Variation of biogenic sulphur compounds in the estuarine and coastal waters of Goa, West coast of India

Abstract

Dimethylsulphide (DMS) originates predominantly from dimethylsulphoniopropionate (DMSP), a metabolite produced by phytoplankton. Through its contribution to the production of new aerosols and cloud condensation nuclei, a high concentration of DMS has the potential to influence the radiation budget of the earth. Estuaries and coastal regions being dynamic may produce significantly high concentrations of DMS and DMSP. The present study aimed to investigate the spatial variation of DMS, its precursor total dimethylsulphoniopropionate (DMSP_t), and its sink total dimethylsulphoxide (DMSO_t) at 7 estuarine locations in 4 rivers and a coastal station in Goa during the North East Monsoon (NEM). Generally, higher concentrations of DMS and DMSP_t were observed at the near mouth stations and the coastal station compared to upstream stations. Though a positive correlation was observed between salinity and DMSP_t, it was not significant, indicating the involvement of other factors influencing DMSP and DMS concentrations. Diatoms were the most abundant group accounting for > 90 % of the phytoplankton. However, higher fractions of dinoflagellates, nano- and picoplankton probably contributed to the DMSP_t, DMS and DMSO_t production at the coastal and near mouth stations. As the wind speeds were low, DMS flux was governed by surface DMS concentrations and varied between 0.07 and 2.11 µmoles S m^-2 D^-1 with an average of 0.92±0.80 µmoles S m^-2 D^-1. In comparison to DMSP_t and DMS, a relatively higher concentration of DMSO_t was observed in the study area. While the high DMSO_t concentration at the estuarine mouths may be attributed to the photo- or biological oxidation of DMS, those in the upper reaches point to an unknown source and warrants further investigation.