Mesoscale features of surface water DMSP and DMS concentrations in the Atlantic Ocean off Morocco and in the Mediterranean Sea

Surface water small-scale variability of dimethylsulfide (DMS), total dimethylsulfoniopropionate (tDMSP) concentrations and supporting parameters were investigated in upwelling cells north of Cap Ghir (Morocco) and during a transect from this area to the eastern Mediterranean Sea in late summer 1999. Off the Moroccan coast, tDMSP concentration was in the range 20–300 nM and was linearly correlated (r2=0.93, n=61) with measurements from a particle counter of the total volume of suspended particles in the range 1.5–100 μm. This suggests that DMSP off the Moroccan coast was homogeneously distributed amongst planktonic communities not resolved at the organismic level. Conversely, the spatial distribution of the DMS-to-tDMSP ratio calculated either from measured or reconstructed tDMSP levels from particle counts was much more heterogeneous. DMS-to-tDMSP ratios were in the range 2–30\%. Four plumes of upwelled waters were clearly identified from maxima in density excess and plankton biomass, and from minima in the partial pressure of CO2 (pCO2) at the constant temperature of 13°C which result mainly from the cumulative biological uptake of CO2. The higher the pCO2 at 13°C, the lower the DMS-to-tDMSP ratio. Thus, DMS was relatively less abundant in recently upwelled waters. The main patterns of DMS variability in the southern Mediterranean Sea in summer were: (1) an eastward unsteady increase of the DMS-to-tDMSP ratio from roughly 10\% in the Alboran Sea up to 40\% in the Ionian Sea and (2) sharp and broad DMS enhancements associated with either cold or warm water masses illustrating the complex origin of DMS in the Mediterranean Sea. There is evidence that shifts in the regime of DMS production-removal occur at the entrances to the western and eastern basins of the Mediterranean Sea.