Sulphate reduction rate in the sediment of the Venice canal network (Italy).

Bacterial sulphate reduction is the predominant process in the cycling of organic matter and anthropogenic contaminants in permanently anoxic sediments such as those of the Venice canal network. Urban materials delivered to the canals and suspended particles transported by tidal currents from the open lagoon cause the silting up of the channel bed at a rate of 2 cm y-1, therefore requiring periodic dredging. As the recent maintenance interventions on canals are accompanied by the extensive improvement of the sewer system, there is an increasing interest for understanding how effective are natural processes in the metabolism of the large amount of organic matter accumulating in the sediment. A specific investigation on sulphate reduction was then started on May 2003. Sulphate reduction rates were measured, as sulphate depletion on incubated sample of the superficial sediment (0-5 cm), in 5 test canals. Measurements were repeated in July and October 2003, to investigate how the rates vary in response to temperature changes. If compared to typical values for estuary and salt marshes, rates measured are among the highest (up to 1.7 mmol l-1 d-1), the limiting factor being only the concentration of S2-. As expected, the highest rates were measured in summer (0.5-1. 7 mmol l-1 d-1). However, significant spatial differences were observable, particularly in the way each site responds to seasonal changes. We interpret major changes as the effect of a non-uniform distribution of bacterial population within the network sediments or a different diffusion of SO42- as a result of local changes in the aggregation state of the sediment. A simple first-order kinetic model applied to S042- concentrations measured in pore waters from 30 cm-long sediment cores finally permitted to better estimate the mean annual sulphate reduction rates which are consistent with values obtained by the incubation of surface sediments.