Abstract
Abstract. We examine the sensitivity of the biogeography of nitrogen fixers to a warming climate and increased aeolian iron deposition in the context of a global earth system model. We employ concepts from the resource-ratio theory to provide a simplifying and transparent interpretation of the results. First we demonstrate that a set of clearly defined, easily diagnosed provinces are consistent with the theory. Using this framework we show that the regions most vulnerable to province shifts and changes in diazotroph biogeography are the equatorial and South Pacific, and central Atlantic. Warmer and dustier climates favor diazotrophs due to an increase in the ratio of supply rate of iron to fixed nitrogen. We suggest that the emergent provinces could be a standard diagnostic for global change models, allowing for rapid and transparent interpretation and comparison of model predictions and the underlying mechanisms. The analysis suggests that monitoring of real world province boundaries, indicated by transitions in surface nutrient concentrations, would provide a clear and easily interpreted indicator of ongoing global change.
Highlights
Nitrogen fixation is a major source of exogenous nitrogen to the global ocean and plays a critical role in the nitrogen cycle and for marine ecosystems
The observed biogeography of diazotrophs can be understood as a function of nutrient supply ratios (Dutkiewicz et al, 2012, Ward et al, 2013) using concepts from Tilman (1982), with the growth of diazotrophs dependent on an excess supply of both iron and phosphorus over nitrogen, relative to the demands of the non-diazotroph community
The theory suggests that the limiting nutrients in the euphotic layer are held at eco-physiologically determined low values, while nonlimiting nutrients accumulate according to the excess supply
Summary
Nitrogen fixation is a major source of exogenous nitrogen to the global ocean and plays a critical role in the nitrogen cycle and for marine ecosystems. Ward et al (2013) showed that these theoretical predictions are consistent with observations along the Atlantic Meridional Transect (Moore et al, 2009), nutrient data from the World Ocean Atlas (Garcia et al, 2006) and in observed diazotroph distribution (Luo et al, 2012). This framework has been used to understand the observed shifts in nutrient gradients (interpreted as province boundaries) that occur due Published by Copernicus Publications on behalf of the European Geosciences Union.
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