Environmental control on the variability of DMS and DMSP in the Mauritanian upwelling region

Dimethylsulphide (DMS) and dissolved and particulate dimethylsulfoniopropionate (DMSPd, DMSPp) were measured in near-surface waters along the Mauritanian coast, Northwest Africa, during the upwelling season in February 2008. DMS, DMSPd and DMSPp surface concentrations of up to 10 nmol L-1, 15 nmol L-1 and 990 nmol L-1, respectively, were measured. However, the DMS concentrations measured are in the low range compared to other upwelling regions. The maximum DMSPp concentration is the highest reported from upwelling regions so far, which might indicate that the Mauritanian upwelling is a hot spot for DMSP. Within the phytoplankton groups, dinoflagellates were identified as important contributors to DMS concentrations, while other algae seemed to have only a minor or no influence on DMS and DMSP concentrations. A pronounced switch from high DMSP to high DMS concentrations was observed when the nitrogen to phosphorus ratio (N:P) was below 7. The high DMS/DMSP ratios at N:P ratios < 7 indicate that nitrogen limitation presumably triggered a switch from DMSP to DMS independent of the species composition. Our results underline the importance of coastal upwelling regions as a local source for surface seawater sulphur.