MEPS 131:275-286 (1996)  -  doi:10.3354/meps131275

Elevated concentrations of dimethylsulfide (DMS) sometimes occur in sea water during phytoplankton blooms. To determine the factors controlling the concentration of DMS in sea water, the development and fate of a bloom dominated by Phaeocystis sp. was studied in relation to the production and fate of DMS and its precursor beta-dimethylsulfoniopropionate (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 hypothesis tested was that high DMS concentrations occur after a mass release of DMSP from algae due to mass sedimentation followed by cell lysis on the mesocosm floor. This was studied by monitoring Phaeocystis cell numbers, chlorophyll a, particulate DMSP, dissolved DMSP and DMS in the water column, and by daily measurements of the sedimentation of Phaeocystis cells. We evaluated a technique for measuring DMSP indirectly 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 50% of the observed loss of Phaeocystis biomass, which indicates that cell 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 growth 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 Phaeocystis cells suggests that, in these mesocosms, lysis after sedimentation was an important mechanism for release of DMSP from cells and, hence, production of DMS. Nevertheless, this mechanism in itself can not explain the strong fluctuations in the DMS concentration observed in this experiment.

Phaeocystis . Sedimentation . DMS . DMSP