Particulate Dimethylsulfoniopropionate Removal and Dimethylsulfide Production by Zooplankton in the Southern Ocean

Abstract

The influence of Antarctic krill, Euphausia superba, on particulate dimethlysulfonio-propionate (DMSP(p)) and dimethylsulfide (DMS) concentrations in surface waters of the Southern Ocean was investigated by shipboard experiments during austral spring near the Antarctic Peninsula. Chlorophyll concentrations were low in the water column, but substan-tially higher in sea ice due to the high biomass of ice algae, predominantly pennate diatoms. A comparison of DMSP(p) concentrations and algal accessory pigments indicated that DMSP(p) was associated primarily with diatoms (fucoxanthin) and to a minor extent with Phaeocystis spp. (19'-hexanoyloxyfucoxanthin) in sea ice algae. Maximum DMSP(p) and fucoxanthin concentrations also occurred in the 100-200 μm size fraction. We interpret this to mean that high biomass of diatoms in sea ice contributes significantly to DMSP(p) pools in the Antarctic. Juvenile krill were the dominant biomass component of the zooplankton community and were often observed grazing along the edge of ice floes. In experiments, krill grazing on phytoplankton or ice algae produced 3 to 16 times the amount of DMS, respectively, relative to that in control bottles without krill. On average, juvenile krill produced 0.64 nmol DMS krill·1 h·1• Control bottles containing autotrophic and microheterotrophic organisms released only 0.03 nmol DMS L·1 h·1 and the net change in DMS due to microbial processes was negligible. Hence, krill appear to be very efficent in converting DMSP(p) to DMS. Krill also egested 1.06 nmol DMSP(p) (mg fecal pellet)·1 h·1• Results from sediment traps however, indicate that little DMSP(p) flux to deep water occurred even though particle fluxes were dominated by fecal pellets. DMSP(p) in fecal pellets may rapidly degrade to DMSP(d) and diffuse from pellets into the surrounding water. Our study indicates that mesozooplank-ton grazing activity plays a significant role in the release of DMSP and DMS from phytoplankton and ice algae. Because krill feed on ice algae during seasons when phyto-plankton production is low, DMS production from grazing occurs during all seasons in the Southern Ocean.