Diatom community shifts across oligotrophic-eutrophic seasonal gradients and bloom variability as a function of mixing depth in the subtropical Northern Red Sea

Diatom blooms dominate nutrient-rich ecosystems. Less is known about the ecology and bloom dynamics of diatom populations in oligotrophic ecosystems. Here, we investigated seasonal succession of planktonic diatoms in the Gulf of Aqaba (GoA) at the northern Red Sea. The GoA is a subtropical ecosystem alternating between stratified, oligotrophic profiles during summer, and deeply mixed, mesotrophic during winter. Diatom density and diversity were lower during the stratified season, dominated by pennate species, and increased at mid-winter as nitrate exceeded [~]0.5 {micro}mol L-1. Diatom density lagged after total phytoplankton and entailed a transition to centric-diatom dominance, suggesting both higher nutrient requirements for diatom growth and ecophysiological differences between morphotypes. Ephemeral blooms were detected at the mixing-to-stratification transition. Under milder conditions, mixing was shallow and diatoms reached [~]98 individuals. mL-1. Small-centric Thalassiosiraceae and several pennates dominated. However, during the following colder year, mixing depth reached [~]700 m. Consequently, nutrient concentrations were higher and diatoms reached [~]390 individuals. mL-1. This enabled emergence of chain-forming species (namely Chaetoceros and Leptocylindrus) along small-centric and pennates, and high spore abundance was detected. Restratification led to rapid bloom decline. These results illustrate diatom community succession and bloom development as a function of nutrient availability in subtropical ecosystems.