Seasonal variation of the d¹³C and d¹⁵N of particulate and dissolved carbon and nitrogen in Lake Lugano: Constraints on biogeochemical cycling in a eutrophic lake
Lehmann, Moritz F., Stefano M. Bernasconi, Judith A. McKenzie, Alberto Barbieri, Marco Simona, and Mauro Veronesi
Limnol. Oceanogr., 49(2), 2004, 415–429

We determined the d¹³C and d¹⁵N 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 d¹³C 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 [CO_2(aq)]. Variations in the d¹⁵N 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 () associated with nitrate assimilation varied with = -1.0‰ ± 0.9 for diatoms and = -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 d¹⁵N of POM, the d¹⁵N of nitrate, and the nitrate concentration in surface waters. Low d¹³C (-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 d¹³C and d¹⁵N 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 d¹⁵N 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 d¹⁵N of POM in eutrophic lakes is of limited use as an indicator of the degree of nitrate utilization and trophic state.