A new recorded of red tide forming species; Heterocapsa triquetra, Gymnodinium impudicum, Heterosigma akashiwo and Thalassiosira rotula in Alexandria Waters, Egypt

The current study is based on daily monitoring observations to follow phytoplankton blooms in the Eastern Harbor (Alexandria) within a year cycle. The harbor ecosystem was harsh and characterized by various dynamics and variable conditions of a multitude of environmental factors that ultimately were considered strong stressors on phytoplankton development. Three red tide blooms reach extremely high biomass leading to water discoloration triggered in late summer-early autumn, and during the last week of December as well. These blooms were considered unique as represented by newly reported red tide causative species of different groups never previously reported in the Egyptian Mediterranean waters as a red tide bloom species; Heterocapsa triquetra and Gymnodinium impudicum (Dinophyceae), Heterosigma akashiwo (Raphidophycea) and the centric diatom Thalassiosira rotula, and under influence of characterized environmental conditions. The highest bloom peak of H. triquetra occurred on 9 August (12.97x 10⁶ cells L^-1), H. akashiwo on 26 August (13.91x10⁶ cells L^-1), G. impudicum on 16 September (7.12x10⁶ cells L^-1) and T. rotula on 28 December (3.25 x 10⁶ cells L^-1). The blooms of the first three species maintained much higher temperature (30.4 - 32.9 ⁰C) and lower salinity (28.6 - 29.3) compared with the winter bloom in December, while nutrient concentrations exhibit considerable variations and the N/P ratio falls down to a minimum with the bloom peak days. The significant contribution of physical forcing rather than chemical on bloom developments was statistically confirmed but failed to define specific controlling factor/s. The winter bloom of Thalassiosira rotula was a surprising and unique first winter red tide bloom in the Eastern Harbor, under minimum annual temperature (18.8°C - 19.6 °C), stresses the strong effect of NO₃, NO₂ and OOM on the bloom development. These blooms maintained higher OOM relative to inorganic nutrients. No fish mortality occurred. The present work offers persuasive evidence for the increased number of newly recorded red tide bloom-forming species in Alexandria waters.