Diagenetic processes and nutrient fluxes at the sediment-water interface, northern Adriatic Sea, Italy

Abstract

Early diagenetic processes that control pore-water chemistry and nutrient fluxes at the sediment-water interface were studied in late summer 1989 and spring 1990 at four sites influenced by fine sediments of the Po River. Parameters on the solid fraction (grain size and C, N and P) and pore- water profiles of SO₄^2-, NO₃^-, NH₃, alkalinity, PO₄^3-, Fe, Mn, Ca and Mg were determined. Data interpretation was supported by application of kinetic and stoichiometric modelling. Nutrient fluxes were calculated by Fick's first law.

In the prodelta station the pore-water profiles showed large depletion in SO₄^2- ions, jointly with the increase with depth of alkalinity, NH₃ and PO₄^3- concentrations, thus indicating anaerobic decomposition of organic matter, mainly due to bacterial sulfate reduction. At the other three sites the results were completely different. Nutrient concentrations in pore water were one order of magnitude lower. The peaks of alkalinity, NH₃ and PO₄^3- values near the interface and the constant presence of SO₄^2- ions indicate aerobic decomposition of organic matter in the uppermost sediment. Nutrient fluxes showed much higher values in the prodelta station, whereas fluxes in the remaining stations decreased southward. In addition, seasonal variations produced higher summertime fluxes.

The clear-cut difference in diagenetic processes as a function of the distance from the main Po River mouth can be explained by the rapid burial of sediments rich in organic matter and inorganic compounds in the prodelta station. At the other stations, the lower sedimentation rate and more efficient bioturbation and resuspension processes cause a prolonged exposure of sediments to aerobic metabolization reactions before burial.