Productivity and Temperature as Drivers of Seasonal and Spatial Variations of Dissolved Methane in the Southern Bight of the North Sea
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Dissolved CH4 concentrations in the Belgian coastal zone (North Sea) ranged between 670 nmol l−1 nearshore and 4 nmol l−1 offshore. Spatial variations of CH4 were related to sediment organic matter (OM) content and gassy sediments. In nearshore stations with fine sand or muddy sediments, the CH4 seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM-rich sediments. In offshore stations with permeable sediments, the CH4 seasonal cycle showed a yearly peak following the chlorophyll-a spring peak, suggesting that in these OM-poor sediments, methanogenesis depended on freshly produced OM delivery. This does not exclude the possibility that some CH4 might originate from dimethylsulfide (DMS) or dimethylsulfoniopropionate (DMSP) or methylphosphonate transformations in the most offshore stations. Yet, the average seasonal CH4 cycle was unrelated to those of DMS(P), very abundant during the Phaeocystis bloom. The annual average CH4 emission was 126 mmol m−2 y−1 in the most nearshore stations (~4 km from the coast) and 28 mmol m−2 y−1 in the most offshore stations (~23 km from the coast), 1260–280 times higher than the open ocean average value (0.1 mmol m−2 y−1). The strong control of CH4 by sediment OM content and by temperature suggests that marine coastal CH4 emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH4 concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area.
Keywordsmethane North Sea sediments eutrophication dimethylsulfide dimethylsulfoniopropionate
We are grateful to the crew of the RV Simon Stevin for assistance during the cruises, to André Cattrijsse and Jonas Mortelmans (VLIZ) for organizing the schedule of cruises, to the Royal Netherlands Institute of Sea Research (Yerseke) and the crew of the RV Luctor for sampling at station WS1, to Bavo De Witte (Instituut voor Landbouw-en Visserijonderzoek-ILVO) for providing a data set of sediment characteristics that allowed a preliminary data analysis, to Thibault Lambert for producing Figure 1, to Marc-Vincent Commarieu, Colin Royer and Adriana Anzil for help with sampling and laboratory analysis, Ruth Lagring (Belgian Marine Data Centre) for help in data mining and two anonymous reviewers for stimulating comments on the previous version of the manuscript. CTD data were provided by VLIZ and acquired in the frame of LifeWatch. This is a contribution to Belgian Science Policy (BELSPO) project 4DEMON (4 decades of Belgian marine monitoring, uplifting historical data to today’s needs, BR/121/A3/4DEMON). The GCs were acquired with funds from the Fonds National de la Recherche Scientifique (FNRS) (2.4.598.07, 2.4.637.10). NG received financial support from the Fonds David et Alice Van Buuren. GS has a Ph.D. Grant from the FRIA (Fund for Research Training in Industry and Agriculture, FNRS). AVB is a senior research associate at the FNRS.
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