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Biogeosciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/bg-2017-74
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
07 Mar 2017
Review status
This discussion paper is under review for the journal Biogeosciences (BG).
Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean
Thomas J. Ryan-Keogh1,2, Sandy J. Thomalla1, Thato N. Mtshali1, and Hazel Little2 1Southern Ocean Carbon and Climate Observatory, Natural Resources and Environment, CSIR, Rosebank, Cape Town 7700, South Africa
2Department of Oceanography, University of Cape Town, Rondebosch, Cape Town 7701, South Africa
Abstract. The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilising a unique experimental design that allowed for 24 hour incubations were performed within the austral summer of 2015/16. The addition of iron can result in the doubling of the photosynthetic parameters αB and PBmax, with subsequent changes in Ek. Mean values for each parameter under iron replete conditions were 1.46 ± 0.55 (μg (μg Chl a)−1 h−1 (μM photons m−2 s−1)−1), 72.55 ± 27.97 (μg (μg Chl a)−1 h−1) and 50.84 ± 11.89 (μM photons m−2 s−1); whereas mean values under the control conditions were 1.25 ± 0.92 (μg (μg Chl a)−1 h−1 (μM photons m−2 s−1)−1), 62.44 ± 36.96 (μg (μg Chl a)−1 h−1) and 55.81 ± 19.60 (μM photons m−2 s−1). There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50° S, with very little difference observed in the sub-Antarctic and Polar Frontal zone. These results emphasize the need for better parameterisation of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

Citation: Ryan-Keogh, T. J., Thomalla, S. J., Mtshali, T. N., and Little, H.: Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean, Biogeosciences Discuss., doi:10.5194/bg-2017-74, in review, 2017.
Thomas J. Ryan-Keogh et al.
Thomas J. Ryan-Keogh et al.
Thomas J. Ryan-Keogh et al.

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Short summary
Primary production in the Southern Ocean is a key contributor to mitigating global anthropogenic carbon dioxide, however, the controlling mechanisms are poorly understood. A series of experiments were performed to look at whether the rates of primary production are limited by the biogeochemically important micronutrient iron. The results demonstrate that any global climate models that do not take into account the effect of iron could underestimate primary production by up to 80 %.
Primary production in the Southern Ocean is a key contributor to mitigating global anthropogenic...
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