In eastern boundary current systems, strong coastal upwelling brings deep, nutrient-rich waters to the surface ocean, supporting a productive food web. The nitrate load in water masses that supply the region can be impacted by a variety of climate-related processes that subsequently modulate primary productivity. In this study, two coastal upwelling regimes along central and southern California were sampled seasonally for nitrogen and oxygen stable isotopes of nitrate (i.e., nitrate isotopes) over several years (2010–2016) on 14 California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruises. Seasonal, interannual, and spatial variations in euphotic zone nitrate isotopes were largely driven by the extent of nitrate utilization, sometimes linked to iron limitation of diatom productivity. Pronounced isotopic enrichment developed with the El Niño conditions in late 2015 and early 2016 which likely resulted from increased nitrate utilization linked to reduced nitrate supply to the euphotic zone. Differential enrichment of nitrogen and oxygen isotopes was observed in the surface ocean, suggesting that phytoplankton increased their reliance on locally nitrified (recycled) nitrate during warmer and more stratified periods. Overall, nitrate isotopes effectively differentiated important euphotic zone processes such as nitrate assimilation and nitrification, while archiving the influence of disparate controls such as iron limitation and climatic events through their effects on nitrate utilization and isotopic fractionation.

中文翻译：

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2015-2016年厄尔尼诺现象的硝酸盐稳定同位素记录效应和硅藻铁对北太平洋东部氮循环的限制

在东部边界洋流系统中，强烈的沿海上升流将富含营养的深层海水带到表层海洋，从而支持生产性食物网。供应该地区的水团中的硝酸盐负荷可能受到各种气候相关过程的影响，这些过程随后会调节初级生产力。在这项研究中，在加利福尼亚州海洋渔业合作调查 (CalCOFI) 的 14 次航行中，在数年（2010 年至 2016 年）期间，对沿加利福尼亚中部和南部的两个沿海上升流区域进行了季节性采样，以获取硝酸盐的氮和氧稳定同位素（即硝酸盐同位素）。透光区硝酸盐同位素的季节性、年际和空间变化主要受硝酸盐利用程度的驱动，有时与硅藻生产力的铁限制有关。2015 年末和 2016 年初的厄尔尼诺现象导致了明显的同位素富集，这可能是由于硝酸盐利用率增加与透光区硝酸盐供应减少有关。在表层海洋中观察到氮和氧同位素的差异富集，这表明浮游植物在更温暖和更分层的时期增加了它们对局部硝化（回收）硝酸盐的依赖。总体而言，硝酸盐同位素有效区分了重要的透光带过程，如硝酸盐同化和硝化作用，同时通过它们对硝酸盐利用和同位素分馏的影响，存档了不同控制因素（如铁限制和气候事件）的影响。