Productivity and temperature as drivers of seasonal and spatial variations of dissolved methane in the Southern Bight of the North Sea, leading to a response from eutrophication and heatwaves

Dissolved CH₄ concentrations in the Belgian coastal zone (BCZ) (North Sea) ranged between 1607 nmol L^-1 near-shore and 4 nmol L^-1 off-shore during field cruises in 2016, 2017, 2018 and 2019. Spatial variations of CH₄ were related to sediment organic matter (OM) content and gassy sediments. In near-shore 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 off-shore 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. The annual average CH₄ emission was 126 mmol m^-2 yr^-1 in the most near-shore stations (~4 km from the coast) and 28 mmol m^-2 yr^-1 in the most off-shore stations (~23 km from the coast), 1,260 to 280 times higher than the open ocean average value (0.1 mmol m^-2 yr^-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. This is also supported by the response to the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe. Average seawater temperature in July was 2.5°C higher than the mean from 2004 to 2017 for same month in the BCZ. The mean dissolved CH₄ concentration in surface waters in July 2018 (338 nmol L^-1) was three times higher than in July 2016 (110 nmol L^-1), and an extremely high dissolved CH₄ concentration in surface waters (1,607 nmol L^-1) was observed at one near-shore station. The high dissolved CH₄ concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH₄ from gassy sediments, both most likely related to warmer conditions. The emission of CH₄ from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 µmol m^-2 d^-1) and by 37% at annual scale (221 versus 161 µmol m^-2 d^-1). The European heatwave of 2018 seems to have led to a major increase of CH₄ concentrations in surface waters and CH₄ emissions to the atmosphere in the BCZ.