Ecological roles of nano-picoplankton in stratified waters of an embayment in the southern Benguela.

Nano-picoplankton are the dominant primary producers during the postupwelling period in St Helena Bay, South Africa. Their dynamics on short timescales are not well-understood and neither are the community composition, structure, and potential functionality of the surrounding microbiome. Samples were collected over five consecutive days in March 2018 from three depths (1, 25, and 50 m) at a single sampling station in St Helena Bay. There was clear depth-differentiation between the surface and depth in both diversity and function throughout the sampling period for the archaea, bacteria, and eukaryotes. Daily difference in eukaryote diversity, was more pronounced at 1 and 25 m with increased abundances of Syndiniales and Bacillariophyta. Surface waters were dominated by photosynthetic and photoheterotrophic microorganisms, while samples at depth were linked to nitrogen cycling processes, with high abundances of nitrifiers and denitrifiers. Strong depth gradients found in the nutrient transporters for ammonia were good indicators of measured uptake rates. This study showed that nano-picoplankton dynamics were driven by light availability, nutrient concentrations, carbon biomass, and oxygenation. The nano-picoplankton help sustain ecosystem functioning in St Helena Bay through their ecological roles, which emphasizes the need to monitor this size fraction of the plankton.