with ɛ = −1.0‰ ± 0.9 for diatoms and ɛ = −3.4‰ ± 0.4 for green algae. The mechanisms controlling the N‐isotopic composition of surface‐water nitrate include the combined processes of nitrate assimilation, nitrification, mixing of water masses, and external nitrate loading. There was no consistent relation between the Δ15N of POM, the Δ15N of nitrate, and the nitrate concentration in surface waters. Low Δ13C (−60‰) of POM from the benthic nepheloid layer (BNL) indicated that as much as 80% of the POM in the BNL consisted of methanotrophic bacteria. The Δ13C and Δ15N of near‐bottom POM allowed tracing a “bacteriocline”, which rose with the expanding volume of bottom‐water suboxia. Associated with the development of anaerobic conditions and coupled to decreasing [NO3+], the Δ15N of bottom‐water nitrate progressively increased from +8‰ to +27‰, indicating active denitrification in the hypolimnion. This study demonstrates the potential of natural‐abundance level measurements of stable carbon and nitrogen isotopes to trace biogeochemical processes in lakes. However, the Δ15N of POM in eutrophic lakes is of limited use as an indicator of the degree of nitrate utilization and trophic state.