Composition of Dinoflagellate Resting Cyst Assemblages and Its Monthly Variability in a Shallow Mediterranean Lagoon (Santa Giusta Lagoon, Sardinia, Italy)

The present study identifies and quantifies the dinoflagellate resting cysts along an annual cycle from a shallow Mediterranean lagoon. Sediment samples were collected and analyzed for the determination of the dinoflagellate cyst density and composition at four sampling stations in Santa Giusta Lagoon (Oristano Gulf) from March 2015 to February 2016. At the same time, selected physicochemical variables and the cell densities of the dinoflagellate species in the water column were measured and determined. The group of Peridiniales dominated the cyst assemblages at spatial and temporal scales, with an almost total prevalence of the autotrophic–mixotrophic taxa. Heterotrophic Peridiniales showed occasional importance, especially in the stations closest to the sea connections, as well as the group of Gonyaulacales and Gymnodiniales. The composition in morphotypes revealed the dominance of Ensiculifera tyrrhenica and Scrippsiella kirschiae in correspondence with the maximum cyst densities in the inner sampling stations. Cysts of Margalefidinium cf polykrikoides reached high density values following an intense summer bloom. Salinity, ammonia, and total nitrogen were the environmental variables more strongly correlated with the multivariate cyst assemblage structure. The monthly time-scale offered a more accurate focus on cyst assemblages changes in respect to the usual single seasonal sampling in Santa Giusta Lagoon. The obtained data provided further knowledge on the relation among cyst assemblage variability, environmental conditions, and dinoflagellate motile stages in the water column relevant for comparisons with similar ecosystems in the Mediterranean and worldwide.