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

The concentrations of dissolved dimethyl sulphide (DMS) were measured on cr uises aboard the R/V Meteor from Las Palmas, Gran Canaria, to Bremen, Germa ny (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 arith metic 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. Th e distribution of dissolved DMS was characterised by higher mesoscale varia bility in July than in September. Patches of elevated DMS concentration wer e consistently associated with frontal regions along the Celtic Sea margin and on the shelf as well as with wind-driven upwelling along the Iberian ma rgin. The pronounced difference between July and September concentrations i s attributed to the seasonal pattern of dissolved DMS in the study area. DM S concentrations showed little correlation with chlorophyll a (chl a), 19'- hexanoyloxy-fucoxanthin, or other selected chemotaxonomic pigment biomarker s. On the contrary, the chlorophyll-normalised DMS concentrations in July 1 995 (45 nmol DMS/mu g chi a) were about a factor of 5.5 higher than those i n September 1994 (8 nmol DMS/mu g chi 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 thes e findings are discussed in relation to seasonal changes in DMS cycling and sea-to-air fluxes along the western European continental margin. (C) 2000 Elsevier Science B.V. All rights reserved.