Diatom community succession and bloom variability as a function of winter-mixing depth in the subtropical Gulf of Aqaba, Red Sea

Although diatoms dominate primary production in high-latitude, nutrient-rich ecosystems, less is known about their ecology in low-latitude, oligotrophic ecosystems. We investigated the seasonal succession of planktonic diatoms for ~2 yr in the subtropical Gulf of Aqaba (Red Sea) by scanning electron microscopy. This ecosystem alternates between stratified oligotrophic conditions during spring/summer and deeply mixed eutrophic conditions during fall/winter. Diatom cell density (reaching <3 cells ml^–1) and diversity were lower during the stratified season. Communities were dominated by small pennate cells, likely due to their higher ability to acquire nutrients during limiting conditions. An increase in diatom density occurred during the mid-winter mixing season (reaching >48 cells ml^–1), caused by a rise in small centric cells (Thalassiosiraceae) as nitrogen concentrations surpassed ~0.6 μmol l^–1. This shift in density occurred months later than the peak in total phytoplankton, suggesting higher nutrient requirements for diatom growth. Nearly 2 wk long diatom blooms were detected during the mixing-to-stratification transition in spring, coinciding with an annual peak in chlorophyll a. Under milder conditions, mixing was shallow and diatom blooms reached ~107 cells ml^–1. Small Thalassiosiraceae and several pennate species dominated. However, during the colder April of 2022, the mixing depth reached ~700 m. Consequently, nutrient concentrations were higher, and blooms reached ~574 cells ml^–1, enabling higher diversity and density of chain-forming species (Chaetoceros and Leptocylindrus) and high Chaetoceros resting spore production. Restratification and nutrient depletion led to rapid bloom decline. This study demonstrates the marked seasonal variability and responses of diatoms to environmental variability in marine subtropical ecosystems.