Phytoplankton in the Arctic Ocean are subject to nitrogen limitation in the summer, however, how severely the nitrogen stress affects phytoplankton physiology remains largely unknown. In the summers of 2015–2018, we examined the distribution of phytoplankton photophysiological properties across two contrasting regions of the Arctic Ocean with distinctly different levels of nitrogen availability in the upper water column. We quantified the extent of nitrogen stress using a highly sensitive fluorescence induction and relaxation system to obtain continuous underway measurements and via discrete sample analyses of phytoplankton physiology, as well as nutrient enrichment incubations. The results revealed vast regions in the Chukchi Sea where phytoplankton photosynthesis was severely nitrogen‐stressed. Thereby, the maximum quantum yield of photochemistry in photosystem II showed only a small decrease (12 ± 9%) relative to its nutrient‐replete values, while the maximum photosynthetic electron transport rates under saturating irradiance were impaired to a greater extent (40 ± 17%). This phytoplankton photosynthesis response is indicative of a severe nitrogen limitation, which results in dramatic reduction in growth and net primary production rates. Nutrient enrichment incubations also revealed a marked increase in large‐size phytoplankton growth (>10 μm) after the nitrogen stress was alleviated, suggesting that the larger cells were more susceptible to nitrogen stress. These results are important for understanding how regional nitrogen fluxes control variability in the primary production and phytoplankton community structure and how these processes might change with rapid climate changes in the Arctic Ocean.

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夏季氮素限制对北冰洋西部浮游植物生理的影响

在夏季，北冰洋的浮游植物受到氮的限制，但是，氮胁迫对浮游植物生理的影响有多严重，目前尚不清楚。在2015–2018年夏季，我们检查了北冰洋两个不同区域浮游植物光生理特性的分布，其中上部水柱中氮的利用率明显不同。我们使用高度灵敏的荧光诱导和弛豫系统对氮胁迫的程度进行了量化，以获得连续进行的测量结果，并通过浮游植物生理学以及营养物富集培养的离散样本分析。结果表明楚科奇海的广大地区浮游植物的光合作用受到严重的氮胁迫。从而，相对于其营养素含量值，光系统II中光化学的最大量子产率仅显示出很小的下降（12±9％），而饱和辐照度下的最大光合电子传输速率受到较大程度的损害（40±17％） 。浮游植物的光合作用反应表明严重的氮限制，这导致生长和净初级生产力的急剧下降。营养富集孵育还显示，缓解氮胁迫后，大型浮游植物的生长（> 10μm）显着增加，这表明较大的细胞更容易受到氮胁迫的影响。