Influence of hydrographic conditions on sediment organic carbon in a coastal area of the NW Adriatic Sea (Mediterranean Sea)

The Adriatic Sea and its coast are vulnerable to climate changes and, especially, to sea-level rise, temperature and salinity anomalies, affecting the biogeochemistry of nutrients and relative exchanges between the water column and the sediments with negative consequences on the primary production. A lack of information characterized seasonal distribution and the potential sources of nutrients in relation to current and future climate change scenarios. To this purpose, water and sediment samples were collected aboard four research cruises conducted as part of the EU Interreg AdriaClim project in order to capture the seasonal changes of hydrographic conditions in the coastal area of Emilia-Romagna region (NW Adriatic Sea), and their influence on the isotopic composition of sediment organic carbon (δ13C). All the coastal area is marked by seasonal freshwaters and nutrient inputs of Po River, the largest Italian river. A Conductivity-Temperature-Depth (CTD) Idronaut OS316 Plus CTD unit was used to produce downward profiles of salinity and temperature at the sampling locations. Surface sediments (0-5 cm) were collected at the same locations with a Van Veen grab and organic carbon and its stable isotopic composition were analysed by EA-IRMS. In our study area, higher temperature marked the second research cruises (Late Spring 2021) with value of 21.02±4.34 °C. Seasonal changes were recorded in the salinity with an increase from the first (Late Autumn 2020) to the last oceanographic campaigns (Early Spring 2021), varying from 32.88±4.34 psu to 37.79±0.39 psu, respectively, and most probably related to the low riverine discharge that characterized the last Winter season. Although sediment OC content was similar among all the research cruises with values within the range of 0.58±0.39% and 0.84±0.41%, remarkable differences were recorded in the isotopic (δ13C) composition of organic matter. Changes in δ13C composition were detected in the surface sediments from the first two ocean campaigns (from -25.12 to -25.22 ‰) to the last two (from -28.08 to -27.19 ‰), respectively, and also in the Po River delta area, underlying a potential shift of organic matter sources driven by hydrographicconditions and climate change.