The Adriatic Sea mucilage: The history of a hidden harmful dinoflagellate bloom

Mucilage events, characterized by the accumulation of gelatinous, polysaccharide-rich organic aggregates in seawater, are irregularly occurring phenomena in the northern Adriatic Sea. While mucilage in other seas is typically linked to blooms of a single phytoplankton or phytobenthic species, Adriatic events have been mostly attributed to complex biogeochemical processes, rapidly leading to the formation of extensive gelatinous masses. Long-term studies on Northern Adriatic phytoplankton communities have revealed a consistent association, since the 1990s, between mucilage events and blooms of a non-toxic dinoflagellate. Originally identified as Gonyaulax fragilis, the ongoing debate regarding its definitive classification leads us to refer to the species as Gonyaulax cf. hyalina. Generally, this species acts as the primary biological trigger for these phenomena, which subsequently develop into a distinct, recurrent mucilage-associated microbial ecosystem. This review provides a comprehensive synthesis of the taxonomic debate, the main hypotheses proposed over the years, historical and recent evidence leading to G. cf. hyalina as the mucilage causative organism, and the ecological and economic impacts of these outbreaks. Regarding the interannual variability of the events, a link with specific environmental conditions could not be assessed; however, based on G. cf. hyalina growth and nutrient dynamics, we speculate about the importance of specific nutritional requirements (i.e., phosphate inputs or still unknown organic compounds) and of competitive interactions. This review would contribute to increasing knowledge on the most recurrent mucilage phenomenon in the Mediterranean Sea, opening the question on the inclusion of the investigated species in the list of harmful algal species.