Diel variations of the DMSP-to-chlorophyll a ratio in Northwestern Mediterranean surface waters

In the framework of the projects DYFAMED and PICASSO, diel variations of particulate dimethylsulfoniopropionate (DMSPp) and of its size fraction higher than 10 μm (DMSPp>10 μm) were studied in surface waters of the central Ligurian Sea in May 1990 and May 1995, and in the harbour of the city of Barcelona (Spain) in July 1998. Time series performed in stratified and nitrate depleted surface waters of the Ligurian Sea revealed that DMSPp>10 μm was undergoing diurnal variations. DMSPp-containing particles in the size range higher than 10 μm also markedly affected the DMSPp-to-chlorophyll (chl) a ratio of surface waters on a daily basis. The ratios were 35% to 72% lower at dawn than at dusk. The fact that the diadinoxanthin (DD)-to-chl a ratio of surface phytoplanktonic populations did not exhibit diurnal cycles suggests that physiological adaptation of cellular DMSP and chl a to the light conditions was not a likely process to account for the diurnal changes of the DMSPp-to-chl a ratio. It is suggested that such diurnal variability resulted from changes in plankton composition due to vertical migrations of DMSP-containing organisms larger than 10 μm. We have demonstrated from samples collected in the harbour of the city of Barcelona that DMSP-containing dinoflagellates are active diel migrants. However, the results obtained in the open sea in May 1990 suggest that dinoflagellates and also ciliates contribute to the pool of DMSPp in the size range larger than 10 μm. The results of May 1995 are ambiguous as to the role of dinoflagellates because, in the absence of specific cell counts, DMSPp>10 μm and the pigment peridinin, which is usually present in dinoflagellates (but peridinin-free dinoflagellates exist) showed very different vertical and temporal patterns.