ECOLOGY OF A NORTH-SEA POCKMARK WITH AN ACTIVE METHANE SEEP

At a large North Sea pockmark, with active methane seeps, surface sediments were found to have higher insoluble sulphide concentrations than sediments from the surrounding area. The fauna of the pockmark was characterized by 2 species which have not previously been reported from the Fladen Ground in the northern North Sea. These species were a bivalve, Thyasira sarsi (which is known to contain endosymbiotic sulphur-oxidising bacteria) and a mouthless and gutless nematode, Astomonema sp., which also contains endosymbiotic bacteria. The nematode was the dominant meiofauna species in the pockmark sediments. Both macro-infauna and total nematodes were in low abundance in samples taken from the base of the pockmark. Sediment samples from the pockmark contained numerous otoliths, implying that substantial winnowing of the sediment had taken place. This was supported by studies on the sulphide concentrations in the sediment which showed multiple layering of the sediments on the sides of the pockmark, suggesting displacement. The carbon isotope compositions (delta-C-13) of the tissues of benthic animals from in and around the pockmark were generally in the range -16 to -20 parts per thousand, indicating that little methane-derived carbon was contributing to their nutrition. T. sarsi had the most C-13-depleted tissues, -31.4 to -35.1 parts per thousand, confirming the nutritional dependence of this species on chemoautotrophic bacteria that utilize reduced sulphur.