Influence of different water masses and biological activity on dimethylsulphide and dimethylsulphoniopropionate in the subantarctic zone of the Southern Ocean during ACE 1

Abstract

Measurements of salinity, temperature, phytoplankton biomass and speciation, dissolved nitrate, dimethylsulfide (DMS) in seawater and air, and dimethylsulfoniopropionate (DMSP), were made in the subantarctic zone of the Southern Ocean from 40°‐54°S, and 140°‐153°E during the southern hemisphere marine First Aerosol Characterization Experiment (ACE 1). DMSP concentrations were highest in subtropical convergence zone (STCZ) waters, intermediate in subantarctic waters, and lowest in polar waters. DMSP appeared to decrease at frontal regions between these major water masses. In subantarctic waters, high levels of DMSP were generally associated with an increase in dinoflagellate biomass and low microzooplankton grazing rates. Lower DMSP concentrations occurred in polar waters when the diatom biomass and grazing rates were high. DMS levels measured on Southern Surveyor ranged from not detectable (nd) to 5.6 nM (mean 1.7 nM), with below average levels in subantarctic waters (mean 1.25 nM), and above average levels (mean = 1.93 nM) in polar waters. Pulses of DMS occurred as Southern Surveyor traveled south into polar waters, with a large pulse (mean = 2.3 nM) highlighted as the vessel traveled back into subantarctic waters (46°‐47°S, 148°‐151°E) in early December. By using the dissolved DMSP (DMSPd) to DMS ratio as an index of the bacterial conversion of DMSPd to DMS some evidence was found that, in polar waters, increased microzooplankton (MZP) grazing in diatom dominated waters, may lead to above average concentrations of DMS. This does not appear to be the case when the biomass was dominated by dinoflagellates in subantarctic waters.