In:
Frontiers in Marine Science, Frontiers Media SA, Vol. 10 ( 2023-1-26)
Abstract:
Methane (CH 4 ) is a key greenhouse gas. Coastal areas account for a major proportion of marine CH 4 emissions. Eutrophication and associated bottom water hypoxia enhance CH 4 production in coastal sediments. Here, we assess the fate of CH 4 produced in sediments at a site in a seasonally anoxic eutrophic coastal marine basin (Scharendijke, Lake Grevelingen, the Netherlands) in spring (March) and late summer (September) in 2020. Removal of CH 4 in the sediment through anaerobic oxidation with sulfate ( S O 4 2 - ) is known to be incomplete in this system, as confirmed here by only slightly higher values of δ 13 C-CH 4 and δD-CH 4 in the porewater in the shallow sulfate-methane-transition zone (~5-15 cm sediment depth) when compared to deeper sediment layers. In March 2020, when the water column was fully oxygenated, CH 4 that escaped from the sediment was at least partially removed in the bottom water through aerobic oxidation. In September 2020, when the water column was anoxic below ~35 m water depth, CH 4 accumulated to high concentrations (up to 73 µmol L -1 ) in the waters below the oxycline. The sharp counter gradient in oxygen and CH 4 concentrations at ~35 m depth and increase in δ 13 C-CH 4 and δD-CH 4 above the oxycline indicate mostly aerobic water column removal of CH 4 . Water column profiles of particulate and dissolved Fe and Mn suggest redox cycling of both metals at the oxycline, pointing towards a potential role of metal oxides in CH 4 removal. Water column profiles of N H 4 + and N O 3 - indicate removal of both solutes near the oxycline. Analyses of 16S rRNA gene sequences retrieved from the water column reveal the presence of aerobic CH 4 oxidizing bacteria ( Methylomonadaceae ) and anaerobic methanotrophic archaea ( Methanoperedenaceae ), with the latter potentially capable of N O 3 - and/or metal-oxide dependent CH 4 oxidation, near the oxycline. Overall, our results indicate sediment and water column removal of CH 4 through a combination of aerobic and anaerobic pathways, which vary seasonally. Some of the CH 4 appears to escape from the surface waters to the atmosphere, however. We conclude that eutrophication may make coastal waters a more important source of CH 4 to the atmosphere than commonly assumed.
Type of Medium:
Online Resource
ISSN:
2296-7745
DOI:
10.3389/fmars.2023.1085728
DOI:
10.3389/fmars.2023.1085728.s001
Language:
Unknown
Publisher:
Frontiers Media SA
Publication Date:
2023
detail.hit.zdb_id:
2757748-X
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