Dimethylsulphide and dimethylsulphoniopropionate in the South-West Indian Ocean sector of East Antarctica from 30 degrees to 80 degrees E during BROKE-West

We investigated the spatial and vertical variations of dimethylsulphide (DMS), dissolved, particulate, and total dimethylsulphoniopropionate (DMSPd, DMSPp and DMSPt) in the south-west Indian Ocean sector of East Antarctica between 30 degrees-80 degrees E and south of 62 degrees S from January to March 2006. Twenty seven sites were examined and mean concentrations in the upper 150 m of the water column were for DMS 8 nM (range nd-63 nM); DMSPd 5 nM (nd-36 nM); DMSPp 11 nM (nd-38 nM), and DMSPt 16 nM (nd-54 nM). The highest concentrations of the sulphur compounds occurred in the western sector of the study area on the Antarctic shelf and across the Antarctic Slope Front region (ASF) of the eastern Weddell Gyre (EWG) in the Cosmonaut Sea. Concentrations were lower north of the ASF region of the EWG and in the eastern sector of the Antarctic Circumpolar Current (ACC) and the greater Prydz Bay Gyre (PBG) circulation in the Cooperation Sea. The influence of melting pack ice on DMS and DMSPd concentrations was apparent in this study but was predominantly restricted to a few stations in the Cosmonaut Sea early in the voyage. DMS concentrations were extremely high at stations in the western sector of the Marginal Ice Zone (MIZ) in the early part of the study (mean 25 nM), reaching concentrations as high as 63 nM at station 42, decreasing to a mean concentration of 5.5 nM in the reduced MIZ to the east. In the Seasonal Mixed-Layer (SML) water (4-50 m) from the ACC and EWG, mean DMS concentrations were 5 nM, whilst very low concentrations of DMS occurred under forming sea ice (0.5 nM). Mean DMSPp concentrations in the top 50 m of the MIZ stations across the survey were closely similar (18-20 nM) and slightly higher than SML waters (13-14 nM), whilst mean DMSPd concentrations were 275 and 100% higher in samples from the MIZ stations (8-15 nM), compared with SML waters (4 nM). In the eastern sector, the coastal region across the shelf break and ASF region was largely free of ice and there was generally no elevation of DMSPd (mean 4 nM). This suggests that biological processes associated with the pack or fast ice in the ASP region of the western sector of the survey, in conjunction with the oceanography of the region, may have caused the high concentrations of DMS and DMSPd observed at latitudes >67 degrees S. Whilst filtration affects did influence DMSPd concentrations at a few sites, it was clear that elevated DMS and DMSPd concentrations occurred in the top 50 m and Winter Water (WW) layers of MIZ stations, and in High-Salinity Shelf Water (HSSW) at specific locations on the Antarctic Shelf, and likely reflected melting sea ice. DMS and DMSPp were also detected below the euphotic zone in Modified Circumpolar Deep Water (MCDW) on the Antarctic slope, with DMSPp possibly transported there in sedimenting detritus and faecal material. The significant concentrations of DMS detected in lower MCDW (0.42-1.1 nM) and Antarctic Bottom Water (AABW, 2.7 nM) were likely to have been transported to these depths in wintertime as dense shelf-water overflows from the Cape Darnley and Prydz Bay source regions. The high concentrations of DMS and DMSPd observed in the surface waters of the MIZ further support the importance of the Antarctic region and the sea-ice zone to the global sulphur cycle.