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

中文翻译：

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南本格拉上升流系统海湾浮游植物产量短期变化的驱动因素

摘要 圣赫勒拿湾 (SHB) 是本格拉南部上升流系统中的一个滞留区，支持该地区 40-50% 的初级生产力。它还经历了有害的藻华 (HAB) 和低氧条件，鉴于亚季节性变化很大，这些条件难以预测。为了更好地了解这种变异性，在夏季（上升流增强的季节）期间，对 SHB 的净初级产量 (NPP)、硝酸盐和铵吸收量以及浮游植物群落组成进行了十天的监测。一段活跃的上升流（第 1-5 天）之后是一段放松期（第 6-10 天）。在上升流期间，混合层比富光区更深，浮游植物受光限制，环境硝酸盐浓度高，NPP 和硝酸盐吸收率低，证明了这一点。在放松期间，分层增加，将浮游植物的生产限制在三天后硝酸盐耗尽的浅层富光区；随后 NPP 和硝酸盐吸收率的下降证实了养分供应迅速成功地成为了生产力的主要控制因素。在整个实验过程中，纳米浮游植物（主要是小硅藻、角毛藻和中肋骨藻）主导了生物量、NPP 和硝酸盐的吸收。我们将此归因于它们对新上涌的硝酸盐快速反应的能力，以及比微型浮游植物和微型浮游植物维持更高的硝酸盐吸收率更长时间的能力。它们还可能参与奢侈的硝酸盐摄取，在高营养条件下将硝酸盐储存在细胞内，并在营养耗尽后吸收。此外，角角藻属。和 S. costatum 是链形成者，受益于小（即作为单个细胞）和大（即聚合成链时）的优势。实验后期表面分层的减弱可能阻止了甲藻，其中一些是 HAB 物种，无法继承硅藻。这样做的一个含义是，了解由主动上升水柱与分层水柱引起的 SHB 浮游植物光和营养限制之间的快速循环，可能会提高我们预测 HAB 和相关缺氧事件的能力。