Stable isotopes of nitrate record effects of the 2015-2016 El Nino and diatom iron limitation on nitrogen cycling in the eastern North Pacific Ocean

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 Nino 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.

Automated summary

In eastern boundary current systems, coastal upwelling plays a vital role in influencing the surface ocean dynamics and productivity. This study investigated the seasonal, interannual, and spatial variations of nitrate isotopes in two coastal upwelling regimes in California. The results revealed that nitrate utilization and iron limitation of diatom productivity were the major drivers of euphotic zone nitrate isotopes. The El Nino conditions in 2015-2016 led to increased nitrate utilization and isotopic enrichment. Moreover, the differential enrichment of nitrogen and oxygen isotopes indicated a shift in phytoplankton's reliance on locally nitrified nitrate during warmer and stratified periods.