Abstract. In this study we report fluxes of chloromethane (CH₃Cl), bromomethane (CH₃Br), iodomethane (CH₃I), and bromoform (CHBr₃) from two sampling campaigns (summer and spring) in the seagrass dominated subtropical lagoon Ria Formosa, Portugal. Dynamic flux chamber measurements were performed when seagrass patches were either air-exposed or submerged. Overall, we observed highly variable fluxes from the seagrass meadows and attributed them to diurnal cycles, tidal effects, and the variety of possible sources and sinks in the seagrass meadows. The highest emissions with up to 130 nmol m⁻² h⁻¹ for CH₃Br were observed during tidal changes, from air exposure to submergence and conversely. Furthermore, during the spring campaign, the emissions of halocarbons were significantly elevated during tidal inundation as compared to air exposure.

Accompanying water sampling performed during both campaigns revealed elevated concentrations of CH₃Cl and CH₃Br, indicating productive sources within the lagoon. Stable carbon isotopes of halocarbons from the air and water phase along with source signatures were used to allocate the distinctive sources and sinks in the lagoon. Results suggest that CH₃Cl was rather originating from seagrass meadows and water column than from salt marshes. Aqueous and atmospheric CH₃Br was substantially enriched in ¹³C in comparison to source signatures for seagrass meadows and salt marshes. This suggests a significant contribution from the water phase on the atmospheric CH₃Br in the lagoon.

A rough global upscaling yields annual productions from seagrass meadows of 2.3–4.5 Gg yr⁻¹, 0.5–1.0 Gg yr⁻¹, 0.6–1.2 Gg yr⁻¹, and 1.9–3.7 Gg yr⁻¹ for CH₃Cl, CH₃Br, CH₃I, and CHBr₃ respectively. This suggests a minor contribution from seagrass meadows to the global production of CH₃Cl and CH₃Br with about 0.1 and 0.7%, respectively. In comparison to the known marine sources for CH₃I and CHBr₃, seagrass meadows are rather small sources.