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Phytoplankton plasticity drives large variability in carbon fixation efficiency

Phytoplankton plasticity drives large variability in carbon fixation efficiency
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  GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1002/, Phytoplankton plasticity drives large variability in 1 carbon fixation efficiency 2 Sakina-Doroth´ee Ayata 1,2 , Marina L´evy 3 , Olivier Aumont 3 , LaureResplandy 4 , Alessandro Tagliabue 5 , Antoine Sciandra 1,2 , and OlivierBernard 1,2,6 Corresponding author: S.-D. Ayata, 1 Sorbonne Universit´es, UPMC Univ Paris 06, UMR 7093, LOV, Villefranche/mer, France. 2 CNRS, UMR 7093, LOV, Villefranche/mer, France. 3 Sorbonne Universit´es, UPMC Univ Paris 06, CNRS/IRD/MNHN, LOCEAN-IPSL, Paris, France. 4 SCRIPPS, La Jolla, California, USA. 5 University of Liverpool, Liverpool, UK. 6 BIOCORE, INRIA, Sophia Antipolis, France. D R A F T November 26, 2014, 3:01pm D R A F T  X - 2  AYATA ET AL.: FLEXIBLE C:N RATIO AND CARBON CONSUMPTION Phytoplankton C:N stoichiometry is highly flexible due to physiological 3 plasticity, which could lead to high variations in carbon fixation efficiency 4 (carbon consumption relatively to nitrogen). However, the magnitude, as well 5 as the spatial and temporal scales of variability, remain poorly constrained. 6 We used a high resolution biogeochemical model resolving various scales from 7 small to high, spatially and temporally, in order to quantify and better un- 8 derstand this variability. We find that phytoplankton C:N ratio is highly vari- 9 able at all spatial and temporal scales (5-12 molC/molN), from meso- to re- 10 gional scale, and is mainly driven by nitrogen supply. Carbon fixation effi- 11 ciency varies accordingly at all scales ( ±  30%), with higher values under olig- 12 otrophic conditions and lower values under eutrophic conditions. Hence phy- 13 toplankton plasticity may act as a buffer by attenuating carbon sequestra- 14 tion variability. Our results have implications for  in situ   estimations of C:N 15 ratios and for future predictions under high CO 2  world. 16 D R A F T November 26, 2014, 3:01pm D R A F T  AYATA ET AL.: FLEXIBLE C:N RATIO AND CARBON CONSUMPTION  X - 3 1. Introduction Marine primary production in surface waters is limited by nutrient availability with ni- 17 trate being the proximate limiting nutrient over most part of the ocean [Tyrrell, 1999; Ar- 18 rigo, 2005; Moore et al., 2013]. The efficiency of the biological carbon pump is determined 19 by the amount of carbon that can be fixed given the available stock of limiting nutrient 20 (assimilation ratio). Up to now, most biogeochemical ocean models have used constant 21 C:N ratio (both for biomass and assimilation), based on Redfield stoichiometry [Redfield, 22 1934]. For instance, most models used in the Coupled Model Intercomparison Project 5 23 (CMIP5) consider a constant C:N stoichiometry of 6.6 molC/molN or 7.62 molC/molN 24 [Bopp et al., 2013]. Such models hence assume a constant carbon fixation efficiency rela- 25 tive to nitrogen. 26 However, numerous observations have revealed large plasticity of phytoplankton phys- 27 iology [Rees et al., 2001; Geider and La Roche, 2002; Arrigo, 2005; Klausmeier et al., 28 2008; Martiny et al., 2013a]. As a consequence, C:N ratios of assimilation and organic 29 matter can deviate from Redfield stoichiometry up to 40% [Sambrotto et al., 1993; Banse, 30 1994; Kortzinger et al., 2001; Koeve, 2004; Martiny et al., 2013a, b; Moore et al., 2013]. 31 Under nutrient limited conditions, flexible C:N ratios allow the production of a relatively 32 higher amount of organic carbon than would be expected from nitrogen uptake according 33 to Redfield stoichiometry. This process is commonly referred as ”carbon overconsump- 34 tion” [Toggweiler, 1993]. Assuming a fixed C:N stoichiometry could then lead to biases 35 in carbon fixation estimates from nutrient uptake. Additionally, previous  in situ   and 36 modeling studies have suggested that C:N stoichiometry of phytoplankton and assimila- 37 D R A F T November 26, 2014, 3:01pm D R A F T  X - 4  AYATA ET AL.: FLEXIBLE C:N RATIO AND CARBON CONSUMPTION tion can strongly vary from mesoscale [Rees et al., 2001; Omta et al., 2007] to seasonal 38 [Fernandez et al., 2005] and regional scales [Kortzinger et al., 2001; Koeve, 2006; Martiny 39 et al., 2013b] (Figure 1). 40 In this context, the aim of the present study is to assess the variability of the phyto- 41 planktonic C:N ratio and of the carbon fixation efficiency in the ocean, and to identify 42 their spatial and temporal scales of variations. For that, we used a high resolution bio- 43 geochemical model resolving various scales from small to high, spatially and temporally, 44 in order to quantify and better understand this variability. 45 2. Methods We used an idealized submesoscale permitting configuration of the North Western At- 46 lantic ocean, reproducing oligotrophic subtropical and productive subpolar regimes. The 47 primitive equation model Nucleus for European Modelling of the Ocean (NEMO) [Madec, 48 2008] was used in conjunction with the biogeochemical model LOBSTER (Lodyc Ocean 49 Biogeochemical System for Ecosystem and Resources) [L´evy et al., 2012a]. The model 50 domain was a rotated rectangle, bounded by vertical walls and by a flat bottom that 51 covered the latitudinal range 15 ◦ N to 50 ◦ N (Figure 2). The circulation was forced by a 52 repeating annual cycle of zonal wind and buoyancy fluxes, which varied seasonally in a 53 sinusoidal manner between winter and summer extrema. The horizontal resolution was 54 submesoscale permitting (1/54 ◦ ). The biogeochemical model LOBSTER solves for phyto- 55 plankton, zooplankton, detritus, dissolved organic matter, nitrate, and ammonium [L´evy 56 et al., 2012a, b]. More detailed information about the model configuration, closures, and 57 parameters can be found in L´evy et al. [2010, 2012a]. 58 D R A F T November 26, 2014, 3:01pm D R A F T  AYATA ET AL.: FLEXIBLE C:N RATIO AND CARBON CONSUMPTION  X - 5 A new version of the LOBSTER model was developed with flexible C:N and Chl:C ratios 59 for the phytoplankton (model version P2.5 in Ayata et al. [2013]). Phytoplankton growth 60 is represented following Geider et al. [1998], so that nitrogen and carbon assimilations are 61 decoupled, allowing a flexible C:N ratio for phytoplankton. Nutrient uptake is constrained 62 by nutrient availability and phytoplanktonic C:N ratio. Carbon fixation is constrained by 63 light and phytoplanktonic C:N ratio. Photo-acclimation is accounted for using a diagnostic 64 chlorophyll:carbon ratio as a function of light and nutrient limitation [Geider et al., 1998; 65 Ayata et al., 2013]. Zooplankton, detritus, and DOM are only represented in nitrogen 66 currency, as in the srcinal LOBSTER model.This means that when the phytoplankton 67 is eaten or dies, the fate of the lost part of carbon is not represented in this model. The 68 equations and the default parameters used in this version of the LOBSTER model with 69 flexible C:N ratio can be found in the Supporting Information. The solution obtained 70 with the physical model at 1/54 ◦ was degraded down to 1/9 ◦ [L´evy et al., 2012b] in order 71 to run off-line this new version of the biogeochemical model over one climatological year. 72 The spin-up state obtained with the srcinal LOBSTER model [L´evy et al., 2010, 2012a] 73 was used as initial conditions. 74 We defined the C:N assimilation ratio as the ratio between the net carbon uptake 75 (primary production minus respiration) and the nitrogen uptake. The carbon fixation 76 efficiency was then defined as the relative difference between the C:N assimilation ratio and 77 the Redfield C:N ratio of 6.6 molC/molN [Redfield, 1934]. The carbon fixation efficiency 78 hence corresponds to the percentage of carbon over-/under-consumption. 79 D R A F T November 26, 2014, 3:01pm D R A F T
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