Harmful epiphytic dinoflagellate assemblages on macrophytes in the Gulf of Tunis

Abstract The spatio-temporal distribution of epiphyticand planktonic microalgae coupled with environmental factors was investigated for a one-year period in the Gulf of Tunis (northeastern Tunisia). Harmful microalgae assemblages were dominated by three toxic epiphyticdinoflagellates: Ostreopsis sp., Prorocentrum lima and Coolia monotis. They were observed, both on macrophytes(1.03 × 10 5 cellsg −1 FW ; 1.3 × 10 4 cells g −1 FW and 865 cells g −1 FW, respectively) and in the water column (2.35 × 10 4 cells L −1 ; 3.72 × 10 3 cells L −1 ; 1.04 × 10 3 cells L −1 , respectively). Species abundances decreased with depth and maximum concentrations were found in shallow waters (0.5–1 m). The highest species abundance was registered both on macroalgae and seagrasswith no special preference observed for either of these substrates. Redundancy analyses (RDA) show significant changes in these species abundances according to sites and seasons. The proliferation of Ostreopsis sp. is widespread in summer, when water temperature is warm, and especially in bay zones. The occurrence of P. lima and C. monotis blooms was mainly correlated to nutrients. In this study, macrophytebeds in the Gulf of Tunis were a reservoir of potentially toxic species that could pose a real threat, both to ecosystems and to public health.