Two-stages bloom of Margalefidinium cf. polykrikoides in a Mediterranean shallow bay (Ionian Sea, Italy)

The emergence of a red tide resulting in yellow-brownish discoloration of waters in Porto Cesareo bay (Italy) during July - August 2018 is reported. The species responsible for the bloom was the dinoflagellate Margalefidinium cf. polykrikoides. Cell densities reached 9.1 x 106 cells L-1 during the initial outbreak. A second peak was observed about three weeks later reaching 6.7 x 105 cells L-1. Study of live specimens showed great variation in cell size and shape, mainly as single cell during the first peak and as new germling cells consisting of two in the second one. Different cyst morphotypes were found in the water samples and in the sediment. For the first time, we followed several stages of the life cycle of M. cf. polykrikoides in natural samples. Fish die-offs in the bay were not observed, however this high-density bloom may have caused consequences on the ecosystem (amount of mucilage on the beach) and in turn, on tourism that is the main activity in the area during the summer season.