METHANE FLUXES IN THE SOUTHERN NORTH-SEA - THE ROLE OF EUROPEAN RIVERS

Methane distributions were measured in the Southern Bight of the North Sea during March 1989 as part of the North Sea Program of the United Kingdom Natural Environment Research Council. Methane concentrations in the open North Sea were at or near solubility equilibrium with the atmosphere, but near the coast of the Netherlands, in the plume of the Rhine, extremely high methane values (as high as 120 times saturation) were observed. Both the Rhine and Scheldt rivers, and possibly coastal sediments, seemed to be significant regional sources of methane. Methane oxidation represented a relatively small sink for methane in the North Sea compared to gas loss to the atmosphere. This was due, in part, to the fact that methane oxidizing bacteria within the river plume appeared to be oxidizing the gas at close to their maximal rate, although offshore bacteria were able to increase oxidation rates as methane concentrations increased. Thus our studies suggested that bacterial methane oxidation kinetics were saturated at much lower methane concentrations in plume or estuarine waters than in open ocean situations. If the Rhine and Scheldt are the source of the methane found in the Southern Bight, river concentrations of methane must be high (order of 50-5000 nM). Although rivers and coastal zones may not represent a major atmospheric source of methane, they can be very important as methane sources to the coastal ocean.