PRODUCTION AND FATE OF DIMETHYLSULFIDE AND DIMETHYLSULFONIOPROPIONATEIN PELAGIC MESOCOSMS - THE ROLE OF SEDIMENTATION

Elevated concentrations of dimethylsulfide (DMS) sometimes occur in se a water during phytoplankton blooms. To determine the factors controll ing the concentration of DMS in sea water, the development and fate of a bloom dominated by Phaeocystis sp. was studied in relation to the p roduction and fate of DMS and its precursor beta-dimethylsulfoniopropi onate (DMSP) in a pelagic mesocosm experiment. The part of this study described here focused on the role of sedimentation of algae as a loss factor for DMSP and as a trigger mechanism for DMS production. A hypo thesis tested was that high DMS concentrations occur after a mass rele ase of DMSP from algae due to mass sedimentation followed by cell lysi s on the mesocosm floor. This was studied by monitoring Phaeocystis ce ll numbers, chlorophyll a, particulate DMSP, dissolved DMSP and DMS in the water column, and by daily measurements of the sedimentation of P haeocystis cells. We evaluated a technique for measuring DMSP indirect ly as acrylate by high-performance Liquid chromatography (HPLC). This method seemed to be hampered by DMSP-lyase activity of Phaeocystis sp. during sample processing, but gave good results when Phaeocystis was not dominant. Sedimentation rates of Phaeocystis sp, were found to be high and constantly related to the standing stock in the water column. The average sinking velocity of Phaeocystis cells was 1.4 m d(-1). At the decline of the bloom, sedimentation accounted for approximately 5 0% of the observed loss of Phaeocystis biomass, which indicates that c ell lysis may have been important as well at this time. The decline of the bloom did not result in an elevated DMS concentration. However, a significant peak in DMS was observed at the end of the exponential gr owth phase of the Phaeocystis bloom. It was concluded that the decline of a Phaeocystis bloom does not lead to an elevated DMS concentration in the water column. The continuously high sedimentation of living Ph aeocystis cells suggests that, in these mesocosms, lysis after sedimen tation was an important mechanism for release of DMSP from cells and, hence, production of DMS. Nevertheless, this mechanism in itself can n ot explain the strong fluctuations in the DMS concentration observed i n this experiment.