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 distri bution of dimethylsulfide (DMS) and its metabolic precursor dimethylsu lfoniopropionate (DMSP) in the Gulf of St. Lawrence in August 1993. Th e horizontal distributions of DMS, DMSP, phytoplankton, microzooplankt on, and mesozooplankton were determined over a grid of 79 stations cov ering 41 000 km(2). Phytoplankton biomass was low (mean = 0.37 mu g ch l a l(-1)) and the community was dominated by unidentified flagellates , Prymnesiophyceae, Chrysophyceae, and Dinophyceae. Maximum plankton b iomass and DMSP were found in the western portion of the grid influenc ed by the outflow of the St. Lawrence Estuary via the Gaspe Current. S urface concentrations of particulate DMSP (DMSP(p)), dissolved DMSP (D MSP(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) w ere positively correlated with the abundances of several phytoplankton species belonging to the classes Dinophyceae, Prymnesiophyceae, and P rasinophyceae and with ciliated protozoans, the dominant microzooplank ton group. Among the groups showing significant correlation with DMSP( p), Gyrodinium aureolum and Chrysochromulina spp. were the only specie s 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 betwe en the distribution of DMSP, DMS, and the abundance of mesozooplankton . Results from 2 shipboard bioassay experiments with Calanus finmarchi cus, the dominant mesozooplankton species, indicate no or low grazing activity (0 and 0.18 ng chl a ind.(-1) h(-1)) and low accumulation rat es of DMSP(d) (5 and 8.9 pmol ind.(-1) h(-1)), and of DMS (0 and 7.7 p mol ind.(-1) h(-1)). When extrapolated to the field conditions, these results indicate that the population of C. finmarchicus may have incre ased ambiant concentrations of DMSP(d) and DMS at maximum rate of 5 pm ol l(-1) h(-1). Mesozooplankton grazing apparently played a minor role in DMSP(d) and DMS accumulation in the Gulf during the cruise. This s uggests that the importance of mesozooplankton in the dynamics of DMSP and DMS may vary in time and space depending on the prevailing food w eb, traditional versus microbial.