PRODUCTION OF DMSP AND DMS DURING A MESOCOSM STUDY OF AN EMILIANIA-HUXLEYI BLOOM - INFLUENCE OF BACTERIA AND CALANUS-FINMARCHICUS GRAZING

We investigated the influence of bacteria and metazooplankton on the p roduction of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DM S) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seaw ater mesocosms. The phytoplankton succession was marked by the rapid c ollapse of an initial Skeletonema costatum (Greville) Cleve bloom foll owed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSP, ) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was foll owed by a rapid decrease in DMSP, concentrations which remained low fo r the rest of the experiment, even during the subsequent collapse of t he E. huxleyi blooms, The absence of DMSP, and DR IS peaks during the declining phase of the E. huxleyi blooms was attributed to the high ba cterial activity prevailing at that time. The influence of metazooplan kton grazing on DMSP and DMS production was investigated by adding mod erate (24 mg dry weight m(-3)) and high (520 mg dry weight m(-3)) conc entrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 mu m mesh net) enclosures during the E. hux leyi blooms. The addition of C. finmarchicus, even in high concentrati ons, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulati on of chlorophyll a and particulate DMSP. These results suggest that c opepods were preying on the herbivorous microzooplankton which, in tur n, was controlling the biomass of nanophytoplankton. DMS production wa s also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton co ntaining DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude o f DMS pulses following phytoplankton blooms.