It is unclear how warming polar marine systems will alter the magnitude of diatom productivity and its fate within the food web. We examined diatom productivity and size-fractionated phytoplankton grazing losses to protozoan grazers in the northern Bering and Chukchi seas during June 2017. Sea ice was nearly absent and water temperatures were unseasonably warm; such conditions may be considered normal in future decades. Among 28 experiments conducted, five were in bloom conditions. Diatom biomass and production rates were similar to previous studies, suggesting the early ice retreat did not lead to appreciably reduced diatom growth. Statistical analyses showed that 77% of the variance in diatom growth rate could be explained by a combination of nutrients, light, and their interaction, but the interactive effect was most important (explaining 66% of the variance). Protozoan grazing intensity on phytoplankton was largely affected by size, specifically, grazing on larger phytoplankton (e.g. diatoms) was highly variable among stations, with many stations having unquantifiable rates. Protozoan grazers consumed an average of 23 ± 35% of growth at bloom stations and 55 ± 102% among non-bloom stations. For smaller phytoplankton, grazing was persistent and less variable spatially, consuming 64 ± 38% of growth at bloom stations and 79 ± 63% at non-bloom stations. Although previous studies (that did not size-fractionate samples) inferred that protozoan grazers control diatom biomass during blooms, our results suggest that diatom productivity largely escaped protozoan grazing losses, especially in bloom conditions, likely due to temporal lag between phytoplankton and protist biomass accumulation. Thus, during bloom conditions, it was estimated that 20–50 times more diatom organic matter was available for higher trophic levels and/or export (as opposed to water column remineralization) than under non-bloom conditions, despite only a 12-fold increase in gross diatom production in the bloom.

目前尚不清楚极地海洋系统变暖将如何改变硅藻生产力的大小及其在食物网中的命运。我们研究了 2017 年 6 月白令海北部和楚科奇海北部原生动物食草动物的硅藻生产力和按大小分级的浮游植物放牧损失。几乎没有海冰，水温异常温暖；这种情况在未来几十年可能被认为是正常的。在进行的 28 次实验中，有 5 次处于开花状态。硅藻生物量和生产率与之前的研究相似，表明早期冰退并没有导致硅藻生长明显减少。统计分析表明，77% 的硅藻生长速度变化可以通过营养、光及其相互作用的组合来解释，但交互效应是最重要的（解释了 66% 的差异）。原生动物对浮游植物的放牧强度在很大程度上受大小影响，特别是在较大的浮游植物（如硅藻）上的放牧在站点之间变化很大，许多站点的放牧率无法量化。原生动物食草动物在开花站消耗了平均 23 ± 35% 的生长量，在非开花站消耗了 55 ± 102%。对于较小的浮游植物，放牧是持久的，空间变化较小，在开花站消耗 64±38% 的生长，在非开花站消耗 79±63%。虽然之前的研究（没有对样品进行尺寸分级）推断原生动物食草动物在开花期间控制硅藻生物量，但我们的结果表明硅藻生产力在很大程度上避免了原生动物放牧损失，尤其是在开花条件下，可能是由于浮游植物和原生生物生物量积累之间的时间滞后。因此，在开花条件下，尽管仅增加了 12 倍，但据估计，与非开花条件相比，可用于更高营养级和/或输出（与水体再矿化相反）的硅藻有机质多 20-50 倍在盛开的总硅藻产量中。