Biogeosciences Discuss., 10, 7367-7412, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
Data-based assessment of environmental controls on global marine nitrogen fixation
Y.-W. Luo1, I. D. Lima1, D. M. Karl2, and S. C. Doney1
1Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
2Center for Microbial Oceanography: Research and Education, University of Hawaii, Honolulu, Hawaii, USA

Abstract. There are a number of hypotheses for the environmental controls on marine nitrogen fixation (NF). Most of these hypotheses have not been assessed against direct measurements on the global scale. In this study, we use ~ 500 depth-integrated field measurements of NF covering the Pacific and Atlantic Oceans to test whether the spatial variance of these measurements can be explained by the commonly hypothesized environmental controls, including measurement-based surface solar radiation, mixed layer depth, sea surface temperature, surface nitrate and phosphate concentrations, surface excess phosphate (P*), atmospheric dust deposition and surface wind speed, as well as minimum dissolved oxygen in upper 500 m to identify possible subsurface denitrification zones. By conducting simple linear regression and stepwise multiple linear regression (MLR) analyses, solar radiation and/or sea surface temperature as well as subsurface dissolved oxygen are identified as the predictors explaining the most spatial variance in the observed NF data, while dust deposition and wind speed do not appear to influence the spatial patterns of NF on global scale. Our study suggests that marine NF is coupled to regional loss of fixed nitrogen induced by subsurface low oxygen concentration, with its magnitude constrained by solar radiation or temperature. By applying the MLR-derived equation, we estimate the global-integrated NF at 71 (error range 49–104) Tg N yr−1 in the open ocean, acknowledging that it could be substantially higher as the 15N2-assimilation method used by most of the field samples underestimates NF. Our conclusion suggests that marine NF will increase in the future if subsurface nitrogen-losses increase as a consequence of developing deoxygenation with the global warming, a projection that will be modulated by other factors such as warming, elevated carbon dioxide, and changes in macro- and micro-nutrient distributions. More field NF samples in the Pacific and Indian Oceans, particularly in the oxygen minimum zones, are needed to reduce uncertainties in our conclusion.

Citation: Luo, Y.-W., Lima, I. D., Karl, D. M., and Doney, S. C.: Data-based assessment of environmental controls on global marine nitrogen fixation, Biogeosciences Discuss., 10, 7367-7412, doi:10.5194/bgd-10-7367-2013, 2013.
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