Seasonal variations of dimethylsulphide in the surface water of the Venice Lagoon

A gaschromatographic method which exploits the purge and trap procedure was set up for the determination of dimethylsulphide (DMS) in brackish lagoon waters (Venice Lagoon, Italy). Calibration experiments carried out using standard solutions (concentration range 146 to 1690 ng/l) in ultrapure water and in lagoon water showed practically the same results, so ultrapure water was used for calibration in routine work. Accuracy of measurements was verified by analysing standard solutions prepared from dimethylsulphoniopropionate (DMSP) after quantitative transformation to DMS. Repeatability of measurements, estimated using DMS-spiked ultrapure water at a concentration of 590 ng/l, and from 16 replicates carried out during ten days of analysis was similar to 14% (RSD). Good stability of lagoon water (stored in the dark and at 4 degrees C) was demonstrated for at least 6 hours. The analyses of 86 samples of lagoon water collected at two sites in Venice trio San Nicolo and Canale della Scomenzera) in the period June-November 1996. showed a general decreasing trend of DMS concentration tin the range 668-25 ng/l) with two major peaks (June and August) and two minor peaks (October and November) superimposed, possibly corresponding to specific events of phytoplankton development. Data also showed a marked relationship between DMS concentration and solar radiation and scarce general influence from tide elevation.