Dynamics of dimethylsulphide and associated compounds in the coastal waters of Goa, west coast of India
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Concentrations of total dimethylsulphoniopropionate (DMSPt), Dimethylsulphide (DMS) and total dimethylsulphoxide (DMSOt) were measured at a coastal time series site, west coast of India, from September 2009 to December 2013. While, DMSPt varied between 0.35 and 252 nM (avg. 24 ± 32.3 nM), DMS varied from 0.5 to 442 nM (avg. 22.5 ± 48.3 nM). The annual phytoplankton regime and succession was observed to play a key role in DMSP and DMS production. DMSPt, DMS and DMSOt showed high temporal variability with maximum concentrations observed during the southwest monsoon (SWM; June to September). While the least DMSP and DMS (<20 nM) concentrations were observed during the northeast monsoon (NEM; December to February) in association with low Chlorophyll a (Chl a), moderate concentrations (40-60 nM) were observed during the spring inter-monsoon (SIM; March to May) in association with diatoms which appear during the senescent phase of the annual Trichodesmium bloom. Moderate concentrations were also observed during the mid-phase of the SWM (July-August) in association with the diatom population. On the other hand, maximum DMSP and DMS observed towards the end of the SWM (September) and fall inter-monsoon (FIM; October) were associated with dinoflagellates, which appear as conditions calm down towards the end of the SWM. There was no correlation between DMSPt and Chl a. High DMS concentrations (up to 442 nM) were observed in the bottom waters during the September of 2009 and 2013 in association with the seasonal anoxia. DMSOt distribution closely resembled that of DMSPt and DMS with an overall variation from 0.56 to 185.9 nM (avg. 27.8 ± 30.1 nM). Though wind speeds were highest during July, high DMS flux was observed during September owing to the high surface DMS concentrations. Average surface DMS during the study period was 18.5 nM, whereas average DMS flux was estimated to be 20.9 mu mol S m-2 D-1. This time series study brings out the importance of hydrography in shaping the phytoplankton population and its influence on the production of DMS.
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