Our results highlight that δ 15N changes in the low latitude twilight zone may provide a useful constraint on emerging changes to nitrogen limitation and NPP over the 21 st century. Model experiments show that ~75% of the low latitude twilight zone develops anomalously low δ 15N by 2060, predominantly due to the effects of climate change that alter ocean circulation, with implications for the nitrogen source-sink balance. Here we show that intensifying nitrogen limitation of phytoplankton, associated with near-term reductions in NPP, causes detectable declines in nitrogen isotopes (δ 15N) and constitutes the primary perturbation of the 21 st century nitrogen cycle. How the nitrogen cycle responds will determine long-term trends in net primary production (NPP) in the nitrogen-limited low latitude ocean, but is poorly constrained by uncertainty in how the source-sink balance will evolve. The open ocean nitrogen cycle is being altered by increases in anthropogenic atmospheric nitrogen deposition and climate change.
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