Functional group dynamics and environmental forcing of microphytoplankton in the Persian Gulf: Temporal patterns from Khark and Qeshm Islands

Phytoplankton communities play a crucial role in marine ecosystem resilience and biogeochemical cycling, yet their functional responses to environmental fluctuations in extreme environmental conditions like the Persian Gulf remain poorly understood. This study investigates the temporal and spatial structuring of microphytoplankton communities around Khark and Qeshm Islands in the Persian Gulf over an annual cycle (November 2021 to December 2022), integrating taxonomic diversity with a trait-based functional approach to assess adaptive strategies under varying physicochemical conditions. Monthly sampling was conducted at six stations (three per island), and phytoplankton functional diversity was quantified based on nine key traits: photosynthetic pigment composition, growth form, body size, motility, nitrogen-fixing ability, trophic strategy, cell wall composition, long projections, and vertical migration. The results reveal distinct ecological regimes between the two islands. The phytoplankton communities in Qeshm Island are taxonomically diverse but functionally stable, reflecting ecological stability under relatively constant environmental conditions. In contrast, the Khark phytoplankton communities exhibit pronounced seasonal shifts in functional diversity shaped by fluctuations in salinity, carbonate chemistry, and nutrient availability. Clustering analysis identified six functional groups with two dominant assemblages: Group 6 (Trichodesmium erythraeum, a diazotrophic cyanobacterium) and Group 4 (chain-forming diatoms such as Chaetoceros spp.). Carbonate saturation state, nitrate availability, and temperature were identified as key environmental factors shaping both taxonomic and functional community composition (as indicated by Canonical Correspondence Analysis (CCA) and Fourth-Corner Analysis). Furthermore, large-scale Trichodesmium erythraeum blooms in Khark during summer and early autumn significantly altered carbonate system equilibrium, a novel insight into phytoplankton trait-environment relationships in the Persian Gulf, highlighting the capacity of diazotrophic cyanobacteria to drive biogeochemical feedback loops through nitrogen fixation, pH modulation, and shifts in carbonate saturation state, with potential implications for regional carbon cycling.