Diazotrophy as a key driver of the response of marine net primary productivity to climate change

Abstract

Abstract. The impact of anthropogenic climate change on marine net primary production (NPP) is a reason for concern because changing NPP will have widespread consequences for marine ecosystems and their associated services. Projections by the current generation of Earth system models have suggested decreases in global NPP in response to future climate change, albeit with very large uncertainties. Here, we make use of two versions of the Institut Pierre-Simon Laplace Climate Model (IPSL-CM) that simulate divergent NPP responses to similar high-emission scenarios in the 21st century and identify nitrogen fixation as the main driver of these divergent NPP responses. Differences in the way N fixation is parameterised in the marine biogeochemical component PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies) of the IPSL-CM versions lead to N-fixation rates that are either stable or double over the course of the 21st century, resulting in decreasing or increasing global NPP, respectively. An evaluation of these two model versions does not help constrain future NPP projection uncertainties. However, the use of a more comprehensive version of PISCES, with variable nitrogen-to-phosphorus ratios as well as a revised parameterisation of the temperature sensitivity of N fixation, suggests only moderate changes in globally averaged N fixation in the 21st century. This leads to decreasing global NPP, in line with the model-mean changes of a recent multi-model intercomparison. Lastly, despite contrasting trends in NPP, all our model versions simulate similar and significant reductions in planktonic biomass. This suggests that projected plankton biomass may be a more robust indicator than NPP of the potential impact of anthropogenic climate change on marine ecosystems across models.

Nutrient uptake plasticity in phytoplankton sustains future ocean net primary production

“…S11A). On the other hand, the rest of the Earth system models (table S2) tend to project overall declining marine NPP with two exceptions that use the same ocean biogeochemistry module: Projected NPP increases in those models result from alleviating P stress by releasing additional PO 4 during N 2 fixation (mimicking an unresolved utilization of DOP pool by diazotrophs) (30), conceptually consistent with a critical role of upper ocean P availability in NPP sustainability advocated in our study. The 21st century projections of POC export out of the euphotic zone also remain distinct between the two groups of Earth system models, with the models of flexible C:N*:P:Fe tending to project less reduction in the biological carbon export (fig.…”

“…5F ). The external source of N mediates the flexible C:P effect on sustaining future ocean NPP through an increased N availability within the euphotic zone, thereby explaining the greater simulated impacts of phytoplankton C:P plasticity on global ocean NPP obtained in our study, as compared to previous studies ( 17 , 18 , 30 ). The coupled changes in C:P ratios and N 2 fixation also explain the disproportionately higher sensitivity of ocean NPP to phytoplankton C:P plasticity in the warm subtropical gyre [where bottom-up nutrient, light, and temperature controls and top-down grazing pressure by zooplankton favor both diazotroph growth and biomass ( 37 )].…”

“…Bopp et al . ( 30 ) identified N 2 fixation as an important driver of future oceanic NPP responses to simulated 21st century climate changes. Our model simulations that include a prognostic representation of micronutrient iron (Fe) cycles (a primary limiting nutrient for N 2 -fixing microbes) and flexible phytoplankton C:Fe ratios ( 39 ) highlight the importance of N 2 fixation and flexible C:Fe ratios in mediating the variable C:P effects on ocean NPP changes.…”

“…The recent generation of Earth system models participating in Coupled Model Intercomparison Project Phase 6 (CMIP6) allows for greater physiological, taxonomical, and functional flexibility of phytoplankton and subsequent cycling of nutrients and carbon than previous generations (28)(29)(30)(31). The increased complexity and flexibility have shifted future projections of marine NPP to an insignificant multimodel mean decline with a large uncertainty of up to ±20% (28).…”

Hotspots and drivers of compound marine heatwaves and low net primary production extremes

“…The application of two different Earth system models facilitated an exploration of how uncertainties in the formulation of NPP manifest themselves in the occurrence (pattern and frequency) of compound events. This should complement work by Kwiatkowski et al (2020) and Bopp et al (2022) in underscoring the challenges faced by the Earth system modeling community given pervasive NPP uncertainty.…”