Controlling factors of harmful microalgae distribution in water column, biofilm and sediment in shellfish production area (South of Sfax, Gulf of Gabes) from southern Tunisia

The aim of this study was to investigate the spatio-temporal distribution of harmful microalgae coupled with environmental factors in the most important area for natural stocks of the grooved carpet shell Ruditapes decussatus in southern Tunisia. Sampling was performed monthly from May 2010 to April 2011 in five stations through the Tunisian National Monitoring Stations Network of Phytoplankton and Phycotoxins along the southern coasts of Sfax (Gulf of Gabes). The presence of harmful microalgae species was explored in three compartments: water column, biofilm and sediment. Our results revealed fourteen species were identified belonging to dinoflagellates and diatoms with higher densities during the summer period. The co-inertia plot analysis exhibited that the seasonal fluctuations of these species were controlled by the temperature as well as the nutrients (particularly nitrogenous). Ternary diagrams showed that biofilm was the most colonized compartment by toxic benthic dinoflagellates species, namely Amphidinium carterae, Prorocentrum rathymum, Prorocentrum concavum, Prorocentrum lima, Ostreopsis cf. ovata and Coolia monotis. In addition, these species were recorded simultaneously in the water column and the sediment, a fact that could be explained by the resuspension of these benthic dinoflagellates from the biofilm by hydrodynamics. The data suggest that harmful microalgae could be the source of toxins in the studied stations, which provide support to the implication of these results on the future sampling strategy of harmful microalgae in shellfish collecting areas in Tunisia.