Extraordinary mucilage event in the northern Adriatic in 2024—a glimpse into the future climate?

Abstract

Large mucilage events occasionally occur along the northern Adriatic coast during late spring and summer, significantly impacting tourism, fisheries, aquaculture, and benthic organisms. However, no such events had been recorded in the last two decades until 2024, when mucus aggregates reappeared in early June and persisted until beginning of September. This paper describes the 2024 event and quantifies the atmospheric, oceanic, and hydrological conditions preceding and during the event using available observing systems (buoys, CTD profiles), satellite observations (sea surface temperature, chlorophyll-a), and reanalysis products (heat fluxes from ERA5 reanalysis). The 2024 mucilage event was preconditioned by an unusually warm winter and early spring, coupled with highly saline residing waters in the northern Adriatic. These conditions led to higher-than-average stratification, initiated earlier than usual, in February. On top of that, several extremes in river discharge (with maximum values up to 5000 m³/s, far exceeding the 90th percentile for a given day) that further strengthened the stratifications were observed from April to July. Discharge peaks in late May and late June resulted in nutrient-rich coastal waters spreading towards the eastern coastline within 1–2 weeks, triggering mucilage blooms along the coast. As the river discharges decreased, the spread of freshened waters over the shelf was limited, leading to a halt in mucilage growth. Given that both air and sea surface temperatures were from January to August 2024 almost constantly 2–4 °C higher than the present climate averages, this event provides a valuable case study for understanding mucilage events under warmer climate conditions.