High-frequency observations during Adriatic mucilage event reveal unique phytoplankton traits and diversity response

Mucilage aggregation is a striking phenomenon in the northern Adriatic Sea, reappearing massively in surface waters on the Istrian coast in 2024 after 20 years. Formed through polymerization of exuded sugars during microphytoplankton blooms, mucilage events are hard to capture with traditional monitoring. Here, we used real-time in situ sensors, satellite data and daily pulse-shape flow cytometry to analyze phytoplankton dynamics during this event. Mucilage formation was linked to rising temperatures and Po River-induced salinity drops. Microphytoplankton species like Cerataulina pelagica, Cylindrotheca closterium, Thalassionema spp., and Gonyaulax fragilis showed as important taxa in each different phase of the phenomenon. Aggregates consisted mainly of single cells. Mucilage periods featured low diversity and thicker, more complex cells, unlike autumn blooms which showed higher diversity, chain-forming colonies, and more pigments. Our findings highlight the key role of microphytoplankton and single cells in mucilage dynamics and the influence of environmental factors like temperature, wind and freshwater inputs on phytoplankton structure and biomass in coastal ecosystems.