Eutrophication and urbanization enhance methane emissions from coastal lagoons

Abstract

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.

Acknowledgments

Funding was provided by the Lithuanian Research Council, Lietuvos Mokslo Taryba (Grant no. S-MIP-22-47) and the Swedish Research Council, Vetenskapsrådet (Grant no. 2022-04710). We thank Elise Lorre for practical support during field work and experiments; the Lithuanian State Border Guard Service for boat and sampling support (Curonian Lagoon); Adam Woźniczka, Agnieszka Szkudlarek-Pawełczyk, and boat crews at the Polish National Marine Fisheries Research Institute for support during sampling (Oder and Vistula lagoons).

References

Abril, G., M.‐V. Commarieu, and F. Guérin. 2007. Enhanced methane oxidation in an estuarine turbidity maximum. Limnol. Oceanogr. 52: 470–475. doi: [DOI: https://dx.doi.org/10.4319/lo.2007.52.1.0470]