Multi-platform study of the extreme bloom of the barrel jellyfish Rhizostoma pulmo (Cnidaria: Scyphozoa) in the northernmost gulf of the Mediterranean Sea (Gulf of Trieste) in April 2021

Abstract. On 7 April 2021, an exceptional bloom of the scyphomedusa Rhizostoma pulmo was observed in the Gulf of Trieste (Italy). Blooms of this species in the northern Adriatic Sea have been reported since the late 1800s, however, the density of jellyfish observed in 2021 reached more than 10 specimens per square metre. In this work, we analyse the bloom from a multiplatform approach using observations and model data at different time scales. We attempt to explain the intensity of the bloom as a consequence of thermohaline and hydrodynamical conditions in the gulf. Meteo-oceanographic conditions that may have contributed to the exceptional aggregation of jellyfish observed along the northernmost coast of the Adriatic Sea are discussed in detail. Specifically, our results indicate that this bloom was enabled by 1) the presence of a high number of jellyfish in the Gulf probably linked to the anomalously warm sea conditions in spring 2020 and winter 2021 which may have favoured a longer reproductive period and enhanced survival of adult R. pulmo respectively; 2) strong wind events, such as the Bora wind for the Gulf of Trieste, which enhanced upwelling and mixing processes in the Gulf bringing the jellyfish from the deeper waters to the surface and clustering them along the coast.