Microalgal characterization during a mucilaginous bloom on deep gorgonian forests of Tavolara Punta Coda Cavallo MPA

Harmful algal blooms (HABs) have increased in frequency, but despite an extreme increase in research on this topic, general knowledge on HABs is still scarce. Mucilaginous algal blooms, in particular, are ever more present and can heavily affect marine ecosystems. Gorgonians appear significantly affected, as their branches are easily entangled (pelagic aggregates) and overgrown (benthic aggregates) by mucilage. Moreover, their proximity to the bottom makes them susceptible to benthic anoxia. In the present study, we investigated the mucilaginous bloom that affected the gorgonian forests of Tavolara Punta Coda Cavallo Marine Protected Area (MPA) (NE coast of Sardinia) in summer 2020. We focused mainly on the characterization of dominant benthic microalgae, identifying those responsible for the bloom. Sampling was performed considering the colonies of Paramuricea clavata present on hard substrata along 2 transects near the Secca del Papa, in the B Zone of the MPA. The characterization of the microalgal community and the count of the most abundant species (>10 cells ml−1) were carried out using the Utermöhl sedimentation method. The community was mainly composed of diatoms (Bacillariophyta) belonging to the genera Pseudo-nitzschia, Nitzschia and Cylindrotheca. C. closterium appeared to be the most abundant species. These results confirm the importance of diatoms during Tyrrhenian mucilaginous blooms, which could be particularly harmful to gorgonians.