A METHANE SEEP AREA IN THE SKAGERRAK, THE HABITAT OF THE POGONOPHORE SIBOGLINUM-POSEIDONI AND THE BIVALVE MOLLUSK THYASIRA-SARSI

On the southern slope of the central Skagerrak there are locally abund ant populations of the pogonophore Siboglinum poseidoni, which lives o n symbiotic methanotrophic bacteria, and the bivalve Thyasira sarsi, w hich lives on symbiotic sulphur-oxidising bacteria. Investigations hav e been made on the environmental factors influencing these populations , notably the methane content of the sediment, the rate of sulphate re duction and the distribution of reduced sulphur species. The subsurfac e gas distribution was investigated using a deep-tow boomer and sedime nt cores were collected from both seep and non-seep areas. Methane-sat urated sediment, detected by acoustic blanking, was found within 3 m o f the sediment surface in water depths from < 55 m to 360 m. At greate r water depths the blanking was less shallow, sharply declining to 100 ms 2-way travel time at 390 m. There was no evidence for outgassing o ver most of the survey area. Seeps and their specialised fauna were fo und on a steeper section of the slope where slumping of the sediment e xposed underlying gas-saturated layers. The seeps were small and local ised and were estimated to occupy 2.3 % of the area of this zone. In s eep sediments the potential aerobic methane oxidation rates were 40-fo ld higher than in non-seep sediments, while the actual rates, measured by a radiotracer technique, were up to 460-fold higher in the surface sediment at the seeps. Within the seep sediments there was a greatly reduced sulphate concentration, down to 6 x 10(3) mug-at. S l-1, in th e upper 10 cm, compared with 30 x 10(3) mug-at. S l-1 at a distance of 30 cm from the seep. Correspondingly, rates of sulphate reduction wer e very much greater at the seeps, 500 to 2000 mug-at. S reduced dm-3 d -1, compared with approximately 50 mug-at. S reduced dm-3 d-1 in non-s eep sediment. Very high methane concentrations were found where S. pos eidoni was abundant, up to 3.4 x 10(3) mumol dm-3 in the upper 10 cm o f the sediment. Dissolved sulphide was also much enriched at the seeps , to 500 mug-at. S l-1 pore water, explaining the abundant populations of T sarsi. These 2 species made up the bulk of the infaunal biomass at the seeps, whereas polychaetes dominated in non-seep sediments. The biomass of the macrofauna was 3-fold higher in seep than in non-seep sediment.