Eutrophication and urbanization enhance methane emissions from coastal lagoons

Coastal lagoons are important nutrient filters and carbon sinks but may release large amounts of methane (CH4) to the atmosphere. Here, we hypothesize that eutrophication and population density will turn coastal lagoons into stronger methane emitters. We report benthic fluxes from 187 sediment cores incubated from three of the largest European lagoons suffering persistent eutrophication. Methane fluxes were mainly driven by sediment porosity, organic matter, and dissolved inorganic carbon (DIC) fluxes. Methane was always supersaturated (250–49,000%) in lagoon waters leading to large, variable emissions of 0.04–26 mg CH4 m−2 d−1. Combining our new dataset with earlier estimates revealed a global coastal lagoon emission of 7.9 (1.4–34.7) Tg CH4 yr−1 with median values of 5.4 mg CH4 m−2 d−1. Lagoons with very highly populated catchments released much more methane (223 mg CH4 m−2 d−1). Overall, projected increases in eutrophication, organic loading and population densities will enhance methane fluxes from lagoons worldwide.