Factors Controlling Benthic Biogeochemistry in Urbanized Coastal Systems: an Example from Venice (Italy)

Benthic biogeochemical processes and their interactions are affected by multiple factors including organic matter load, season, and hydrology. We analyzed benthic biogeochemistry in two canals in the urban area of Venice (Italy), to assess the effects of contrasting tidal conditions on organic matter processing in the heavily modified lagoonal ecosystem. We measured sediment oxygen demand, bacterial sulfate reduction, denitrification, dissimilative nitrate reduction to ammonia, dissolved inorganic carbon, and inorganic nutrient fluxes across the sediment-water interface under different seasonal (late winter and summer) and tidal (spring and neap tide) conditions. Sediments were highly organic and strongly reduced. Organic matter mineralization was mainly driven by bacterial sulfate reduction, whereas denitrification was limited by both nitrate availability and competition with dissimilatory nitrate reduction to ammonium. While the elevated benthic metabolism can be largely explained by organic enrichment, contrasting tidal conditions can be a significant driver of intra-seasonal variability of benthic biogeochemistry. Under neap tide, dissolved inorganic carbon production increased up to threefold, sulfides attained up to 6.4 mM in porewater, denitrification became inefficient, and the ratio of nitrogen recycling to dissipation increased by ~70 % compared to spring tide. Additionally, increased efflux of inorganic nitrogen from sediments fed back to promote further eutrophication. We infer that human modifications to reduce fortnightly tidal flushing can impact benthic processes, impairing sediment functioning and water quality.