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 abundant populations of the pogonophore Siboglinum poseidoni, which lives on symbiotic methanotrophic bacteria, and the bivalve Thyasira sarsi, which lives on symbiotic sulphur-oxidising bacteria. Investigations have been made on the environmental factors influencing these populations, notably the methane content of the sediment, the rate of sulphate reduction and the distribution of reduced sulphur species. The subsurface gas distribution was investigated using a deep-tow boomer and sediment cores were collected from both seep and non-seep areas. Methane-saturated sediment, detected by acoustic blanking, was found within 3 m of the sediment surface in water depths from < 55 m to 360 m. At greater 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 over most of the survey area. Seeps and their specialised fauna were found on a steeper section of the slope where slumping of the sediment exposed underlying gas-saturated layers. The seeps were small and localised and were estimated to occupy 2.3 % of the area of this zone. In seep sediments the potential aerobic methane oxidation rates were 40-fold 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 the 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 were 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-seep sediment. Very high methane concentrations were found where S. poseidoni was abundant, up to 3.4 x 10(3) mumol dm-3 in the upper 10 cm of 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.