Benthic harmful algal blooms occurrence in the south coast of Madeira Island, Portugal, from 2018 to 2020

Ciguatera fish poisoning episodes have been recently reported in the Madeira Archipelago, requiring actions to protect both public health and economic sectors such as tourism and fisheries. Surveys on the occurrence of benthic harmful algal bloom (bHAB) species were performed during three consecutive years (from 2018 to 2020), at Cais do Carvão Bay, Funchal, Madeira. Samples collected on artificial substrates were analyzed for the presence of bHAB species, the seasonal succession of the bHAB community and the effects of abiotic factors on BHAB dynamics. The bHAB community was characterized by the presence of five genera: Gambierdiscus, Ostreopsis, Coolia, Prorocentrum, and Amphidinium. The abundance of the different genera showed marked differences. Ostreopsis displayed cell abundances several orders of magnitude higher than the other genera (841.05 x 103 cells 100 cm-2 (September 2018) and was recorded at all sampling dates. The second most abundant genus was Coolia, followed by Prorocentrum and Amphidinium. Gambierdiscus always showed the lowest cell densities (max. 0.43 x 103 cells 100 cm-2 (September 2018). During summer, Ostreopsis abundances exceeded, in several sampling dates the limits of level of concern for respiratory syndrome outbreaks (20 x 103 cells 100 cm-2) as referred to in the literature. In situ recorded abiotic data suggests that the increase of Ostreopsis densities could be linked to an increase in water temperature during summer. These results confirm Cais do Carvão Bay, as a potential high-risk area for bHAB development, especially concerning the genus Ostreopsis.