Anomalous Po River flood event effects on sediments and the water column of the northwestern Adriatic Sea

The aim of this study was to investigate the spreading pattern and trophic impact of the anomalous Po River flood in the fall of 1994 in the marine area offshore of the Po River delta. The study was carried out by surveying the areal distribution of physical and biogeochemical parameters (pH, redox potential [Eh], water content, grain-size, mineralogy, organic and inorganic C, total N, total organic and inorganic P) in surficial and sub-surficial sediments, and physical-chemical properties (salinity, temperature and dissolved O2) of the water column. Water column investigations highlighted a fresh, turbid and relatively cold Po River water wedge on the basin surface that extended more than 40 km offshore, well above the usual limits of the Po River plume influence, which was a result of slow geostrophic currents and no wave motion in the basin. The distribution of minerals in sediments (such as serpentine and dolomite), derived from river catchments, provided cues on the maximum extension of Po and northern river influences. Biogeochemical tracers such as organic C and organic P, together with C:N ratios, provided indirect evidence on the development of algal blooms owing to the Po River flood run-off. Indeed, they indicated areas where there was the deposition of autochthonous reactive organic matter, derived from fresh plankton biomass. Their location suggests that the maximum development of plankton blooms occurred along the external edge of the plume, where warm basin waters mixed with colder nutrient-rich waters from the river. The data also suggest that significant amounts of newly formed and degrading organic matter reached bottom waters at the centre of the basin, thus increasing the degree of hypoxia.