How extreme river discharge events shape coastal biogeochemical dynamics: the 2024 northern Adriatic case

In 2024, the northern Adriatic Sea experienced marked levels of eutrophication, as indicated by unusually high surface chlorophyll concentrations observed by remote sensing and in-situ measurements. In addition, exceptionally high Po river discharges, in the form of repeated and intense events, were recorded from March to June and in October. We used Copernicus Marine Service satellite and model products for the Mediterranean Sea, together with a high-resolution (500 m) Italian coastal model, to quantify chlorophyll anomalies in the area with respect to 1999–2022 climatology and investigated the marine ecosystem functioning in connection with river discharges, phosphate input and sea surface temperature as key drivers. We found that chlorophyll values exceeded 100 % above climatological levels in early spring, followed by sustained anomalies in summer and autumn. Primary production remained elevated from the early spring onward, with a significant anomaly peaking in early summer, concurrent with renewed Po river discharges and rising sea surface temperatures. Our findings highlight the central importance of riverine freshwater and nutrient inputs in controlling the northern Adriatic ecosystem, in combination with circulation and thermal variability. Moreover, this study shows how forecasting systems play a pivotal role in monitoring eutrophication and potential biomass accumulation in the context of episodic river extremes and increasing climate variability, with high-resolution coastal modeling providing additional valuable insights into spatial patterns and localized eutrophication dynamics.