Dynamique du peuplement phytoplanctonique le long de l’axe côtier Casablanca – El Jadida

Abstract

Our taxonomic study from March 2014 to November 2015 along the atlantic coast between Casablanca and El Jadida revealed that phytoplanktonic structure is mainly represented by Diatoms (Bacillariophyceae), Dinoflagellates (Dinophyceae), Silicoflagellates (Dictyophyceae) and Euglenophyceae with a clear dominance of Diatoms and Dinoflagellates. A total of 101 taxa of planktonic algae have been identified revealing a relatively diversified taxocenosis. In terms of respective diversity, the Diatoms are represented by 62 taxa (61.4%), the Dinoflagellates by 36 taxa (35.6%) whereas the other two groups of Silicoflagellates and Euglenophyceae are only represented by 3 species (3.0%). Regarding potentially toxic species, more than fifteen taxa have been identified, most of which were Dinoflagellates and Diatoms (Pseudo-nitzschia australis and Pseudo-nitzschia cuspidata). The total phytoplankton densities exhibited great spatial and temporal variations as shown by analyses of diversity (H') and equitability (E) indices at the different coast sites investigated throughout the 2014-2015 years. Massive proliferation of some toxic species (e.g. Pseudo-nitzschia australis and Pseudo-nitzschia cuspidata, Lingulodinium polyedrum, Karenia mikimotoi) was also noticed. In addition, our study revealed the presence of opportunistic species (e.g. Eutreptiella, Thalassiosira, Prorocentrum scutellum) and of the new ectoparasite Dinoflagellate Amyloodinium ocellatum for the first time in Morocco. The detection of such diversity of toxic species, sometimes with alarming concentrations, should prompt the competent authorities to broaden the spectrum and frequency of biomonitoring to uncontrolled seafood harvesting sites.