Sulphur compounds, methane, and phytoplankton: interactions along a north–south transit in the western Pacific Ocean
Here we present results of the first comprehensive study of sulphur compounds and methane in the oligotrophic tropical western Pacific Ocean. The concentrations of dimethylsuphide (DMS), dimethylsulphoniopropionate (DMSP), dimethylsulphoxide (DMSO), and methane (CH 4), as well as various phytoplankton marker pigments in the surface ocean were measured along a north–south transit from Japan to Australia in October 2009. DMS (0.9 nmol L −1), dissolved DMSP (DMSP d, 1.6 nmol L −1) and particulate DMSP (DMSP p, 2 nmol L −1) concentrations were generally low, while dissolved DMSO (DMSO d, 4.4 nmol L −1) and particulate DMSO (DMSO p, 11.5 nmol L −1) concentrations were comparably enhanced. Positive correlations were found between DMSO and DMSP as well as DMSP and DMSO with chlorophyll a, which suggests a similar source for both compounds. Similar phytoplankton groups were identified as being important for the DMSO and DMSP pool, thus, the same algae taxa might produce both DMSP and DMSO. In contrast, phytoplankton seemed to play only a minor role for the DMS distribution in the western Pacific Ocean. The observed DMSP p : DMSO p ratios were very low and seem to be characteristic of oligotrophic tropical waters representing the extreme endpoint of the global DMSP p : DMSO p ratio vs SST relationship. It is most likely that nutrient limitation and oxidative stress in the tropical western Pacific Ocean triggered enhanced DMSO production leading to an accumulation of DMSO in the sea surface. Positive correlations between DMSP d and CH 4, as well as between DMSO (particulate and total) and CH 4, were found along the transit. We conclude that DMSP and DMSO and/or their degradation products might serve as potential substrates for CH 4 production in the oxic surface layer of the western Pacific Ocean.