Climate change has led to a ~ 40% reduction in summer Arctic sea-ice cover extent since the 1970s. Resultant increases in light availability may enhance phytoplankton production. Direct evidence for factors currently constraining summertime phytoplankton growth in the Arctic region is however lacking. GEOTRACES cruise GN05 conducted a Fram Strait transect from Svalbard to the NE Greenland Shelf in summer 2016, sampling for bioessential trace metals (Fe, Co, Zn, Mn) and macronutrients (N, Si, P) at ~ 79°N. Five bioassay experiments were conducted to establish phytoplankton responses to additions of Fe, N, Fe + N and volcanic dust. Ambient nutrient concentrations suggested N and Fe were deficient in surface seawater relative to typical phytoplankton requirements. A west-to-east trend in the relative deficiency of N and Fe was apparent, with N becoming more deficient towards Greenland and Fe more deficient towards Svalbard. This aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in + N treatment near Greenland and + N + Fe near Svalbard. Collectively these results suggest primary N limitation of phytoplankton growth throughout the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. We suggest that the supply of Atlantic-derived N and Arctic-derived Fe exerts a strong control on summertime nutrient stoichiometry and resultant limitation patterns across the Fram Strait region.

自 20 世纪 70 年代以来，气候变化已导致夏季北极海冰覆盖面积减少约 40%。由此产生的光利用率的增加可能会提高浮游植物的产量。然而，目前限制北极地区夏季浮游植物生长的因素缺乏直接证据。GEOTRACES 游轮 GN05 于 2016 年夏季从斯瓦尔巴群岛到格陵兰岛东北部陆架进行了弗拉姆海峡横断面测量，在约 79°N 处采样了生物必需的微量金属（铁、钴、锌、锰）和常量营养素（氮、硅、磷）。进行了五次生物测定实验，以确定浮游植物对添加铁、氮、铁+氮和火山尘的反应。环境营养物浓度表明，相对于典型浮游植物的需求，表层海水缺乏氮和铁。氮和铁的相对缺乏呈现出明显的从西到东的趋势，格陵兰岛的氮缺乏程度更高，斯瓦尔巴特群岛的铁缺乏程度更高。这与生物测定实验中浮游植物的反应一致，生物测定实验表明，格陵兰岛附近的+ N 处理和斯瓦尔巴群岛附近的+ N + Fe 处理中叶绿素-a 增加最多。总的来说，这些结果表明整个研究区域浮游植物生长的主要氮限制，而弗拉姆海峡东部的条件可能接近次要铁限制。我们认为，来自大西洋的氮和来自北极的铁的供应对整个弗拉姆海峡地区夏季养分化学计量和由此产生的限制模式产生了强有力的控制。