he phytoplankton abundance, chlorophyll-a and diversity cycle in the Mediterranean sea water, west of Alexandria (Naubaria, reference site), El Mohandessin, Marakia and Marabella tourism villages were studied during the
perivd from March 2005 to Jan. 2006 and the results were discussed in relation to some environmental physico-chemical parameters. A total of 208 taxa were recorded in the study area and characterized by different ecological affinities
extending from typical marine forms (107 taxa), to freshwater euryhaline forms (101 taxa). The Naubaria site was the most diversified community (176 taxa) followed by El Mohandessin village site (90 taxa), Marakia village site (83 taxa)
and Marabella village site (54 taxa). Bacillariophyceae was the predominant group in the study area except in Marabella village site, where Chlorophyceae was predominant in September (99.53%). Few species were responsible for
abundance peaks, namely: Cyclotella kütziginiana, Rhizosolenia fragellissima (July), Rhizosolenia setigera (September), Skeletonema costatum (November) in Naubaria site; Skeletonema costatum (November) in El Mohandessin site;
Rhizosolenia setigera (September) in Mrakia site; Carteria sp. (September) in Marabella site. Chlorophyll-a varied greatly from an average of 0.77 μg/L at Marabella site to 2.66 μg/L at Naubaria site, classifying Naubaria and El-
Mohandessin sites between mesotrophic to oligotrophic status and Marakia and Marabella sites as oligotrophic. Species diversity had a negative effect on phytoplankton abundance, ranging between 0.53 and 2.99 nats at Naubaria site,
0.13 and 2.39 nats at El Mohandessin site, 0.56 and 2.54 nats at Marakia site,and between 0.03 and 2.79 nats at Marabella site. The blooms of few dominant species were accompanied with low diversity under the prevailing ecological
conditions reflecting the effect of wastewater effluents discharged in Naubaria, El-Mohandessin and Marabella sites. Thus, they should be kept under investigation to control pollution, whereas the co-dominance of several species,
accompanied with low nutrient concentrations at Marakia village site suggests free conditions.