First detection of a bloom forming Peridiniopsis species from a Sardinian reservoir (NW Mediterraean Sea)

In the Mediterranean region, reservoirs represent the main sources of water supply for various human demands. The main issues in these ecosystems are linked to eutrophication and, consequently, to the development of Cyanobacteria blooms. However, freshwater red tides caused by dinoflagellates are becoming a new emergent problem for water treatments. In Sardinian reservoirs, dinoflagellate blooms have been detected since the seventies and were mainly caused by Ceratium hirundinella. Recently, other dinoflagellates have given rise to similar events. One of the most intense was recorded in Cedrino Lake in February 2012. The causative dinoflagellates were firstly recognized as Peridinium species. In this study, fixed samples collected during this event were analyzed in more detail with optical and scanning electron microscopy. Furthermore, surface sediment samples were taken and analyzed for the presence of resting cysts. The plate formula of the recovered cells was po, x, 4’, 6’’, 5C, 4S, 5’’’ and 2’’’’. Scattered pores ornamented the theca and numerous spines the hypotheca. Based on the tabulation features, this dinoflagellate was assigned to the Peridiniopsis genus, ‘penardii’ section. The cysts were slightly irregular in shape, and most of them maintained the external theca. The cyst content was grainy with yellowish to reddish accumulation bodies. Some inconsistencies (on cell and cyst morphology) found in the literature, have not permitted the certain determination of the species. Those difficulties fall in a larger scientific context because many of the known freshwater dinoflagellates have been described before the coming of electron microscopy and molecular methods.