Role of zooplankton in the mesoscale distribution of surface dimethylsulfide concentrations in the Gulf of St Lawrence, Canada

We investigated the influence of mesozooplankton grazing on the distribution of dimethylsulfide (DMS) and its metabolic precursor dimethylsulfoniopropionate (DMSP) in the Gulf of St. Lawrence in August 1993. The horizontal distributions of DMS, DMSP, phytoplankton, microzooplankton, and mesozooplankton were determined over a grid of 79 stations covering 41 000 km(2). Phytoplankton biomass was low (mean = 0.37 mu g chl a l(-1)) and the community was dominated by unidentified flagellates, Prymnesiophyceae, Chrysophyceae, and Dinophyceae. Maximum plankton biomass and DMSP were found in the western portion of the grid influenced by the outflow of the St. Lawrence Estuary via the Gaspe Current. Surface concentrations of particulate DMSP (DMSP(p)), dissolved DMSP (DMSP(d)), and DMS ranged from 6 to 117 nM, 1.7 to 23.1 nM, and < 0.9 to 9 nM, respectively. The surface concentrations of DMSP, and DMSP(d) were positively correlated with the abundances of several phytoplankton species belonging to the classes Dinophyceae, Prymnesiophyceae, and Prasinophyceae and with ciliated protozoans, the dominant microzooplankton group. Among the groups showing significant correlation with DMSP(p), Gyrodinium aureolum and Chrysochromulina spp. were the only species to represent a substantial part of the biomass. The distribution of DMS was not statistically related to the distribution of phytoplankton and microzooplankton. We also found no significant correlations between the distribution of DMSP, DMS, and the abundance of mesozooplankton. Results from 2 shipboard bioassay experiments with Calanus finmarchicus, the dominant mesozooplankton species, indicate no or low grazing activity (0 and 0.18 ng chl a ind.(-1) h(-1)) and low accumulation rates of DMSP(d) (5 and 8.9 pmol ind.(-1) h(-1)), and of DMS (0 and 7.7 pmol ind.(-1) h(-1)). When extrapolated to the field conditions, these results indicate that the population of C. finmarchicus may have increased ambiant concentrations of DMSP(d) and DMS at maximum rate of 5 pmol l(-1) h(-1). Mesozooplankton grazing apparently played a minor role in DMSP(d) and DMS accumulation in the Gulf during the cruise. This suggests that the importance of mesozooplankton in the dynamics of DMSP and DMS may vary in time and space depending on the prevailing food web, traditional versus microbial.