A methane seep area in the Skagerrak, the habitat of the pogonophore Siboglinum poseidoni and the bivalve mollusc Thyasira sarsi

Abstract

On the southern slope of the central Skagerrak there are locally abundant populations of the pogonophore Sjboglinum pose~doni, 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 blanlung, was found wthin 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 dechning 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 sedirnents the potential aerobic methane oxldahon rates were 40-fold h g h e r than in non-seep sedim e n t ~, while the actual rates, measured by a radiotracer technique, were up to 460-fold higher in the surface sedlment at the seeps. Within the seep sediments there was a greatly reduced sulphate concentration, down to 6 X 103 pg-at. S I-', in the upper 10 cm, compared with 30 X 103 pg-at. S 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 pg-at. S reduced drn-3 d-', compared with approxin~ately 50 pg-at. S reduced dm-3 d-' in non-seep sediment. Very high methane concentrations were found where S, poseidoni was abundant, up to 3.4 X 103 pm01 dm-3 in the upper 10 cm of the sediment. Dissolved sulphide was also much ennched at the seeps, to 500 pg-at. S I-' pore water, explaining the abundant populations of 7: sarsi. These 2 species made up the bulk of the infaunal biomass at the seeps, whereas polychaetes dominated in non-seep sedirnents. The biomass of the macrofauna was 3-fold higher in seep than in non-seep sediment.