Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 3 2010 10.5194/bg-7-1043-2010 http://www.biogeosciences.net/7/1043/2010/ http://www.biogeosciences.net/7/1043/2010/bg-7-1043-2010.html http://www.biogeosciences.net/7/1043/2010/bg-7-1043-2010.pdf 1043 1064 2010-03-18 Regional impacts of iron-light colimitation in a global biogeochemical model E. D. Galbraith eric.galbraith@mcgill.ca A. Gnanadesikan J. P. Dunne M. R. Hiscock AOS Program, Princeton University, Princeton, NJ 08544, USA NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08542, USA now at: Department of Earth and Planetary Science, McGill University, Montreal, QC, Canada Laboratory and field studies have revealed that iron has multiple roles in phytoplankton physiology, with particular importance for light-harvesting cellular machinery. However, although iron-limitation is explicitly included in numerous biogeochemical/ecosystem models, its implementation varies, and its effect on the efficiency of light harvesting is often ignored. Given the complexity of the ocean environment, it is difficult to predict the consequences of applying different iron limitation schemes. Here we explore the interaction of iron and nutrient cycles in an ocean general circulation model using a new, streamlined model of ocean biogeochemistry. Building on previously published parameterizations of photoadaptation and export production, the Biogeochemistry with Light Iron Nutrients and Gasses (BLING) model is constructed with only four explicit tracers but including macronutrient and micronutrient limitation, light limitation, and an implicit treatment of community structure. The structural simplicity of this computationally-inexpensive model allows us to clearly isolate the global effect that iron availability has on maximum light-saturated photosynthesis rates vs. the effect iron has on photosynthetic efficiency. We find that the effect on light-saturated photosynthesis rates is dominant, negating the importance of photosynthetic efficiency in most regions, especially the cold waters of the Southern Ocean. The primary exceptions to this occur in iron-rich regions of the Northern Hemisphere, where high light-saturated photosynthesis rates allow photosynthetic efficiency to play a more important role. In other words, the ability to efficiently harvest photons has little effect in regions where light-saturated growth rates are low. Additionally, we speculate that the phytoplankton cells dominating iron-limited regions tend to have relatively high photosynthetic efficiency, due to reduced packaging effects. 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