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Regional Impacts of Iron-Light Colimitation Biogeosciences Discuss., 6, 7517–7564, 2009 Biogeosciences www.biogeosciences-discuss.net/6/7517/2009/ Discussions BGD © Author(s) 2009. This work is distributed under 6, 7517–7564, 2009 the Creative Commons Attribution 3.0 License. Biogeosciences Discussions is the access reviewed discussion forum of Biogeosciences Regional impacts of iron-light colimitation E. D. Galbraith et al. Title Page Regional impacts of iron-light colimitation Abstract Introduction in a global biogeochemical model Conclusions References Tables Figures E. D. Galbraith1,*, A. Gnanadesikan2, J. P. Dunne2, and M. R. Hiscock1 1AOS Program, Princeton University, Princeton, NJ 08544, USA J I 2NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08542, USA J I *now at: Department of Earth and Planetary Science, McGill University, Montreal, QC, Canada Back Close Received: 10 June 2009 – Accepted: 3 July 2009 – Published: 27 July 2009 Correspondence to: E. D. Galbraith ([email protected]) Full Screen / Esc Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 7517 Abstract BGD Laboratory and field studies have revealed that iron has multiple roles in phytoplankton physiology, with particular importance for light-harvesting cellular machinery. However, 6, 7517–7564, 2009 although iron-limitation is explicitly included in numerous biogeochemical/ecosystem 5 models, its implementation varies, and its effect on the efficiency of light harvesting Regional impacts of is often ignored. Given the complexity of the ocean environment, it is difficult to pre- iron-light colimitation dict the consequences of applying different iron limitation schemes. Here we explore the interaction of iron and nutrient cycles using a new, streamlined model of ocean E. D. Galbraith et al. biogeochemistry. Building on previously published parameterizations of photoadapta- 10 tion and export production, the Biogeochemistry with Light Iron Nutrients and Gasses (BLING) model is constructed with only three explicit tracers but including macronutri- Title Page ent and micronutrient limitation, light limitation, and an implicit treatment of community Abstract Introduction structure. The structural simplicity of this computationally inexpensive model allows us to clearly isolate the global effects of iron availability on maximum light-saturated Conclusions References 15 photosynthesis rates from those of photosynthetic efficiency. We find that the effect Tables Figures on light-saturated photosynthesis rates is dominant, negating the importance of photo- synthetic 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, J I where high light-saturated photosynthesis rates cause photosynthetic efficiency to play J I 20 a more important role. Additionally, we speculate that the small phytoplankton dominat- ing iron-limited regions tend to have relatively high photosynthetic efficiency, such that Back Close iron-limitation has less of a deleterious effect on growth rates than would be expected Full Screen / Esc from short-term iron addition experiments. Printer-friendly Version 1 Introduction Interactive Discussion 25 In large surface regions of the open ocean, macronutrients remain in considerable abundance throughout the year, a puzzle that has engaged the interest of oceanogra- 7518 phers for many decades. These regions contrast with coastal environments in which the surface ecosystem handily strips out macronutrients, despite high resupply rates. BGD Although many factors have been implicated in the maintenance of the nutrient-rich 6, 7517–7564, 2009 surface oceans, limitation by micronutrients – principally iron – clearly plays a central 5 role. Over the past decades, numerous experiments have shown that adding iron to Regional impacts of macronutrient-rich regions of the ocean produces plankton blooms (see Boyd et al., iron-light colimitation 2007 for a review). On a physiological level, this appears to be largely due to the role of iron in the electron transport pathways that accomplish photosynthesis (Raven, E. D. Galbraith et al. 10 1990; Maldonado et al., 1999); cells that are replete in iron can build more photo- synthetic reaction centers and utilize the light they collect more efficiently (Greene et Title Page al., 1991; Strzepek and Harrison, 2004). Iron is also required for other cellular pro- cesses, including the reduction of nitrate to ammonia (Raven, 1990; Price et al., 1991). Abstract Introduction Laboratory studies support this, reporting large decreases in growth rates under iron Conclusions References 15 limitation (Price et al., 1991; Greene et al., 1991; Sunda and Huntsman, 1997; Tim- mermans et al., 2004). In addition, iron deficiency has been shown to significantly Tables Figures reduce the chlorophyll to carbon ratio, θ, in almost all cases, due to the requirement for iron in chlorophyll biosynthesis (e.g. Greene et al., 1991; Sunda and Huntsman, 1997; J I Marchetti and Harrison, 2007). 20 While the impact of iron on photosynthesis is clearly important, the manner in which J I its effects should be implemented in numerical models is less clear. Recent repre- Back Close sentations of algal physiology in biogeochemical models have often relied on the pho- toadaptation scheme of Geider et al. (1997), used in numerous global models (e.g. Full Screen / Esc Moore et al., 2002; Aumont and Bopp, 2006). This scheme is built around a common C 25 expression for the carbon-specific photosynthesis rate, P , as a function of irradiance, Printer-friendly Version I, Interactive Discussion C C P = Pm 1 − exp(−I/Ik ) (1) C where Pm is a macronutrient-limited, temperature-dependent, light-saturated carbon- 7519 −1 specific photosynthesis rate (s ) and Ik is a scaling term that determines the degree of light-limitation (W m−2). Note that photosynthesis is always light limited to some degree BGD in this formulation (since {1 − exp(−I/Ik )} is always less than 1), and that for a given 6, 7517–7564, 2009 I, photosynthesis decreases with increasing Ik . In the model of Geider et al. (1997), 5 θ adapts to a phytoplankter’s nutritional status, temperature, and light environment in Regional impacts of a way that is consistent with laboratory experiments. This leads to a formulation for Ik C chl iron-light colimitation as a function of Pm , θ and α , the latter of which is the initial slope of the chlorophyll- −1 −1 2 −1 a specific photosynthesis-light response curve (units of g C g chl W m s ). This E. D. Galbraith et al. latter term governs how rapidly photosynthesis (relative to chlorophyll) increases with 10 increasing light at low light levels, essentially a metric for the yield of usable photons harvested by each molecule of chorophyll under low light (Frenette et al., 1993). Sub- Title Page stituting the Geider et al. (1997) formulation for θ in their equation for Ik , we rearrange to obtain Abstract Introduction P C I Conclusions References I = m + mem (2) k chl α θmax 2 Tables Figures −1 15 where θmax is a maximum chlorophyll-to-carbon ratio (g chl gC ) and Imem is the irradi- ance to which the phytoplankton are accustomed. This equation captures the capacity J I of phytoplankton to adjust their photosynthetic machinery to their environment, in order J I to maximize photosynthesis rates while minimizing metabolic costs. Within this widely-applied conceptual framework, the experimental evidence sug- Back Close 20 gests that iron limitation could impact the growth of phytoplankton in three obvious C Full Screen / Esc ways. First, it could reduce the light-saturated growth rate Pm . This would represent the need for iron in proteins that mediate photosynthetic electron transport (Raven, 1990) Printer-friendly Version and thereby determine the maximum yield of electrons for photosynthesis when light is abundant. Additionally, this term accounts for the utility of iron for non-photosynthetic Interactive Discussion C 25 processes such as nitrate reduction (Price et al., 1991). However, if Pm is the only iron- dependent term, low iron will have the effect of making the plankton less light-limited, as they will need less light to match the other cellular functions (see the first term of Eq. 2). 7520 In other words, because Fe limitation reduces the maximum achievable photosynthetic rate, the utility of photons would decrease under Fe limitation, making light availability BGD less important. This tendency would be counteracted by including iron dependencies 6, 7517–7564, 2009 in the two other obvious terms: αchl, so that photosynthetic efficiency decreases at low 5 light under iron limitation, representing a reduction of the reaction centers available for light harvesting elements; and θmax, so that iron deficiency reduces chlorophyll synthe- Regional impacts of sis and thereby cause θmax to decrease. Note that the second and third mechanisms iron-light colimitation are numerically linked in the photosynthesis formulation, through modification of Ik as chl E. D. Galbraith et al. the product 1/α θmax. 10 Applying iron dependencies to these three terms appears to broadly reflect the avail- able measurements of photosynthetic parameters made during bottle incubations and Title Page mesoscale iron fertilization experiments. For example, in bottle incubations of natural samples from the Drake Passage, Hopkinson et al. (2007) reported that iron addition Abstract Introduction chl B increased α , θ and Pm (the light-saturated chlorophyll-specific growth rate, equal C Conclusions References 15 to Pm /θ). Marchetti et al. (2006a) also reported large increases in these three pa- rameters, within an iron-fertilized patch of the subarctic Pacific during the SERIES Tables Figures experiment. Hiscock et al. (2008) presented observations from the SOFeX mesoscale chl iron enrichment experiment, showing an increase of α by about 70% while quantum J I yield (effectively αchl θ) increased by about twice as much, a factor of ∼2.5. How- B J I 20 ever, in their case the chlorophyll-specific photosynthesis rate Pm remained relatively C unchanged, indicating that any change in Pm was approximately balanced by a corre- Back Close sponding change in θ.
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