The effects of methane seepage at an intertidal/shallow subtidal site on the shore of the Kattegat, Vendsyssel, Denmark

Abstract

The effects of methane gas seepage were studied at an intertidal/shallow subtidal site on the Kattegat coast of Denmark. A 30 m wide zone, containing approximately 65 gas seeps, extended over 70 m seawards from the shore. This was characterised by the presence of slabs, pavement and small pillars of carbonate-cemented sandstone which formed a partially buried reef. The escaping gas contained 91-100% methane with some carbon dioxide, 0.6--0.9%, and hydrogen sulphide. The hydrogen sulphide concentration varied over time and between individual seeps. Gas flow rates of individual seeps ranged up to 211 h-1 and the estimated total flow was 110 I h-1• Seeps were often stopped by sand movement, but the overall gas flow from the site appeared to be constant. The escaping gas generated an interstitial water circulation and drew overlying seawater into the sediment. Water pumped out by the seeps was enriched in phosphate and ammonia. Sulphate reduction rates in the seep area were 1.1-17.1 m moles sulphate reduced and aerobic methane oxidation rates were 0.2 - 5.5 m moles methane consumed m-2 day-1• The composition of the flora and fauna surrounding the seeps was affected by the presence of hard substrate (the cemented sandstone). Epifauna was more abundant in the seep zone than else­where, whereas the macrobenthic infauna was reduced in the seep zone, possibly due to the cementation. The sediment was almost devoid of meiobenthic organisms, except nematodes. Nematode species numbers, abundance and biomass were lower at the seeps than 5-20 cm away. The nematode fauna penetrated deeper into the sediment close to the seeps than at the seeps themselves. This is explained by the interstitial water circulation at and close to the seeps. 14C measurements showed that little methane carbon was entering the food web surrounding the seeps.