INFLUENCE OF PHOSPHORUS-CONTAMINATED SEDIMENTS IN THE ABUNDANCE OF POTENTIALLY TOXIC PHYTOPLANKTON ALONG THE SFAX COASTS (GULF OF GABES, TUNISIA)

This is the first work performed in the shellfish production area located along the Sfax coasts (southern Tunisia) on the spatial and temporal patterns of toxic phytoplankton. It relates the excessive introduction of phosphorus in coastal waters from sediments contaminated with that nutrient. A multivariate approach was applyed using data derived from the National Phytoplankton Monitoring Program (REPHY) (2006-2009). We also examine if there is a direct relationship between the abundance of toxic phytoplankton and physical and chemical parameters. This study is based on phytoplankton composition and abundance, as well as physical and chemical data to evaluate the ecological status of the Sfax coasts, at shellfish farms. A total of 13 taxa included in the Intergovernmental Oceanographic Commission (IOC) toxic algae checklist and well-known bloom formers were identified in REPHY. Higher nutrient spring samples were distinguished from those of lower nutrient summer waters. The Redundancy Analysis (RDA) separated the toxic species into two groups related to nutrients availability. The large amounts of phosphorus and organic matter affected the toxic phytoplankton structure, due to the pollution of chemical origin underlining an organic load hardly biodegradable in Sfax coasts. Many of these species recorded in the water column were benthic dinoflagellates, a fact that could be explained by the resuspension of these organisms by hydrodynamics. The knowledge obtained in this study can be used to develop best management practices of the sediment compartment as well as the water column, which is crucial in the framework of any phytoplankton monitoring program.