Nutrients limiting phytoplankton growth in the ocean are a critical control on ocean productivity and can underpin predicted responses to climate change. The extensive western subtropical North Pacific is assumed to be under strong nitrogen limitation, but this is not well supported by experimental evidence. Here, we report the results of 14 factorial nitrogen–phosphorus–iron addition experiments through the Philippine Sea, which demonstrate a gradient from nitrogen limitation in the north to nitrogen–iron co-limitation in the south. While nitrogen limited sites responded weakly to nutrient supply, co-limited sites bloomed with up to ~60-fold increases in chlorophyll a biomass that was dominated by initially undetectable diatoms. The transition in limiting nutrients and phytoplankton growth capacity was driven by a gradient in deep water nutrient supply, which was undetectable in surface concentration fields. We hypothesize that this large-scale phytoplankton response gradient is both climate sensitive and potentially important for regulating the distribution of predatory fish.

中文翻译：

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副热带西北太平洋的营养共同限制

限制海洋中浮游植物生长的营养物质是对海洋生产力的关键控制，可以支撑对气候变化的预测反应。广泛的西副热带北太平洋被认为受到强烈的氮限制，但这并没有得到实验证据的充分支持。在这里，我们报告了通过菲律宾海进行的 14 个阶乘氮-磷-铁添加实验的结果，这些实验证明了从北部的氮限制到南部的氮-铁共限制的梯度。虽然氮限制的地点对养分供应的反应很弱，但共同限制的地点开花，叶绿素a增加了约 60 倍以最初无法检测到的硅藻为主的生物量。限制养分和浮游植物生长能力的转变是由深水养分供应的梯度驱动的，这在地表浓度场中是无法检测到的。我们假设这种大规模的浮游植物响应梯度既对气候敏感，对调节捕食性鱼类的分布也可能很重要。