Dimethylsulphide, DMSP-lyase activity and microplankton community structure inside and outside of the Mauritanian upwelling

We investigated the relationship between microplankton composition and dimethylsulphide (DMS) concentration in the Mauritanian upwelling in July/August 2006. As well as DMS and DMSP-lyase activity (DLA), we made some accompanying measurements of the precursor of DMS, DMSP (dimethylsulphoniopropionate) in the particulate and dissolved fraction, and photosynthetic pigments. At well-stratified offshore stations, chlorophyll-a concentrations were low (<0.5 mg m^-3) with microplankton biomass dominated by coccolithophores and dinoflagellates. Water-column stratification became less pronounced closer inshore and the increased CO₂ fugacity and lowered sea surface temperature indicated significant upwelling at around 20°N. The highest chlorophyll-a levels were found in this region and in general, much higher chlorophyll-a and microplankton biomass was encountered inshore, and the microplankton assemblage was composed of a greater abundance and diversity of diatoms with planktonic ciliates, dinoflagellates, flagellates, and coccolithophores a smaller proportion of the total biomass. DMS was generally <2 nM at the most offshore stations whilst more variable (up to 14 nM) closer inshore, reflecting the influence of upwelling and higher microplankton biomass. DLA was highest in the surface mixed layer, and the highest recorded value of DLA (45 nM DMS h^-1) was recorded at the surface alongside elevated DMS at the 20°N upwelling region. These data are the first coupled measurements of DMS, DLA, DMSP and microplankton species composition in an upwelling system and emphasise the patchy nature of DMS production and its complex relationship with microplankton species composition. This study provides a characterisation of DMS and community composition in a productive upwelling system, a system which is predicted to undergo an intensification of upwelling due to climate change.