Methane in the southern North Sea: Low-salinity inputs, estuarine removal,and atmospheric flux

Dissolved CH4 was measured in coastal waters of the southern North Sea, in two adjacent U.K. estuaries with well-defined turbidity maxima (Humber and Tyne) and in their associated river catchments, during a series of campaign s covering the period 1993-1999. In general, samples from all three environ ments were significantly to highly CH4 enriched relative to atmospheric air . Observed river water concentrations, similar to 33-152 nmol L-1 (940-4305 % saturation) for the Humber river catchment and similar to 3-62 nmol L-1 ( 86-1754% saturation) in the river Tyne, were within but toward the low end of the range of CH4 concentrations in river waters world wide. In sea water s from the outer Wash estuary (U.K, coast) and adjacent to the Dutch coast, CH4 was highly but nonlinearly correlated with salinity, consistent with s trong CH4 removal from river and/or estuarine CH4 sources influencing these locations. In transects along the Humber and Tyne estuaries, CH4 was highl y negatively nonconservative, confirming the estuarine removal hypothesis. For both estuaries, highest CH4 concentrations, similar to 190-670 nmol L-1 (6000-21,000% saturation) in the Humber and similar to 650 nmol L-1 (21,80 0% saturation) in the Tyne, were observed at very low salinity in the vicin ity of the turbidity maximum. Importantly, these concentrations greatly exc eeded measured river water values, implying for both situations the existen ce of a large in situ CH4 supply associated with high turbidity. Time serie s measurements at two locations in the upper Tyne subsequently confirmed th e strong correspondence of dissolved CH4 and turbidity in the vicinity of t he turbidity maximum. CH4 removal estimated for the Humber, Tyne, Wash, and Rhine-Scheldt estuaries was similar to 90% of the low-salinity CH4 input. On the basis of this and river discharge data, similar to 7.1 x 10(8) mol C H4 may be removed annually in estuaries bordering the southern North Sea. O f this, similar to 6.6 x 10(8) mor may be lost by air-sea exchange. This re presents an additional atmospheric CH4 flux from the North Sea unaccounted for in previous work, which may have, consequently, underestimated this sou rce by similar to 50%. Upward scaling of this estimate based on the mean of reported river water CH4 concentrations implies a previously unaccounted f or similar to 6.3-24 x 10(9) mol (i.e., similar to 0.1-0.4 x 10(12) g) CH4 yr(-1) which may be lost globally to gas exchange in estuaries, increasing previous such estimates by similar to 8-50 %. However, as it is based on da ta that exclude the possibility of elevated CH4 levels at estuarine turbidi ty maxima, even this revision is likely to be conservative. Detailed studie s of CH4 distributions in major world estuaries would now be required in or der to successfully reevaluate the CH4 budget of the coastal marine atmosph ere.