Quantifying the Impact of Climate Change on Marine Diazotrophy: Insights From Earth System Models

Lewis Wrightson,Alessandro Tagliabue

doi:10.3389/fmars.2020.00635

Abstract

Nitrogen fixation is a major source of new nitrogen to the ocean, supporting biological productivity in the large nitrogen-limited tropical oceans. In Earth System Models, the response of nitrogen fixation to climate change acts in concert with projected changes to physical nitrogen supply to regulate the response of primary productivity in nitrogen-limited regions. We examine the response of diazotrophy from nine Earth System Models and find large variability in the magnitude and spatial pattern of nitrogen fixation in both contemporary periods and future projections. Although Earth System Models tend to agree that nitrogen fixation will decrease over the next century, strong regional variations exist, especially in the tropical Pacific which may counteract the response of the Atlantic and Indian oceans. As the climate driven trend of nitrogen fixation emerges by mid-century in the RCP8.5 scenario, on regional scales it may modulate the broad climate trends in productivity that emerge later in the century. The generally poor skill and lack of agreement amongst Earth System Models indicates that the climate response of nitrogen fixation is a key uncertainty in projections of future ocean primary production in the tropical oceans. Overall, we find that the future evolution of nitrogen fixation plays an important role in shaping future trends in net primary production in the tropics, but the poor skill of models highlights significant uncertainty, especially considering the role of multiple concurrent drivers.

Highlights

Future warming is projected to enhance ocean stratification, reducing the vertical supply of nutrient-rich subsurface waters to the nutrient-depleted surface waters of the euphotic zone in the tropical ocean in particular (Steinacher et al, 2010; Laufkötter et al, 2015)
Some models (CanESM2, CESM1-BEC, IPSL-CM5A-LR, IPSL-CM5A-MR, and IPSL-Quota) imposed a minimum temperature restriction, which restricts diazotrophs to low latitudes, but for those models without thermal limits, diazotrophy can occur over the entire global ocean this may be an imperfect solution to account for diazotrophy in the high latitudes
Current models project different spatial patterns and magnitudes of diazotrophy in the contemporary ocean, models, broadly fall into two groups based upon their spatial distribution of N2 fixation

Summary

Introduction

Future warming is projected to enhance ocean stratification, reducing the vertical supply of nutrient-rich subsurface waters to the nutrient-depleted surface waters of the euphotic zone in the tropical ocean in particular (Steinacher et al, 2010; Laufkötter et al, 2015). As dinitrogen (N2) fixation is a globally significant N supply mechanism (68–164 Tg N yr−1, Gruber and Sarmiento, 1997; Jickells et al, 2017; Luo et al, 2014; Luo et al, 2012; Tang et al, 2019a; Wang et al, 2019), future changes in N2 fixation by diazotrophs may modulate any N limitation arising from enhanced stratification This means that future changes to diazotrophy, and the supply of new fixed N, will be an important component of the response of tropical ocean productivity to climate change in Earth System Model projections. In N limited regions of the ocean, diazotrophs are only able to outcompete and proliferate provided that sufficient Fe and P are available (Moore et al, 2009; Dutkiewicz et al, 2012)

Methods

Results

Conclusion