Salinity variations in the Venice Lagoon, Italy: Results from the SIOSED Project, May 2005-February 2007

Abstract As a component of the Scripps Institution of Oceanography Sediment Research Project (SIOSED) in the Venice Lagoon, we studied the variability of the salinity regime with time at three principal locations in the south-central part of the Lagoon. We carried out measurements of salinity near the bottom of the Lagoon at these stations, as well as measurements of the chloride concentrations of the sedimentary pore fluids with depth into the sediments. The principal aim of these combined studies was to examine the relationship between the chloride contents of the overlying waters with those in the pore fluids. Salinity variations were measured several times over month-long periods during the SIOSED program. Occasionally these measurements overlapped with the taking of sediment cores from which pore fluids were extracted for studies of interstitial water chemistry. Generally chloride concentrations in the upper 2–4 cm of the sediments reflected the average chlorides in the overlying water column, estimated from average salinities measured over the sites of the cores, notwithstanding the short-term variability of the salinities as a function of tidal currents. Chloride concentration-depth profiles are interpreted in terms of exchange with overlying waters as well as in terms of potential influences of underlying aquifers. Though cores were taken at a higher frequency than the salinity measurements, we feel justified to use this data set together with the pore fluid data in recording the time change in chlorides of the near-bottom waters of two major SIOSED sites from May 2005 through February 2007. Results provide a better understanding of the salinity dynamics of the Venice Lagoon, and revealed that deep pore water gradients in salinity likely reflect diffusive exchange with underlying freshwater aquifers. These findings highlight the need for additional long cores, and will guide subsequent geochemical investigations of the Lagoon. Indications of the influences of aquifers on pore waters will help prevent misinterpretation of pore water trends as indications of gradual changes in the salinity of the tidal prism over time.

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