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Abstract

Seasonal variation in plankton community structure (phytoplankton and zooplankton), chlorophyll a and some physico-chemical properties of Rabigh Lagoon were studied during 2015. The study area included seven sites situated between latitudes 22.9978° to 22.8519° and longitudes 38.8339° to 38.9616°E. The ecological parameters comprise some physical and chemical parameters as Temperature, Secchi Disc, pH, DO, S%, Nutrients “Nitrite, NO2-N; Nitrate, NO3-N; Ammonia, NH3-N; total nitrogen, T-N; Reactive phosphate, PO4-P and total phosphorus, T-P”. A total of 103 species belonging to five phytoplanktonic groups were identified. The most leadings group is Bacillariophyceae (45.05%) then Cyanophyceae (33.83%) followed by Dinophyceae (20.99%). On the other hand, Chrysophyceae (0.07%). and Chlorophyceae (0.05) are considered a rare group. Chlorophyll a showed almost the same pattern with the standing crops of the phytoplankton ranged between 12.33 μg l-1 during spring and a lowest value of 5.49 μg l-1 during summer. The most dominant group of zooplankton was Copepoda (89.00%) followed by Protista (7.30%) and Rotifera (3.70%). The maximum peak of Copepoda was recorded during all the investigation period except autumn. The Diversity Index fluctuated between 2.785 during autumn and 3.568 during summer. The calculating of trophic status index according to Secchi Disc methods was found to be better than chlorophyll a and total phosphorus methods. The analysis showed that Rabigh Khore is a mesotrophic ecosystem. Canonical correspondence analysis (CCA) was additionally completed to break down the relations between the physico- Chemical parameters and the plankton. It showed that the most important factors affecting plankton distribution.

Keywords: Rabigh Lagoon; Phytoplankton; Zooplankton; Trophic status index; Diversity index; Canonical corresponding analysis