Po River plume on the Adriatic continental shelf: Dispersion and sedimentation of dissolved and suspended matter during different river discharge rates

Abstract

Suspended matter and dissolved nutrients were studied in the prodelta of the Po River (northern Adriatic Sea) immediately after one of the greatest floods of the last century, which occurred in October 2000, and in four successive seasonal conditions. Hydrology, total suspended matter (TSM), particulate organic carbon (POC) and particulate nitrogen (PN) content, organic carbon stable isotopes ( δ 13C POC), particle grain size spectra, phytoplankton, and dissolved inorganic nutrients (i.e., nitrate, nitrite, ammonium, phosphate, and silicate) were measured in different discharge conditions to describe processes occurring in the Po River delta. The main effect of high river discharge was a large volume of low salinity water at the surface that extended over the entire study area. The concentration of most sampled variables decreased along a dilution gradient from the Po River distributaries’ mouths to the offshore. For TSM at the surface, riverine input and dilution were the main factors affecting its distribution. At the bottom, however, increased TSM in the nearshore was due to rapid sinking of suspended matter at the river mouth, while resuspension and advection dominated the transport of near bottom sediments offshore at 15–20 m depth. In the offshore where resuspension was active, an increase in ammonium and decrease in oxygen supported the presence of regeneration processes occurring at the bottom. During the October 2000 flood event, high nutrient concentrations promoted new production, triggering an algal bloom in the southern region of the study area. The origin of particulate organic carbon in the river delta was then estimated from a mixing model based on δ 13C POC values and carbon : nitrogen (C / N) molar ratios, and as the contribution of three distinct sources of organic carbon were determined (i.e., marine, riverine, and terrestrial). The riverine end-member accounted for ∼50% of the surface POC and generally decreased towards the offshore. The terrestrial carbon contribution was generally the least abundant (20–28% at the surface and at depth, respectively) and exhibited peak values (30–40%) during the October 2000 flood. Last, the marine contribution was the most abundant at the bottom (∼40%) and generally increased offshore. During the October 2000 flood, the fluvial contribution of particulate organic carbon, that is the terrestrial and riverine fractions, represented approximately 80% of the total particulate organic carbon in the Po River delta.