Detecting and attributing change in net primary production using carbon and nitrogen isotopes.

Net primary production (NPP) sets the foundation for the ocean’s basic ecosystem services: fisheries production and carbon sequestration. While warming is predicted to reduce tropical NPP and stimulate polar NPP, the evolution of NPP in a future ocean remains highly uncertain and challenging to detect, owing to the multifarious impacts of different drivers of change. Here, we examine NPP under an RCP8.5 climate change scenario through the lens of carbon and nitrogen isotopes (δ¹³C and δ¹⁵N), as simulated within a state-of-the-art ocean general circulation and biogeochemistry model. When used together, these isotopes are highly sensitive detectors and attributors of change. Their value lies in predictable fractionation associated with carbon fixation and gas exchange, nutrient uptake, nitrogen fixation and nitrogen loss via denitrification. Moreover, the invasion of anthropogenic carbon and nitrogen have unique isotopic signatures. Shifts in how carbon and nitrogen cycle through the ocean therefore imprint strongly on the δ¹³C and δ¹⁵N of dissolved and organic material. To this end, we use δ¹³C and δ¹⁵N to detect change in NPP and attribute these changes to shifts in carbon and nitrogen cycling. In particular, we focus on changes occurring at ocean time-series sites and repeat hydrography lines to facilitate comparison with observational datasets. Our results demonstrate that δ¹³C and δ¹⁵N measurements will be useful in the coming decades to detect important shifts in marine ecosystems and NPP.