High biomass blooms in beaches: are resting cysts maintained in situ?

Data derived from an extensive sampling (74 beaches in summer 2012) along the Sardinian coasts have allowed the identification of Alexandrium taylorii, Gymnodinium instriatum and Barrufeta bravensis as the most widely distributed cyst-forming harmful species. Statistical analyses indicated that the distribution of harmful algal species was correlated with gravel and medium-fine sand substrata. This relationship suggested that vegetative cells might be recruited from cyst beds in beach sediments. Therefore, the objective of this study was to verify the presence of resting cysts in the near shore beach sediments. Sediment cores (40-50 cm long) were collected in May 2013 from three target beaches. Cores were sampled at three stations (foreshore, trough, bars) along a perpendicular transect at each beach. Cores were sectioned every 5 cm. Alexandrium taylorii cysts were observed in the three beaches, always below a depth of 15 cm of sediment. Cyst presence was assessed at the trough and bar stations in Platamona and Villasimius, whereas at the foreshore station in Bosa. Gymnodinium instriatum cysts were detected only in Bosa at the bar station, at a depth below 20 cm. Naked-like dinoflagellate cysts, probably belonging to Barrufeta bravensis, were observed at the same station and depth. The found cyst distributions reflected the different morphodynamic features of the beaches. The discovery of resting cysts in the near shore sediments supports the hypothesis of a potential ‘in situ’ source of vegetative cell inoculum.