F. Corami

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

Abstract 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.

Temporal evolution of cadmium, copper and lead concentration in the Venice Lagoon water in relation with the speciation and dissolved/particulate partition

In order to study the role of sediment re-suspension and deposition versus the role of organic complexation, we investigated the speciation of cadmium (Cd), copper (Cu) and lead (Pb) in samples collected in the Venice Lagoon during several campaigns from 1992 to 2006. The increment in Cd and Pb concentration in the dissolved phases, observed in the central and northern basins, can be linked to important alterations inside the lagoon caused by industrial and urban factors. The study focuses on metal partition between dissolved and particulate phases. The analyses carried out in different sites illustrate the complex role of organic matter in the sedimentation process. While Cd concentration in sediments can be correlated with organic matter, no such correlation can be established in the case of Pb, whose particulate concentration is related only to the dissolved concentration. In the case of Cu, the role of organic complexation remains unclear.

Free phenolic compounds in waters of the Ross Sea

The presence of free phenolic compounds (PC) in Antarctic sea water has been investigated to explain their source and particle size distribution in the atmospheric aerosols, as determined in our previous research. The sea water samples were filtered to distinguish the PC concentrations in the particulate and dissolved fractions. Two sample preparation procedures were developed to quantify nine PC in both fractions. The highest concentrations were found in the dissolved fraction of Ross Sea water, with vanillin, vanillic acid, acetovanillone and p-coumaric acid being the most abundant PC. Dissolved PC were mainly found in the upper part of water column. This facilitated the sea water-air exchange by bubble busting processes. In the aerosol, they were mainly found in the fine fraction, where these compounds have a higher degree of oxidation than PC detected in seawater, suggesting that they were newly emitted and they have been not yet oxidized. These results supported our previous hypothesis that PC were locally emitted into the atmosphere from the Ross Sea. Three different possible sources of PC are hypothesized for Antarctic sea waters: 1) from the intrusion of Modified Circumpolar Deep Water that may transport oceanic lignin; 2) from phytoplankton biomass that may be a source of PC in Antarctic waters since diatoms produce exudates that contain vanillic acid, p-coumaric acid and syringic acid; 3) from the melting of glaciers and sea ice: glaciers contain lignin that can be degraded, while in the sea ice there are diatoms that may release PC. Statistical analysis and the low value of vanillic acid/vanillin ratio indicated that the most plausible source for PC in the dissolved fraction was the senescence of phytoplankton. As a contrast, particulate PC with higher vanillic acid/vanillin ratios were ascribed to degraded lignin or the sorption of diagenically oxidized material on particles.