Dimethylsulfide (DMS) production by size-fractionated particles in the Labrador Sea

We measured the production of dimethylsulfide (DMS) by size-fractionated particles during a cruise in the Labrador Sea in May-June 1997. The experiments were conducted at 2 stations characterised by low levels of nitrate and high levels of phytoplankton biomass and particulate dimethylsulfoniopropionate (DMSPp). Samples were size fractionated to assess the size distribution of DMSPp and the potential DMS production associated with the different size fractions. The potential for DMS production was estimated by incubating the filters in pre-filtered and boiled seawater amended with 500 nM of dissolved DMSP (DMSPd). At both stations, the highest DMSPp concentrations were measured in the 2 to 11 and >20 mu m size fractions. Elevated potential net DMS production rates were also associated with these 2 size fractions, which were responsible for 40 to 53 % and 23 to 31% of the cumulative production, respectively. Only 4% of the potential net DMS production was measured in the 0.7 to 2 mu m fraction, which presumably contained many of the free-living bacteria. The potential net DMS production rates of the different size fractions were linearly related (Spearman correlation coefficient = 0.86) to the concentrations of DMSPp in the fractions. These results suggest that DMSP-cleaving activity was spatially associated with DMSP-producing algae or DMSP-rich detritus (e.g. faecal pellets, marine snow).