Regional variation in the particulate organic carbon to nitrogen ratio

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Transcript of Regional variation in the particulate organic carbon to nitrogen ratio

  • Regional variation in the particulate organic carbon to nitrogenratio in the surface ocean

    A. C. Martiny,1,2 Jasper A. Vrugt,1,3 Francois W. Primeau,1 and Michael W. Lomas4

    Received 22 March 2013; revised 25 June 2013; accepted 30 June 2013.

    [1] The concept of constant elemental ratios in plankton communitiesthe Redfield ratiois of central importance to ocean biogeochemistry. Recently, several studies havedemonstrated regional differences in the plankton C:P and N:P ratio. However, less isknown about potential systematic variations in the C:N ratio. Here we present an analysisof the particulate organic carbon to nitrogen ratio of 40,482 globally distributed samplesfrom the upper 200m of the ocean water column. Particulate organic carbon and nitrogenconcentrations are highly correlated (R2 = 0.86) with a median value of 6.5. Using anartificial neural network analysis, we find regional variations in the C:N ratio linked todifferences in environmental conditions. The ratio is lower in upper latitude cold water aswell as upwelling regions in comparison to the warm oligotrophic gyres. We findsubstantial differences between ocean gyres that might be associated with differences inthe nutrient supply ratio. Using cell sorting, we also quantified the C:N ratio ofProchlorococcus, Synechococcus, and picoeukaryotic field populations. The analysisdemonstrates that picophytoplankton lineages exhibit a significantly higher ratio than thebulk particulate material but are only marginally significantly different from each other.Thus, the dominance of picophytoplankton in ocean gyres may contribute to the elevatedratios observed in these regions. Overall, the median C:N ratio derived from 40,482samples is close to the canonical Redfield ratio, but significant regional deviations fromthis value are observed. These differences could be important for marine biogeochemistryand the regional coupling between the ocean's carbon and nitrogen cycles.

    Citation: Martiny, A. C., J. A. Vrugt, F. W. Primeau, and M. W. Lomas (2013), Regional variation in the particulateorganic carbon to nitrogen ratio in the surface ocean, Global Biogeochem. Cycles, 27, doi:10.1002/gbc.20061.

    1. Introduction

    [2] Nearly 80 years ago, Alfred Redfield observed thatocean surface plankton and deep dissolved nutrient concen-trations shared similar C:N:P ratios [Redfield, 1934]. ThisRedfield ratio has later become a central tenet for oceanbiogeochemistry and underlies our understanding of thelinkages between the nutrient cycles, conditions for nutrientlimitation, extent of nitrogen fixation or loss, and otherimportant oceanographic processes [Deutsch and Weber,2012;Mills and Arrigo, 2010]. Recently, several studies havedemonstrated that the N:P and C:P ratios vary in different

    ocean regions [Martiny et al., 2013; Weber and Deutsch,2010]. This variation in N:P and C:P is attributed to regionalvariations in environmental conditions and plankton biodi-versity. Despite the importance of elemental ratios in manybiogeochemistry and modeling studies, not much is knownabout systematic regional variations in the C:N ratio.[3] The particulate organic carbon to nitrogen ratio

    (referred to as the C:N ratio) is important for linking the carbonand nitrogen cycles [Oschlies et al., 2008; Schneider et al.,2004; Tagliabue et al., 2011] as well as trophic processingof material [Sterner et al., 1992] and is a basic assumptionfor many concepts in ocean biogeochemistry like the f-ratioand new production [Dugdale, 1967]. Several reviews havereported variations in the C:N ratio but with means above thecanonical Redfield ratio of 6.63 [Fleming, 1940; Geiderand La Roche, 2002; Schneider et al., 2003; Sterner et al.,2008]. Part of this variation can be linked to an elevated C:Nratio in the deep ocean compared to surface waterpossiblydue to a preferential remineralization of organic N over C[Schneider et al., 2003]. Moreover, some variation in the C:N ratio has been observed in regional studies. For example,Krtzinger and colleagues observed a ratio ~5 in eutrophiccompared to ~7 in nitrogen depleted regions along a NorthAtlantic transect [Kortzinger et al., 2001]. In contrast, othersfound little regional variation [Copin-Montegut and Copin-Montegut, 1983].

    Additional supporting information may be found in the online version ofthis article.

    1Department of Earth System Science, University of California, Irvine,California, USA.

    2Department of Ecology and Evolutionary Biology,University ofCalifornia, Irvine, California, USA.

    3Department of Civil and Environmental Engineering, University ofCalifornia, Irvine, California, USA.

    4Bigelow Laboratory for Ocean Sciences, East Boothbay, Maine, USA.

    Corresponding author: A. C. Martiny, Department of Earth SystemScience, University of California, 3208 Croul Hall, Irvine, CA 92697,USA. ([email protected])

    2013. American Geophysical Union. All Rights Reserved.0886-6236/13/10.1002/gbc.20061

    1

    GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 27, 19, doi:10.1002/gbc.20061, 2013

  • [4] Despite the progress made, relatively little is yet knownabout any systematic regional variations in the C:N ratio. Theexistence of such variations should not be surprising. Ingeneral, protein and nucleic acids are both rich in N comparedto C (C:N~4). In contrast, lipids in the cell membrane andwall, carbohydrates, and storage molecules like glycogen orpolyhydroxy-butyrate are rich in C. Thus, the relativebiochemical allocation to different biopolymers can influencethe cellular stoichiometry and result in variations in C:Nbetween plankton lineages and environments [Geider and LaRoche, 2002; Sterner and Elser, 2002]. Indeed, at least fivedifferent mechanisms can induce variations in the particulateC:N ratio. As a first mechanism, the N cell quota declineswhen cells are N limited [Droop, 1983]. Thus, N limitationcommonly results in an increased cellular C:N ratio[Goldman and Peavey, 1979; Vrede et al., 2002]. The secondmechanism links light limitation with a lower cellular C:Nratio as phytoplankton growing under low light irradiancemay accumulate less carbon storage polymers [Chalup andLaws, 1990; Cronin and Lodge, 2003]. The third mechanismis based on a negative relationship between cellular C:N ratioand growth rate [Chalup and Laws, 1990]. This is particularlythe case if the growth rate is controlled by nutrient availability[Bertilsson et al., 2003; Goldman and Peavey, 1979; Vredeet al., 2002]. Although the exact mechanism for this relation-ship is unknown, it is likely due to an increase in the concen-tration of both proteins and nucleic acids compared to othercellular components as well as an increase in nutrient storagein fast-growing cells. The fourth mechanism suggests phylo-genetically constrained elemental ratios derived from differ-ences in the structural biochemical composition of the cell.This mechanism has been invoked as contributing to regionaldifferences in the C:P and N:P ratios, whereby coexisting phy-toplankton taxa have unique elemental stoichiometries[Martiny et al., 2013]. Phylogenetic differences in the C:N ratiohave not been studied in detail. Generally, it is plausible thatsmaller cells can have a higher C:N ratio compared to largecellsi.e., more skin to flesh [Kroer, 1994]. The fifth mecha-nism is that dead plankton material or detritus can influencethe observed particulate elemental ratios. Recently, we ob-served that living cells constituted the majority of particulatematerial in open ocean waters [Martiny et al., 2013] but detritusis likely a nontrivial component of the particulate pools in someregions. However, our quantitative knowledge of detritus con-centration [Jackson and Williams, 1985] and elemental compo-sition [Verity et al., 2000] in the ocean is rather limited.Furthermore, both heterotrophs and autotrophs can preferen-tially scavenge specific elements from this pool. Thus, processesof both detritus formation and degradation can influence the el-emental ratio of marine particles [Loh and Bauer, 2000].[5] While these mechanisms can lead to deviations from

    the canonical Redfield value of 6.63 among ocean planktonand communities, variations in the global ocean C:N ratiohave hitherto not been studied in detail. To address this, weask the following questions for the global upper ocean: (i)What are the median and (ii) global variation in the C:Nelemental ratio?

    2. Data Set

    [6] To assess the variation in the particulate C:N ratio inthe top 200m of the water column, we compiled a data set

    of particulate organic carbon (POC) and nitrogen (PON)measurements from 72 previously published and publiclyavailable cruises or time series (Table S1 in the supportinginformation). Unless otherwise stated, all mathematicaloperations were done in Matlab (Mathworks, Natick,MA). Our combined dataset contained 40,482 paired sam-ples of POC and PON covering both open-ocean and coastalsamples. We combined this information with data on longi-tude, latitude, temperature, sampling and absolute watercolumn depths, as well as nitrate, phosphate, and chloro-phyll concentrations. Missing temperature, nitrate, or phos-phate data were estimated from the World Ocean Atlas[Boyer et al., 2006]. The water column depth was estimatedbased on the ETOPO2v2 bathymetry (National GeophysicalData Center, 2006). Samples with a C:N ratio below 2 andabove 20 were excluded from our data set. Based on empir-ical observations and the ratios of plankton, such valueswere deemed unrealistically small or large and could cor-rupt the observed mean ratios and further analysis. We de-fined a station as a unique depth profile in time (1 day)and space (within a 1 1 box). This resulted in 5383 sta-tions. We purposely defined this margin to allow for mea-surements of different environmental parameters onseparate casts at the same station. Note that we assumedour samples to be i