Drivers of short-term variability in phytoplankton production in an embayment of the southern Benguela upwelling system

摘要: Abstract St Helena Bay (SHB), a retention zone in the southern Benguela upwelling system, supports 40–50% of region's primary productivity. It also experiences harmful algal blooms (HABs) and hypoxic conditions that are difficult to predict given high sub-seasonal variability. To better understand this variability, net production (NPP), nitrate ammonium uptake, phytoplankton community composition were monitored for ten days SHB during summer, season enhanced upwelling. A period active (days 1–5) was followed by one relaxation 6–10). During upwelling, mixed layer deeper than euphotic light-limited, evidenced ambient concentrations low rates NPP uptake. relaxation, stratification increased, restricting shallow which exhausted after three days; subsequent decline uptake confirm nutrient availability rapidly succeeded light as dominant control on Nanophytoplankton (mainly small diatoms, Chaetoceros spp. Skeletonema costatum) dominated biomass, NPP, throughout experiment. We attribute their ability respond quickly newly-upwelled sustain elevated longer pico- microphytoplankton. They may engage luxury storing intracellularly under high-nutrient assimilating it later when nutrients depleted. Additionally, S. costatum chain-formers, benefitting from advantages being (i.e., individual cells) large aggregated into chains). weakening surface late experiment have prevented dinoflagellates, some HAB species, succeeding diatoms. One implication is understanding rapid cycling between limitation phytoplankton, induced an actively-upwelling versus stratified water column, improve our capacity anticipate HABs associated events.