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 (N 2 O) to the atmosphere. We hypothesize that widespread nitrogen‐driven coastal eutrophication will enhance N 2 O production and emissions from coastal lagoons. Here, we quantified spatial and temporal patterns of sediment‐water and water‐air N 2 O fluxes in three large eutrophic lagoons in Europe. Annual sediment N 2 O 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 N 2 O demand for denitrification. The water‐air N 2 O fluxes were comparable with benthic fluxes, indicating that sediment was the main source of N 2 O to the atmosphere. The hypereutrophic Curonian Lagoon had the largest N 2 O 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) N 2 O 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 N 2 O emissions globally.