Anthropogenic marine eutrophication is one of the biggest risks to the health of marine environments, causing ecosystem disruption. In recent years, eutrophication events, together with climate change, have accelerated the development of potentially toxic algal species, causing severe harmful algal blooms (HABs) to be more common in coastal areas.Dinoflagellates, which make up a significant portion of marine phytoplankton, could create hazardous algal blooms.The results revealed that the phytoplankton community in Alexandria's Eastern Harbor (EH) consisted of 26 species from 24 genera, with dinoflagellates dominating (14 species). The total phytoplankton standing crop showed an average count of 5.277 x 10⁶ units L^-1, and biomass (Chlorophylla) of 5.63µg L^-1. Dinoflagellates accounted for 98.95% of the total phytoplankton abundance and were dominated by Scrippsiella trochoidea, followed by Pyrophacus horologium, Gyrodenium fusiforme and Gymnodinum catenatum.Scrippsiella trochoidea exhibited an extremely high count of 5.152 x 10⁶ units L^-1,contributing for 98.66% of the total dinoflagellates, 97.62% of the total numerical standing crop. S. trochoidea was the causative species of brown tide blooms and water discoloration that suddenly appeared in the EH. These blooms killed a vast number of fishes, resulting in severe economic losses in the study area.