Abstract
Phytoplankton are key indicators of marine ecosystem health and drivers of biogeochemical processes, yet their seasonal patterns in the southern Arabian Gulf are insufficiently understood. This study investigates inter-seasonal variability in phytoplankton composition and abundance along the coast of the United Arab Emirates (UAE), focusing on waters surrounding the region’s largest oyster beds. Surface samples were collected at nine sites during four seasons between 2020 and 2022. Phytoplankton dynamics were analyzed together with water chemistry parameters using multivariate techniques, i.e. Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA). A non-parametric Bray-Curtis dissimilarity analysis with PERMANOVA was also used to analyze seasonal differences in the phytoplankton abundance and composition and it confirmed DCA findings on clear community changes between January and June. Results revealed distinct seasonal assemblages: cyanobacteria prevailed during warmer months (May–June), while diatoms and cryptophytes dominated in cooler periods (November–January). Spatial differences in composition were evident, even between nearby locations, and likely reflect the influence of localized anthropogenic pressures and environmental gradients. Temperature, dissolved oxygen, chemical oxygen demand (COD), and oxidation-reduction potential (ORP) were identified as the principal environmental drivers, jointly explaining about 30% of phytoplankton variation. The unexplained variance in the phytoplankton data may be attributed to the impact of unmeasured water chemistry variables such as Si and toxic pollutant influx together with ecological variables, e.g. zooplankton grazing. Several potentially harmful taxa including dinoflagellates Scrippsiella spp. , Heterocapsa spp., and Blixaea quinquecornis , were detected albeit at low abundances. At the same time, a potentially toxic diatom group Pseudo - nitzschia spp. occurred between November and May at considerably higher concentrations, which are comparable with the European bloom threshold levels although no actual harmful algal blooms (HABs) were observed. This study contributes to understanding phytoplankton ecology in the Gulf coastal waters and highlights the need for long-term monitoring supporting sustainable marine resource management under growing environmental and climatic stressors.