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The impact of the decline of the vernal bloom on the bacterial metabolism of dimethyl- sulfoniopropionate (DMSP), the precursor of dimethyl- sulfide (DMS), was investigated during a 7 d Lagran- gian study conducted in the Northwest Atlantic in spring 2003. Daily variations in bacterial leucine incor- poration, dissolved DMSP (DMSPd) uptake and DMS production rates were measured in the surface mixed layer (SML) and in the deep chlorophyll a maximum (DCM) that formed as the bloom collapsed. Seawater samples were amended with 35 S-DMSPd, and the prod- ucts of bacterial DMSPd degradation were measured during 3 h on-board incubations. The gradual decrease in phytoplankton biomass and diatom abundance measured in the SML was accompanied by a sharp doubling of the bacterial abundance and a peak in leucine incorporation rate on Day 2, suggesting that bacteria responded to a transient pulse in dissolved organic matter. Bacterial DMSPd uptake and DMS production were highest on Days 1 and 2 (1.2 and 0.10 nmol l -1 h -1 , respectively), but rapidly decreased by Day 3, suggesting that DMSPd was becoming a less important substrate for the growing bacterial assem- blage as other substrates became available. Bacterial DMSPd uptake and DMS production rates were also low in the DCM despite very high DMS yields (40 to 50%), showing that neither the decline of the diatom spring bloom in the SML nor the accumulation of cells in the DCM resulted in a stimulation of bacterial DMSP metabolism or accumulation of DMS. The pre- sent study provides new field evidence for the po- tential uncoupling between bacterial production and DMS dynamics likely due to variations in the availabil- ity of other S-containing organic compounds released during the decay of phytoplankton blooms.

Distributions of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) were surveyed in surface waters of the NW Atlantic in May 1998, a few weeks after the spring bloom. A tri- angular transect extending from Nova Scotia to Bermuda and northeast toward Newfoundland encompassed 4 major oceanic biogeochemical provinces: Northwest Atlantic Continental Shelves (NWCS), Gulf Stream (GFST), North Atlantic Subtropical Gyre (NAST) and the North Atlantic Drift (NADR). Surface concentrations of DMS and DMSP were highest in the NADR, with peaks up to 8.9 nM DMS, 44.1 nM dissolved DMSP (DMSPd) and 240 nM particulate DMSP (DMSPp). The phyto- plankton assemblage throughout the study area was dominated by dinoflagellates and prymnesio- phytes. Statistically significant correlations were observed between the abundance of dinoflagellates and prymnesiophytes and the concentrations of DMS and DMSP along the transect. Size-fractionation of DMSPp revealed the 2 to 11 µm fraction to be the most important contributor to total DMSP (mean 72%, range 27% to 91% of total). In the region of the highest DMS(P) concentrations, the phyto- plankton assemblage was dominated by prymnesiophytes and dinoflagellates, with Chrysochro- mulina spp. and Gyrodinium flagellare being the most abundant. The abundance of these taxa showed a marked correlation with total DMSPp and with the 2 to 11 µm size fraction of DMSP in which these cells are found. This plankton assemblage was observed both early and late in the transect, which may indicate that it is a persistent feature along the northern side of the Gulf Stream at this time of year. Sea-air flux of DMS was calculated based on 2 different models. The results showed peaks in flux corresponding to the peaks in DMS concentration in surface water. Pooling and averaging the values for each biogeochemical province reveals DMS concentrations lower than the average values of an existing global DMS database for the same regions and times.

The dynamics of the cleavage of dissolved dimethylsulfoniopropionate (DMSP d ) to dimethylsulfide (DMS) were measured experimentally in the surface waters of the Labrador Sea in spring 1997. At in situ DMSP d concentrations, DMS production and consumption processes were generally in balance. Two stations in the central Labrador Sea displayed net DMS production of approximately 2 nmol l -1 h -1 , DMSP d net consumption of 3.48 nmol l -1 h -1 and a net DMS production yield from DMSP d of 60 % at near in situ DMSP d concentrations. Similar to general bacterial substrate utilization in cold waters, DMS production in the Labrador Sea seemed to be temperature and substrate limited. Following DMSP d additions, linear and non-linear net DMS production were observed. The non-linear response was characterized by a lag in DMS production and was associated with the cold, polar waters of the Labrador and West Greenland Currents. Net DMS production rates measured after DMSP d addition were proportional to the added amount of DMSP d , No saturation of the net DMS production rate was observed for concentrations up to 5000 nmol DMSP d l -1 , First order rate constants determined for these DMS production kinetics suggest an average turnover time of DMSP d by cleavage to DMS of 3.8 d (2.7 to 5.2 d). At water temperatures of -1.3 to 8°C, potential net DMS production rates measured following DMSP d additions were comparable and even higher than those previously published for temperate and warm oceanic and coastal regions. The net DMS production potential varied by 1 order of magnitude (1.7 to 18.4 nmol DMS l -1 h -1 ) throughout the study area. Causal links established with path analysis indicate that this potential seemed to be controlled by water temperature and chlorophyll a concentrations.

Production and consumption of dimethylsulfide (DMS) were studied in surface waters of the northwest Atlantic between latitude 32° and 45° N during May 1998. The kinetics of DMS production by the whole planktonic community were studied in short-term (3 h) experiments using additions of dissolved dimethylsulfoniopropionate (DMSP d ) (0 to 3000 nM). Measurements of DMS production and DMSP d consumption showed that the DMS production rate increased in direct proportion to the concentration of added DMSP d . This rate relationship did not saturate, suggesting acclimation of the microbial community to DMSP d concentrations much higher than the average for bulk seawater. Longer-term experiments were performed in which DMSP d consumption and DMS production were measured over 48 to 60 h. The DMSP d consumption rate decreased as the concentration of DMSP d decreased during the incubations. However, the DMS production rate was initially constant for the first 36 h. When DMSP d concentrations fell below 50 nM, DMS production stopped, even though DMSP d consumption continued. A possible explanation is that DMSP cleavage might dominate the total DMSP consumption at high DMSP d concentrations while DMSP demethylation and other processes dominate at low DMSP d concentrations. DMS consumption was measured both directly and by using the DMS consumption inhibitors dimethyl disulfide (DMDS) and methyl butyl ether (MBE). DMS consumption was generally undetectable except at 1 station dominated by a dense population of the DMSP-producing phytoplankton Chrysochromulina sp. and where ambient DMS concentrations were high. This suggests that the potential for DMS consumption is highest where ambient DMS levels are elevated. Pooling results from these experiments with earlier results from more northerly waters revealed an inverse exponential relationship (r 2 = 0.75, p < 0.0001) between the potential rate constant and chlorophyll a standing stock across a wide area of the Northwest Atlantic. This finding is potentially useful for the development of DMS production models.

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 km2 Phytoplankton biomass was low [mean = 0.37 pg chl a I-') and the community was dominated by unidentified flagellates, Prymnes~ophyceae, Chrysophyceae, and Dlnophyceae. Maximum plankton biomass and DMSP were found in the western portion of the grid influenced by the outflow of the St. Lawrence Estuary vla the Gaspe Current. Surface concentrations of particulate DMSP (DMSP,,), dissolved DMSP (DMSP,), and DMS ranged from 6 to l17 nM, 1.7 to 23.1 nM, and