Phytoplankton distribution in the Nador lagoon (Morocco) and possible risks for harmful algal blooms

摘要

1 - Phytoplankton populations and environmental parameters were analysed seasonally in the lagoon of Nador (Morocco) over an annual cycle (2006-2007) at six stations. Seven phytoplankton Classes were observed: Diatomophyceae, Dinophyceae, Raphidophyceae, Prasinophyceae, Euglenophyceae, Dictyochophyceae and Cyanophyceae. Seven taxa were identified that may present a risk of inducing harmful blooms i.e. Alexandrium minutum, Pseudonitzschia delicatissima and P. fraudulenta, Prorocentrum minimum, Prorocentrum rhatymum, Dinophysis sacculus, Gymnodinium catenatum and Karenia mikimotoi. Physico-chemical analyses were carried out on water samples: Dissolved Organic Carbon (DOC) and nutrients (NH4 +, NO3-, PO4 3- and SiO2).2 - Favourable and unfavourable factors for bloom development were identified. The possible relations between environmental parameters and the occurrence of potentially harmful phytoplankton blooms were investigated through a Factorial Discriminant Analysis. The proliferation of potentially harmful phytoplankton was linked to the availability of various nutrients (N, P, Si). Prorocentrum minimum tolerated waters with high nitrate contents, while Alexandrium minutum tolerated nutrientpoor waters. The most frequently observed taxa, Pseudonitzschia species and Dinophysis sacculus, were adapted to variations of water quality. Excessive levels of nutrients appeared as inhibiting for proliferation of potentially harmful phytoplankton which disappeared for the benefit of a very dense bloom of Tetraselmis sp. (Prasinophyceae).3 - A risk of toxicity was observed during all samplings throughout the lagoon, it was particularly pronounced during the summer sampling in the northern continental part of the lagoon with proliferation of Pseudonitzschia species.