Dissolved dimethylsulphide, dimethylsulphoniopropionate and dimethylsulphoxide in western Mediterranean waters

Spring-summer concentrations of the three main dissolved dimethyl sulphur species (DMSSd), dimethylsuphide (DMS), dimethylsulphoniopropionate (DMSSd) and dimethylsulphoxide (DMSO), have been determined in western Mediterranean Sea water with some spatial and temporal resolution. Depth profiles showed that the three DMSSd were mostly confined to the euphotic layer. In surface waters, DMSO was generally the dominant DMSSd regardless of the site and the season. Concentration averages resulted in a DMS:DMSPd:DMSO proportion of l: 1: 6 (2.9: 3.0: 16.6 nM). DMSSd concentrations exhibited a great variability, but were higher on the continental shelf than in open seawaters, as were chlorophyll concentrations. Some hot spots with the highest levels were identified off the main continental outflows. None of the DMSSd correlated significantly with chlorophyll. In open seawaters (water column depth > 200 m) DMS increased as chlorophyll declined between April and July. This was attributed to changes in the biological community from spring development to summer decay and ecological succession. The temporal variation of the three DMSSds suggests that they are subject to a tight cycling of production and consumption where the composition of the entire planktonic community, as well as its physiological state, play a significant role. A similar conclusion was achieved from the study of the DMSSd concentrations in a transect between oligotrophic and eutrophic waters in the top NW area. Finally, the western Mediterranean has been assessed as a source of atmospheric sulphur. The moderate DMS emission flux estimates (mean: 5.4 μmol m−2 day−1), together with the moderate DMS surface concentrations, were consistent with the low productivity of the Mediterranean Sea.