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.