Effects of reduced ultraviolet radiation on aqueous concentrations of dimethylsulfoniopropionate and dimethylsulfide during a microcosm study in the Lower St. Lawrence Estuary

In August 1994, microcosm experiments were conducted with the natural phytoplankton community from the Lower St. Lawrence Estuary, Canada, in order to determine the influence of ultraviolet radiation (UVR) on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS). The planktonic community was exposed for 42 h to 2 different Light regimes: natural light conditions and UVR reduced by 95% using UF3 filters. Throughout the experiments, flagellates dominated the algal community. During the first day, the production rate of DMS under reduced UVR was 4 times greater than under natural light conditions, suggesting a decrease in the loss rate of DMS under reduced UVR. There were no significant effects of the light regimes on particulate DMSP (DMSPp), dissolved DMSP (DMSPd), DMSPp/chlorophyll a and DMSPp/total algal cell number ratios during the first 24 h. During the second day, DMSP, concentrations, DMSPp/chlorophyll a and DMSPp/total algal cell number ratios increased significantly under reduced UVR whereas these variables decreased in the microcosms exposed to natural light conditions. These results suggest that the reduction of UVR favoured the accumulation of DMSP in algal cells. We conclude that the reduction of UVR affects the DMSP/DMS dynamics in seawater at 2 levels depending on the time frame considered: on a short-term basis (<24 h), it increases the DMS concentration, probably by decreasing its removal; on a long-term basis (>24 h), it increases algal DMSP content, probably by stimulating the synthesis and/or by inhibiting the excretion of DMSP.