Nitrate‐driven eutrophication supports high nitrous oxide production and emission in coastal lagoons

Abstract

Under current circumstances, coastal lagoons are net emitters of nitrous oxide (N2O) to the atmosphere. We hypothesize that widespread nitrogen-driven coastal eutrophication will enhance N2O production and emissions from coastal lagoons. Here, we quantified spatial and temporal patterns of sediment-water and water-air N2O fluxes in three large eutrophic lagoons in Europe. Annual sediment N2O fluxes ranged between −0.3 ± 0.3 (summer) and 10.6 ± 2.0 μmol m−2 d−1 (spring). In spring, conspicuous sediment effluxes were mainly supported by high nitrate concentrations (89–202 μM) and incomplete denitrification. In summer, a small sediment influx was related to nitrate limitation (0–9 μM), potentially leading to N2O demand for denitrification. The water-air N2O fluxes were comparable with benthic fluxes, indicating that sediment was the main source of N2O to the atmosphere. The hypereutrophic Curonian Lagoon had the largest N2O emission at 4.9 ± 2.1 μmol m−2 d−1, while the less eutrophic Oder and Vistula lagoons emitted 2.5 ± 1.0 and 2.0 ± 0.7 μmol m−2 d−1, respectively. Our observations, combined with earlier measurements in coastal lagoons worldwide, revealed a lagoon median (Q1–Q3) N2O emission of 14.2 (2.7–29.8) Gg yr−1, which is about 48% higher than previous estimates. Eutrophication driven by large nitrogen inputs is thus a significant driver of coastal N2O emissions globally.