Ocean dinitrogen fixation and its potential effects on ocean primary production in Earth system model simulations of anthropogenic warming

Abstract

Dinitrogen fixation (DNF) provides a large fraction of the 'new' nitrogen supporting upper ocean productivity, and is associated with environmental conditions likely to show substantial change under anthropogenic warming. For example, surface warming induces stronger stratification, weaker nutrient supply and more rapid nutrient depletion. Using six Earth System Models, we have examined spatial patterns and trends of DNF in the CMIP5 historical and RCP 8.5 experiments. Four models (CanESM2, CESM1-BGC, IPSL-CM5R-LR, and UVicESCM) show high DNF rates in warm, stratified waters mostly associated with the western parts of the ocean basins, while GFDL-ESM2M and MPI-ESM-LR show elevated rates near the eastern boundaries because of coupling of DNF and denitrification. Despite a growing body of data, the spatial pattern of DNF is still insufficiently resolved by available observations, and none of the models agrees well with the observations. Modelled and observed rates are mostly in the same general range except for UVicESCM, and frequency distributions are similar, but spatial pattern correlations are weak and in most cases not statistically significant. Only a few models show strong trends in DNF and primary production in a warming climate, and the signs of the trends are inconsistent. Observations of primary production at the benchmark subtropical station ALOHA (22.75°N, 158°W) and proxies for historical DNF from the same region appear to corroborate trends in CanESM2 that are not present in other models. However, the CanESM DNF parameterization does not include any limitation by P or Fe, so modelled future trends may not materialize due to nutrient limitation. Analysis of available models and observations suggests that our understanding of environmental controls on ocean DNF remains limited and future trends are highly uncertain. Long-term global simulations of DNF will only be meaningful if we maintain long-term observations and extend coverage to undersampled regions.Keywords: ocean dinitrogen fixation; Earth system modelling; anthropogenic climate change Riche and Christian: Ocean dinitrogen fixation and its potential effects on ocean primary production in Earth system model simulations of anthropogenic warming Art. 16, page 2 of 18 of phosphorus or iron are available, with opposing effects on primary production (Karl et al., 1995(Karl et al., , 1997Karl, 1999). As diazotrophic phytoplankton thrive in warm, stratified waters (Sohm et al., 2011), investigating the potentially enhanced role of diazotrophs in tropical and subtropical ocean ecosystems in a warming climate is important. Proxy δ 15N measurements of the DNF contribution to the N inventory in the subtropical Pacific suggest that the recent apparent enhancement discussed by Karl (1999) is a long-term trend throughout the 20 th century, possibly associated with anthropogenic warming (Sherwood et al., 2014).We have examined trends in DNF in a variety of Earth System Models associated with the CMIP5 project (Taylor et al., 2012) to determine which if any sh...

Machine Learning Estimates of Global Marine Nitrogen Fixation

“…N 2 fixation in IPSL‐CM5A‐LR was scaled to balance denitrification, which may have been overestimated (Séférian et al, ). We therefore rescaled N 2 fixation in IPSL‐CM5A‐LR by 0.17 following Riche & Christian, . In our comparisons, we included models obtained from four additional studies: (1) Luo et al () derived estimates of global N 2 fixation based on a stepwise MLR between observed N 2 fixation rates and predictors; (2) Landolfi et al () combined resource competition theory and a coupled ecosystem‐circulation model to explain the high N 2 fixation rates in nitrogen‐rich waters of the North Atlantic but also provided a model for the global ocean; (3) Jickells et al () evaluated the effects of atmospheric nitrogen deposition on marine N 2 fixation using the output from the PlankTOM model (Buitenhuis et al, ; Le Quéré et al, ); Finally, (4) Paulsen et al () incorporated a prognostic representation of diazotrophs in MPI‐ESM to map marine N 2 fixation.…”

“…N 2 fixation in IPSL-CM5A-LR was scaled to balance denitrification, which may have been overestimated (Séférian et al, 2013). We therefore rescaled N 2 fixation in IPSL-CM5A-LR by 0.17 following Riche & Christian, 2018. In our comparisons, we included models obtained from four additional studies: (1) Luo et al (2014) derived estimates of global N 2 fixation based on a stepwise MLR between observed N 2 fixation rates and predictors; (2) Landolfi et al (2015) combined resource competition theory and a coupled ecosystem-circulation model to explain the high N 2 fixation rates in nitrogen-rich waters of the North Atlantic but also provided a model for the global ocean;…”

“…Current estimates of global marine N 2 fixation vary widely, with most ranging from less than 100 to over 200 Tg N/year (Gruber & Galloway, 2008;Luo et al, 2012). These estimates are derived using a variety of approaches including direct extrapolation from scarce field observations (Luo et al, 2012) and models (Riche & Christian, 2018). In some prognostic models, N 2 fixation is parameterized relying on N 2 fixers' ecophysiology (Hood et al, 2001;Monteiro et al, 2010;Paulsen et al, 2017).…”

Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections

“…The uptake of carbon by the oceans affects marine chemistry via ocean acidification (Gattuso and Buddemeier, 2000;Orr et al, 2005;Doney et al, 2009), a process that increases seawater concentrations of CO 2 , H + , and HCO − 3 and reduces pH and CO 2− 3 ion concentrations. The oceans have absorbed approximately 30 % of anthropogenic carbon emissions since the pre-industrial era (Sabine et al, 2004;Khatiwala et al, 2009Khatiwala et al, , 2013Gruber et al, 2019), resulting in global surface pH declines of approximately 0.1 units (Bindoff et al, 2007). Declines in global open ocean surface pH are 0.018 units per decade over 1991-2011 (Lauvset et al 2015), with individual time series stations exhibiting declines of 0.017 to 0.027 units per decade (Bindoff, et al, in press).…”

Stable isotopic evidence for the differential contribution of diazotrophs to the epipelagic grazing food chain in the mid‐Pacific Ocean

“…In the coming decades, the oligotrophic subtropical biomes are predicted to expand poleward as a result of global warming (Polovina, Dunne, Woodworth, & Howell, 2011), while the effect of environmental change on the diazotrophic activity is still in debate owing to the complex response to shift of limiting nutrients (phosphate and iron) supply and increase of CO 2 concentration (Eichner, Rost, & Kranz, 2014;Riche & Christian, 2018). Nevertheless, the outlined changes in subtropical waters may affect the diazotroph community composition in the future ocean because the nutrient requirement and the response to enriched CO 2 conditions differ among diazotrophic species (Eichner et al, 2014;Riche & Christian, 2018). As mentioned above, the transfer pathway of the biologically fixed nitrogen in the grazing food chain differs depending on the diazotroph species (Bonnet, Baklouti, Gimenez, Berthelot, & Berman-Frank, 2016).…”