A halocarbon survey from a seagrass dominated subtropical lagoon, Ria Formosa (Portugal): flux pattern and isotopic composition

摘要

In this study we report fluxes of chloromethane (CH₃Cl), bromomethane(CH₃Br), iodomethane (CH₃I), and bromoform (CHBr₃) from twosampling campaigns (summer and spring) in the seagrass dominated subtropicallagoon Ria Formosa, Portugal. Dynamic flux chamber measurements wereperformed when seagrass patches were either air-exposed or submerged.Overall, we observed highly variable fluxes from the seagrass meadows andattributed them to diurnal cycles, tidal effects, and the variety of possiblesources and sinks in the seagrass meadows. The highest emissions with up to130 nmol m?2 h?1 for CH₃Br were observed during tidalchanges, from air exposure to submergence and conversely. Furthermore, duringthe spring campaign, the emissions of halocarbons were significantly elevatedduring tidal inundation as compared to air exposure.Accompanying water sampling performed during both campaigns revealed elevatedconcentrations of CH₃Cl and CH₃Br, indicating productive sourceswithin the lagoon. Stable carbon isotopes of halocarbons from the air andwater phase along with source signatures were used to allocate thedistinctive sources and sinks in the lagoon. Results suggest that CH₃Clwas rather originating from seagrass meadows and water column than from saltmarshes. Aqueous and atmospheric CH₃Br was substantially enriched in13C in comparison to source signatures for seagrass meadows and saltmarshes. This suggests a significant contribution from the water phase onthe atmospheric CH₃Br in the lagoon.A rough global upscaling yields annual productions from seagrass meadows of2.3–4.5 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, andCHBr₃ respectively. This suggests a minor contribution from seagrassmeadows to the global production of CH₃Cl and CH₃Br with about 0.1 and 0.7%, respectively. In comparison to the known marine sourcesfor CH₃I and CHBr₃, seagrass meadows are rather small sources.