The seasonal variations of dimethyl sulphide and carbon disulphide in surface waters of the Venice lagoon

The seasonal variation in the concentration of dimethyl sulphide (DMS) and carbon disulphide (CS2) in the surface water of the Venice lagoon was determined at two stations in the period from 3 June 1996 (3 March 1997 for CS2) to 26 November 1997. Other parameters such as chlorophyll a, water temperature, and wind speed were also measured. DMS concentration (ranges 0.85–15.0 nmol S l−1 at Stn. 1 and 0.40–16.3 nmol S l−1 at Stn. 2) showed two relative maxima in the spring–summer period, probably connected to phytoplanktonic and macro-algal blooms. Low and constant values were observed in autumn, whereas the maximum concentration was observed in the late winter period. The CS2 concentration (ranges 0.17–2.8 nmol S l−1 at Stn. 1 and 0.08–2.0 nmol S l−1 at Stn. 2) increased in the spring, was maximal in summer and then decreased in autumn, with a different trend from that of DMS, suggesting that the production and the fate of CS2 and DMS in water are different. Interpretation of the seasonal trends, based on current knowledge of possible formation and transformation processes, is reported. The mean flux of DMS to the atmosphere is estimated to be 0.34 μmol S (DMS) m−2 day−1, which is about one order of magnitude lower than that observed in the open sea, due mainly to lower wind speed. The CS2 flux (March to November average 0.086 μmol S(CS2) m−2 day−1) may represent, at least in lagoon environments, an appreciable fraction of the total natural reduced sulphur emitted to the atmosphere.