Distribution and air-sea gas exchange of dimethyl sulphide at the European western continental margin

Abstract The concentrations of dissolved dimethyl sulphide (DMS) were measured on cruises aboard the R/V Meteor from Las Palmas, Gran Canaria, to Bremen, Germany (7–22 September 1994) and aboard the R/V Valdivia from Cork, Ireland, to Hamburg, Germany, via Cascais, Portugal (14 July–1 August 1995). The arithmetic means (ranges) of the DMS concentrations were 2.8 nM (0.6–33.4 nM, n =208) in September 1994 and 7.2 nM (2.9–38.5 nM, n =393) in July 1995. The distribution of dissolved DMS was characterised by higher mesoscale variability in July than in September. Patches of elevated DMS concentration were consistently associated with frontal regions along the Celtic Sea margin and on the shelf as well as with wind-driven upwelling along the Iberian margin. The pronounced difference between July and September concentrations is attributed to the seasonal pattern of dissolved DMS in the study area. DMS concentrations showed little correlation with chlorophyll a (chl a ), 19′-hexanoyloxy-fucoxanthin, or other selected chemotaxonomic pigment biomarkers. On the contrary, the chlorophyll-normalised DMS concentrations in July 1995 (45 nmol DMS/μg chl a ) were about a factor of 5.5 higher than those in September 1994 (8 nmol DMS/μg chl a ). Our results and a compilation of presently available data suggest that the DMS-to-chlorophyll ratios follow a sinusoidal pattern with peak values around July. The implications of these findings are discussed in relation to seasonal changes in DMS cycling and sea-to-air fluxes along the western European continental margin.