The counterintuitive effect of summer-to-fall mixed layer deepening on eukaryotic new production in the Sargasso Sea
The counterintuitive effect of summer-to-fall mixed layer deepening on eukaryotic new production in the Sargasso Sea
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DOI：
10.1002/2013gb004579
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发表时间：
2014-02-01
影响因子：
5.2
通讯作者：
Sigman, Daniel M.
中科院分区：
文献类型：
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作者：
Fawcett, Sarah E.;Lomas, Michael W.;Sigman, Daniel M.
马尾藻海的特点是夏季层化强烈，被认为是导致营养不良的原因，但出口产品与主要营养物质供应充足的高纬度地区惊人地相似。在这里，我们使用马尾藻海西北部夏季到秋季的序列来研究上层海洋层化和浮游植物氮(N)吸收之间的关系。用流式细胞仪对7、10、12月份采集的真光带颗粒物进行分选，并对分离出的原核生物和真核浮游植物的N-15/N-14进行分析。原氯球菌和聚球藻的N-15/N-14始终较低，表明对循环氮的依赖程度一致。在7月和两次秋季剖面中，真核浮游植物的N-15/N-14较高，反映了对表层下硝酸盐的消耗。在其他三个秋季剖面中，真核生物的N-15/N-14与原核生物的N-15/N-14相似，表明向更完全依赖循环N的转变。从夏季到秋季，混合层的逐渐加深，尽管减小了从表层到深层的密度对比，但增加了整个真光带与营养丰富的下层水的密度差异，这可能在秋季真光带硝酸盐供应下降的原因中起到了作用。当混合层最浅时，夏季硝酸盐向真光层的明显供应可能有助于解释副热带和热带海洋令人惊讶的高出口产量。
The Sargasso Sea is characterized by strong summertime stratification that is thought to drive oligotrophy, but export production is surprisingly similar to that of high-latitude regions with ample major nutrient supply. Here we use the summer-to-fall progression in the northwestern Sargasso Sea to investigate the relationship between upper ocean stratification and phytoplankton nitrogen (N) uptake. Euphotic zone particles collected in July, October, and December were sorted by flow cytometry, and the N-15/N-14 of separated prokaryotic and eukaryotic phytoplankton was analyzed. The N-15/N-14 of Prochlorococcus and Synechococcus was always low, indicating uniform reliance on recycled N. In July and in two fall profiles, the N-15/N-14 of eukaryotic phytoplankton was high, reflecting consumption of subsurface nitrate. In three other fall profiles, eukaryotic N-15/N-14 was similar to prokaryote N-15/N-14, suggesting a shift toward more complete reliance on recycled N. The progressive deepening of the mixed layer from summer to fall, although reducing the surface-to-deep density contrast, increases the density difference of the euphotic zone as a whole from underlying nutrient-rich waters, which may play a role in the observed decline in euphotic zone nitrate supply into the fall. The apparent summertime nitrate supply to the euphotic zone, when the mixed layer is shallowest, may help to explain the surprisingly high export production of the subtropical and tropical ocean.