First occurrence of Margalefidinium cf. polykrikoides blooms in Ionian Sea, Italy

Harmful algal blooms (HABs) are natural phenomena, however some microalgal blooms can cause harm to humans and other organisms. These HABs have direct impacts on human health and negative influences on human wellbeing, mainly through their consequences to coastal ecosystem services (fisheries, tourism and recreation). These events can be favoured by anthropogenic pressures and eutrophication in coastal areas (Sarkar, 2018). Global warming and associated changes in the oceans could affect HAB occurrences and toxicity as well, although forecasting the possible trends is still speculative and requires intensive multidisciplinary research (Wells et al., 2015). Dynamics of HABs vary from one site to another, depending on the hydrographic and ecological conditions but also the complexity of the algal life cycle, which is composed of discrete life stages whose morphology, ecological niche, function, and lifespan vary (Glibert et al., 2018). Margalefidinium polykrikoides (Margalef) F. Gómez, Richlen & D.M. Anderson 2017 is a cosmopolitan dinoflagellate notorious for causing fish-killing HABs. The red tide forming dinoflagellate genus Margalefidinium appears to be expanding globally, as well as blooming and/or causing more economic losses within its previously reported geographic distribution (Kudela and Gobler, 2012). Margalefidinium has an adaptive capability conducive to rapid colonization of newly opened ecological niches, which may partially explain the apparent global expansion of its geographic range and bloom frequency. This species has a wide distribution in the Asian and European waters, it is responsible for high mortalities of wild and farmed fish and causes large economic losses in the Pacific, Atlantic, and Indian oceans (Matsuoka et al., 2008). In the Mediterranean Sea, the Italian coast is affected since the late 1990s and the Black Sea since 2001 (Zingone et al., 2006). In this work, we described an exceptional yellow-brownish discoloration of the waters in the shallow Porto Cesareo Bay (Mediterranean, Ionian Sea, Italy) in the July - August 2018 caused by a bloom of M. cf. polykrikoides. Cell densities reached 9.1 x 106 cells L-1 during the initial outbreak. A second bloom was observed about three weeks later reaching 6.7 x 105 cells L-1. The assessment of the anthropogenic pressures and expected impacts on the marine-coastal waters was carried out. The study of live specimens showed great variation in cell size and shape, different cyst morphotypes were also found in the water samples and in the sediment. For the first time, we followed the life cycle of M. cf. polykrikoides in natural samples. The high densities of this species did not lead to fish die-offs in the bay, however it caused consequences on ecosystem and in turn, on tourism. The need for improving risk assessment to manage and prevent the occurrence of harmful blooms is globally enhancing. This is particularly true for dominant HAB-causing species after their persisting with low abundance in the phytoplankton assemblages but are worldwide considered as expanding their distribution, as the case of Margalefidinium polykrikoides.