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556 Edwards et al.Fig. 1. Scaling relationships between primary nutrient utilization traits and cell volume. All variables are log 10 -transformed. Marinespecies are larger open circles, freshwater species are smaller filled circles. Lines plotted are from the SMA regressions in Table 1. If thereis no significant difference in slopes between marine and freshwater species, as reported in Table 1, a single solid line is plotted. If theslopes differ significantly, a dotted line is plotted for the marine species and a dashed line is plotted for the freshwater species. If there isno significant correlation between the variables, no line is plotted. (A) V N max , (B) KN , (C) Q N min , (D) V P max , (E) KP , (F) Q P min .fewer than two species. Therefore, the following groupswere compared for freshwater species: cyanobacteria,diatoms, desmids, and non-desmid chlorophytes (desmidswere only present for the phosphate traits). For the marinespecies, the following groups were compared: diatoms,dinoflagellates, chlorophytes, haptophytes, and raphidophytes.We also used t-tests to compare mean trait valuesfor freshwater and marine diatoms, because diatoms fromboth environments were relatively well represented.We tested for correlations between the primary nutrientutilization traits, which could indicate physiological traitconstraints or correlated selection pressures. Because mosttraits are correlated with volume, we also tested for partialcorrelations that controlled for the combined effect ofvolume on each pair of traits. Correlations between thecomposite traits are the subject of a separate manuscript, inwhich we use techniques for multivariate analysis in thepresence of missing data to better estimate the correlationsbetween cell volume, uptake affinity, and scaled uptakeaffinity for N and P (Edwards et al. 2011).ResultsTrait–volume scaling—All six of the primary nutrienttraits tend to increase with cell volume (Fig. 1). Four ofthese traits have no significant difference in slope betweenfreshwater and marine species: V N max , Q N min , V P max , and Q P min(Table 1; Fig. 1). In contrast, K N and K P both havesignificantly different slopes for freshwater vs. marinespecies, with freshwater species exhibiting a steeper slopefor K N , but exhibiting no significant relationship withvolume for K P (Fig. 1B,E). Of the four composite nutrienttraits, P affinity, N affinity, and scaled N affinity all haveslopes that are indistinguishable for freshwater and marinespecies. Both P affinity and N affinity increase with cellvolume, whereas scaled N affinity decreases with cellvolume (Fig. 2A,B,D). Scaled P affinity exhibits significantlydifferent slopes for freshwater and marine species,with marine species showing a negative relationship withvolume, and freshwater species showing no significantrelationship (Fig. 2C). Maximum growth rate tends todecline with cell volume (Fig. 3A), and marine species showa significantly shallower slope than freshwater species(Table 1).Between-taxon trait differences—m max differs significantlybetween taxa for both freshwater and marine species,with and without controlling for volume (Table 2;Fig. 3C,D). Because of the greater amount of data form max , we also tested for differences in the allometricexponent between taxa; SMA slopes did not differ bytaxon for freshwater (likelihood ratio 5 6.46, p 5 0.09) ormarine species (likelihood ratio 5 0.58, p 5 0.96). Formarine species, after controlling for volume, mean maximum

Reviews in L&O 557Table 1. Coefficients for SMA regression of nutrient utilization traits and mmax with cell volume. Intercepts (int.) and slopes are listed for separate regressions offreshwater and marine species, and the common intercepts and slopes are listed for regressions that combine freshwater and marine species. 95% confidence intervals for eachparameter are in parentheses. ns is listed for traits not significantly correlated with volume. The final column lists the p value for a test of whether the freshwater and marinespecies have different slopes. p values , 0.05 are marked with an asterisk.Freshwater int. Freshwater slope Marine int. Marine slope Common int. Common slope Slope pV N max28.8(210, 27.4) 1.3(0.82, 2.1) 28.1(28.8, 27.3) 0.82(0.65, 1) 28.0(28.5, 27.4) 0.82(0.68, 0.98) 0.069K N 20.71(21.1, 20.28) 0.52(0.37, 0.73) 20.84(21.2, 20.44) 0.33(0.24, 0.45) 20.61(20.88, 20.34) 0.33(0.26, 0.42) 0.044*Q N min28.7(29.2, 28.2) 0.68(0.51, 0.93) 29.2(29.6, 28.7) 0.88(0.77, 1) 29.0(29.3, 28.8) 0.84(0.76, 0.94) 0.13V P max28.4(28.9, 27.9) 0.81(0.64, 1) 29.1(210, 28.2) 1.0(0.8, 1.3) 28.7(29.2, 28.3) 0.94(0.81, 1.1) 0.17K P ns ns 21.4(21.8, 20.88) 0.41(0.29, 0.56) 21.5(21.9, 21.1) 0.53(0.42, 0.67) 0.0097*Q P min210.5(210.8, 210.2) 0.86(0.74, 1) 210.6(211.1, 210.1) 0.97(0.84, 1.1) 210.7(210.9, 210.4) 0.96(0.87, 1.1) 0.3P affinity 29(29.7, 28.2) 1 .0(0.76, 1.3) 28.1(28.8, 27.5) 0.73(0.58, 0.94) 28.5(29, 28.1) 0.85(0.72, 1) 0.089N affinity 28.6(29.8, 27.3) 1 .0(0.6, 1.7) 28.2(29, 27.4) 0.75(0.58, 0.98) 28.1(28.7, 27.6) 0.75(0.61, 0.92) 0.33P scaledaffinity ns ns 3.6(2.8, 4.3) 20.55(20.38, 20.8) 3.6(3.1, 4.1) 20.65(20.5, 20.83) 0.046*N scaledaffinity 20.96(22.1, 0.2) 0.78(0.43, 1.4) 2.9(1.9, 3.9) 20.63(20.44, 20.91) 2.4(1.8, 3.0) 20.57(20.43, 20.78) 0.54m max 0.69(0.52, 0.86) 20.36(20.30, 20.43) 0.70(0.54, 0.85) 20.24(20.20, 20.29) 0.65(0.53, 0.76) 20.28(20.25, 20.32) 0.0014*growth rates for diatoms are greater than those ofchlorophytes, dinoflagellates, and cyanobacteria (Tukeypairwise comparison p , 0.001 for all tests). This meansthat at a given cell size, diatoms grow faster than othertaxonomic groups. In addition, dinoflagellates and haptophyteshave greater growth rates than cyanobacteria whencontrolling for volume (p 5 0.0081 and p 5 0.0078,respectively). For freshwater species, cyanobacteria havelower growth rates than diatoms, desmids, non-desmidchlorophytes, and cryptomonads when controlling forvolume (p , 0.001 for all tests).Most nutrient traits differ significantly between taxawhen cell volume is not included as a covariate (Table 2;Figs.4–7).However,fewtraits differ between taxa whenthe effect of volume is accounted for (Table 2; Figs. 4–7).For freshwater species, values of primary nutrient traitstend to be greatest for the desmids and smallest for thecyanobacteria, with diatoms and non-desmid chlorophyteshaving intermediate values (Fig. 4). For marinespecies, dinoflagellates and raphidophytes tend to havethe largest trait values, whereas diatoms have intermediatevalues and haptophytes and chlorophytes have thesmallest values (Fig. 6). Because few between-taxondifferences remain when cell volume is controlled for,these differences are evidently driven by differencesbetween taxa in typical cell size. For freshwater species,the exceptions are K P , P affinity, and scaled N affinity, allof which differ between taxa while controlling for volume(Table 2; Figs. 4, 5). Tukey tests show that for K P ,desmids have significantly greater values than the otherthree taxa, and non-desmid chlorophytes have significantgreater values than cyanobacteria (p , 0.01 for all tests).For P affinity, desmids have significantly greater valuesthan cyanobacteria (p 5 0.034). For scaled N affinity,diatoms and non-desmid chlorophytes both have significantlygreater values than cyanobacteria (p 5 0.036 and0.026, respectively). For marine species, the traits withsignificant between-taxon variation when controlling forvolume are K N and scaled N affinity (Table 2; Figs. 6, 7).For K N , chlorophytes have greater values than diatoms (p, 0.001), dinoflagellates (p , 0.001), haptophytes (p ,0.001), and raphidophytes (p , 0.037). For scaled Naffinity, although there is significant between-taxonheterogeneity, no pairwise comparisons are significantlydifferent at p , 0.05.Diatoms—We compared log-transformed mean traitvalues for freshwater and marine diatoms using those traitsmeasured on at least five species from each environment(m max , K N , Q N min , V P max , KP , Q P min, P affinity, and scaled Paffinity). Marine diatoms have higher m max than freshwaterdiatoms (back-transformed means of 1.0 vs. 0.28, respectively;p 5 0.001). Marine diatoms have lower K N (backtransformedmeans of 1.6 vs. 3.5; p 5 0.018), higher V P max(back-transformed means of 2.8 3 10 26 vs. 2.5 3 10 27 ; p 50.029), and higher K P (back-transformed means of 1.1 vs.0.23; p 5 0.035). Marine diatoms are also larger on average(means of 2090 vs. 436 m 3 ; p 5 0.009), but the othersignificant trait differences were unchanged in analyses ofcovariance that included volume as a covariate.

558 Edwards et al.Fig. 2. Scaling relationships between composite nutrient traits and cell volume. All variablesare log 10 -transformed. Lines plotted are from the SMA regressions in Table 1. If there is nosignificant difference in slopes between marine and freshwater species, as reported in Table 1, asingle solid line is plotted. Symbols as in Fig. 1. (A) P affinity (aff.), (B) N aff., (C) scaled P aff.,(D) scaled N aff.Correlations between traits—Raw correlations betweenprimary nutrient utilization traits are dominated by strongpositive correlations (Table 3). For the marine species, allpairs of traits are significantly positively correlated. For thefreshwater species, most pairs of traits either are significantlypositively correlated or show no significant correlation.The single exception to this pattern is a significantnegative correlation between K P and K N among thefreshwater species (Table 3). The dominance of positivecorrelations among traits is likely driven by the fact that alltraits tend to increase with cell volume (Fig. 1). A similarargument applies to the fact that all of the raw correlationsof nutrient traits with m max are negative (Table 3). Wetherefore tested for partial correlations, which control forthe simultaneous effect of cell volume on each pair of traits.In this analysis, positive correlations between nutrient traitsstill dominate, as do negative correlations between nutrienttraits and m max (Table 3). Among marine species, five pairsof traits still show significant positive correlations betweennutrient traits; among freshwater species, four pairs oftraits show significant positive correlations, whereas K Pand K N again show a negative correlation. As discussedbelow, the preponderance of positive correlations thatremain while controlling for volume may indicate that theeffect of volume is not fully removed.Q N min : Q P min ratios—We tested whether the ratio Q N min :Q P mintended to differ between marine and freshwaterhabitats. Both habitats showed broad variation in thisratio, but freshwater species had a significantly higher ratioon average (Fig. 8; freshwater median molar ratio, 24.4;marine median molar ratio, 13.6; two-sample Wilcoxontest, p 5 0.032).DiscussionScaling relationships and marine–freshwater differences—We find that V max , K, and Q min all tend to increase as cellvolume increases, although this is not the case for K Pamong freshwater species (Fig. 1; Table 1). It is noteworthythat marine and freshwater slopes are indistinguishable forthose traits that appear to be the most strongly constrainedby volume, V max and Q min (Fig. 1). In contrast, the halfsaturationconstants for uptake appear to be less stronglyconstrained by volume, and have significantly differentslopes for freshwater and marine species (Fig. 1B,E),possibly reflecting different selective pressures from N orP limitation. If P limitation is more prevalent in freshwaterenvironments, it may select for low K P and thus decreasethis trait’s dependence on cell volume. The generallypositive effect of volume on all of these traits is consistent

Reviews in L&O 559Fig. 3. Allometric scaling and between-taxon differences in m max . (A) m max vs. cell volumefor freshwater and marine species. The fitted SMA regressions a plotted for marine (dashed line)and freshwater (dotted line) species. (B) Violin plots of m max (on a linear scale). The violin plotscombine a box plot (showing 10th, 25th, 50th, 75th, and 90th quantiles) with a density plot. (C)m max vs. volume for freshwater species, coded by taxon. (D) m max vs. volume for marine species,coded by taxon. Chrys, chrysophytes, crypt, cryptophytes; diat, diatoms; cyan, cyanobacteria;chlor, chlorophytes; dino, dinoflagellates; hapt, haptophytes.with prior analyses (Shuter 1978; Grover 1989; Litchmanet al. 2007). These results are also largely consistent withthe idea that marine and freshwater phytoplankton havesimilar physiological ecology (Kilham and Hecky 1988),although the different environments may modify traitpatterns to some extent. For example, diatoms in marineand freshwater systems appear to occupy somewhatdifferent ecological niches and exhibit different strategies.Marine diatoms are a fast-growing group compared toother marine taxa and appear to be adapted to highnutrient conditions. In contrast, freshwater diatoms haveintermediate growth rates and low K P compared to otherfreshwater taxa. They also have a relatively high P affinityamong freshwater groups, likely being adapted to P-limitedconditions. When marine and freshwater diatoms arecompared, marine diatoms have significantly higher growthrates and K P than freshwater diatoms; conversely, marinediatoms also tend to have lower K N than freshwaterdiatoms. These differences possibly reflect different selectivepressures in marine and freshwater environments.Because of the heavy silica frustules of diatoms, in marineenvironments diatoms benefit from mixing conditionsassociated with high nutrients that select for high growthrates. In freshwaters, diatoms also are associated withmixing conditions, but in shallow lakes mixed layer depthsare too small and strongly select against diatoms (Ptacniket al. 2003). Therefore, freshwater diatoms may be moreprevalent in deeper lakes that also tend to be lower in P(Dillon and Rigler 1974), causing freshwater diatoms to begood P competitors.Uptake affinity is often used as a proxy of the ability togrow or compete under nutrient limitation; therefore, it isimportant to note that uptake affinity on a per-cell basisincreases with cell volume (Fig. 2A,B), because V max scales

560 Edwards et al.Table 2. Tests of between-taxon trait differences. For each trait, the F statistic and p value are listed for a one-way analysis of variance testing the effect of taxon inisolation (‘‘Taxon only’’ rows). Numerator and denominator degrees of freedom are in parentheses. In addition, the Wald (W) statistic and p value are listed for a test ofcommon intercepts across taxa in an SMA regression with cell volume (‘‘Taxon with volume [w/vol.]’’ rows). P values , 0.05 are marked with an asterisk.N scaledaffinity m maxP scaledaffinityV N max K N Q N minV P max K P Q P minP affinity N affinityMarine speciesTaxon only F 4.5(4,25) 5.3(4,32) 6.3(3,19) 0.9(4,22) 0.6(4,22) 5.2(4,23) 0.81(4,20) 6.5(4,25) 3.5(4,16) 5.8(3,12) 12.2(4, 91)Taxon only p 0.0075* 0.0022* 0.0038* 0.48 0.67 0.0038* 0.53 0.001* 0.031* 0.011* 5.6310 28 *Taxon w/vol. W 1.5 8.8 0.83 1.2 1.6 1.2 1.6 3.3 0.43 0.51 30Taxon w/vol. p 0.24 7.10310 25 * 0.5 0.35 0.21 0.35 0.22 0.029* 0.78 0.68 4.5310 26 *Freshwater speciesTaxon only F 9.2(2,13) 0.77(2,24) 3.1(2,17) 5.5(3,34) 5.1(3,31) 14(3,40) 1.7(3,31) 4.2(2,12) 2.8(3,28) 3.3(2,11) 2.3(4, 112)Taxon only p 0.0032* 0.48 0.074* 0.0034* 0.0056* 2.80310 26 * 0.19 0.042* 0.06 0.077 0.061Taxon w/vol. W 3.2 1.6 2 2.4 13 0.57 3.1 1.1 1.2 5.2 35Taxon w/vol. p 0.075 0.22 0.17 0.088 1.50310 25 * 0.64 0.043* 0.38 0.35 0.028* 5.0310 26 *more steeply with volume than K. In contrast, scaleduptake affinity tends to decrease with cell volume, althoughstrong support for this relationship is present only formarine species. Among freshwater species, scaled P affinityshows no relationship with cell volume (Fig. 2C). Furthermore,although the slopes for scaled N affinity cannot bedistinguished for freshwater and marine species, thefreshwater species occupy a limited range of volumes, andby themselves show no correlation with volume (Kendallrank correlation 0.03, p 5 0.91; Fig. 2D). Scaled uptakeaffinity should be a more appropriate proxy for competitiveability under nutrient limitation, because it standardizesuptake ability by the nutrient requirement for growth. Thisis supported by our comparison of scaled uptake affinitieswith the outcome of P-limited chemostat experiments (WebAppendix, Table A2). Our results therefore support theexpectation that the ability to compete for limitingnutrients tends to decline with increasing cell volume(Chisholm 1992), although this relationship may be weakeror nonexistent in freshwater habitats. These results are alsoconsistent with a study of marine species that measuredscaled P affinity directly, as the biomass-specific P turnoverrate, and found that scaled P affinity declines withincreasing equivalent spherical radius (Tambi et al. 2009).Maximum growth rate tends to decline with increasing cellvolume, consistent with prior results (Banse 1976; Chisholm1992). To our knowledge, this is the first study comprehensivelycomparing marine and freshwater species, and we findthat the allometric slope is shallower for marine species(Table 1; Fig. 3). The consequence of this difference in slopesis that, on average, marine phytoplankton of a certain sizegrow faster than freshwater species of the same size, and thisdifference is greater as cell size increases (Fig. 3A). Theallometric slopes of m max for freshwater and marine species are20.36 and 20.24 respectively, and the slope when all speciesare combined is 20.28 (Table 1). These slopes are steeperthan some previously reported slopes (Banse 1976; Tang1995), but this may be due to our use of SMA regression.Lines fit using ordinary least squares (OLS) have slopesbiased towards zero (Warton et al. 2006), and if we fit ourdata using OLS the slopes are 20.10 and 20.096 forfreshwater and marine species, respectively. Because allometricexponents are used to parameterize ecosystem models(Irwin et al. 2006), and to test hypothesized mechanisms forscaling relationships (DeLong et al. 2010), we recommend theuse of SMA regression in comparative analyses, as discussedin detail by Warton et al. (2006).In general, there are a number of limitations to ourcomparisons of freshwater and marine species. Thefreshwater species in our dataset cover a smaller range ofcell volumes, which may limit our ability to detect scalingrelationships and therefore obscure potential effects ofvolume on scaled uptake affinity. However, freshwaterphytoplankton may have a smaller cell size range comparedto marine phytoplankton in general (Litchman et al. 2009;Kamenir et al. 2010). In addition, the species in ourcompilation are those that phytoplankton ecologists havechosen to study in the lab, rather than a random or sizestructuredsample from natural communities. Future workon trait–volume scaling relationships would benefit from a

Reviews in L&O 561Fig. 4. Between-taxon variation in primary nutrient traits for freshwater species. All variables are log 10 -transformed. For eachtaxon, mean trait value and mean volume are plotted, 6 1 SE. The SMA fits from Fig. 1 are plotted as dotted lines. cyan, cyanobacteria;diat, diatoms; chlor, non-desmid chlorophytes; desm, desmids. (A) V N max , (B) KN , (C) Q N min , (D) V P max , (E) KP , (F) Q P min .more comprehensive sampling of natural communities,including endpoints of the size spectrum, from picoplanktonto giant diatoms.Between-taxon differences—We find strong differencesbetween taxa in average nutrient trait values (Figs. 3–6;Table 2). At the same time, few between-taxon differencesremain when controlling for cell volume (Table 2),indicating that taxonomic variation in nutrient utilizationtraits is largely driven by size variation. Considering rawtrait values (not corrected for volume), among marinespecies differences in scaled uptake affinities suggest thatthe relatively small haptophytes and prymnesiophytesshould be better competitors under P limitation, whereasthe relatively large dinoflagellates and raphidophytesshould be poorer competitors (Fig. 7C). The same trendsoccur for scaled N affinity, although there are no data forchlorophytes (Fig. 7D). Among freshwater species, thedata for scaled P affinity suggest that cyanobacteria tendto be better competitors, whereas the desmids are poorcompetitors, with diatoms and non-desmid chlorophytesin between (Fig. 5C). There are fewer data for scaledN affinity among freshwater species, but the presentpatterns suggest that cyanobacteria may be poorercompetitors for nitrate (Fig. 5D); however, only twocyanobacteria are present in this analysis, and one ofthese species can fix nitrogen, which may compensate forlow nitrate affinity.There is significant variation in mean m max between taxa(Table 2), with most variation due to the fact that amongmarine species, diatoms have high growth rates relativeto their cell volume, whereas among freshwater species,cyanobacteria have low growth rates relative to their cellvolume (Fig. 3C,D). However, because of the relativelysmall size of cyanobacteria cells, their raw growth rates arestill relatively high among freshwater species, on average(Fig. 3B).Q N min : Q P minratios—Our analysis of the structural N : Pratio supports prior work showing broad variation in thisratio across phytoplankton (Rhee and Gotham 1980;Klausmeier et al. 2004). We also find that Q N min : Q P min isgreater on average for freshwater species. If the evolutionof this ratio is driven by the relative availability of N and P,the difference between habitats may be due to greaterprevalence of P limitation in freshwater systems (Elser et al.2007). Alternatively, if the evolution of this ratio is drivenby allocation to resource acquisition proteins vs. ribosomes(Klausmeier et al. 2004), higher ratios among freshwaterspecies may indicate a greater tendency for nutrient or lightlimitation in freshwater systems, and a greater tendency fornear-maximal growth rates in marine systems (Goldmanet al. 1979).Comparison of scaling exponents to theoretical predictions—Theobserved scaling exponents of nutrient utilization

562 Edwards et al.Fig. 5. Between-taxon variation in composite traits for freshwater species. All variables arelog 10 -transformed. For each taxon, mean trait value and mean volume are plotted, 6 1 SE. TheSMA fits from Fig. 2 are plotted as dotted lines. cyan, cyanobacteria; diat, diatoms; chlor, nondesmidchlorophytes; desm, desmids. (A) P affinity, (B) N affinity, (C) scaled P affinity, (D)scaled N affinity.traits can be compared to those expected under amechanistic model of nutrient uptake (Aksnes and Egge1991). If V max is determined by the number of uptake siteson the cell surface, and if the density of uptake sites doesnot vary with cell volume, then cell-specific maximumuptake rate should be proportional to surface area. Forspherical cells, this will result in a scaling exponent of 2/3for V max relative to cell volume. Although a prior analysisof nitrate traits in marine species found a scaling exponentconsistent with two-thirds scaling (Litchman et al. 2007),with the current, larger dataset, both V N max and V P max scalemore steeply than 2/3 (95% confidence intervals are [0.68,0.98] and [0.81, 1.1], respectively; Table 1). However, aninterspecific comparison has found that species withgreater cell volume tend to show greater deviation froma spherical shape, and as a result the surface area : volumeratio declines with increasing cell volume, but it declinesmore slowly than expected for spherical cells (Reynolds2006). Therefore, our results for the scaling of V max arenot inconsistent with a scaling relationship driven bysurface area.K appears to be less constrained by cell volume thaneither V max or Q min , and unlike these traits, K exhibitsdifferent scaling patterns for freshwater and marine species,with K P showing no correlation with volume for freshwaterspecies (Table 1; Fig. 1). Therefore, although cell volumemay affect K by altering the mass transfer coefficient(Aksnes and Egge 1991), our results suggest that otherfactors beyond cell volume strongly affect K.We hypothesized earlier (Litchman et al. 2007) thatallometric exponents for Q min may range from 2/3 to 1, ifwe assume that the minimum nutrient quota is contained inthe cytoplasm that occupies most of the cell volume insmall cells (isometric scaling), or is distributed along the cellwall in cells with large vacuoles (thus scales with cellsurface, 2/3 of volume). We are, however, unaware ofmechanistic predictions for whether Q min scaling exponentsshould be different for N and P. Intriguingly, Q N min scalesallometrically with volume (95% confidence interval [0.76,0.94]; Table 1), whereas the scaling of Q P minis indistinguishablefrom isometric (95% confidence interval [0.87,1.1]; Table 1). This also implies that the ratio Q N min : Q P minmay decline as volume increases. In our dataset, this is truewhen marine and freshwater species are combined (Kendallrank correlation: 20.28, p 5 0.013), although not whenmarine and freshwater species are analyzed separately(marine correlation 20.059, p 5 0.77; freshwater correlation20.25, p 5 0.14).The allometric scaling of uptake affinities and scaleduptake affinities are consistent with the scaling of the

Reviews in L&O 563Fig. 6. Between-taxon variation in primary nutrient traits for marine species. All variables are log 10 -transformed. For each taxon,mean trait value and mean volume are plotted, 6 1 SE. The SMA fits from Fig. 1 are plotted as dotted lines. hapt, haptophytes; diat,diatoms; chlor, chlorophytes; dino, dinoflagellates; raph. raphidophytes. (A) V N max , (B) KN , (C) Q N min , (D) V P max , (E) KP , (F) Q P min .component parameters V max , K, and Q min . V max tends toincrease more steeply with volume than K, resulting in anincrease in cell-specific uptake affinity with volume (Fig. 2).When uptake affinity is scaled by Q min , this results in adecrease in scaled uptake affinity with increasing volume,because the scaling exponents of V max and Q min tend to besimilar. The exception to this trend is scaled P affinity forfreshwater species, which does not decline with increasingvolume; this may be due to the fact that K P does not declinewith volume.Potential constraints among nutrient utilization traits—Araw correlation analysis reveals a preponderance of positivecorrelations among nutrient traits (Table 3). These positivecorrelations may partially constrain the evolution ofcompetitive ability, because for a given nutrient, a positivecorrelation between V max and K, or between V max andQ min , constrains the evolution of equilibrium competitiveability, whereas a positive correlation between K and Q minhas the opposite effect (Litchman et al. 2007). However, ifthese trait correlations are driven solely by cell size, thenthe only relevant constraints for trait evolution are thosethat will constrain size evolution, i.e., the trait–volumescaling parameters (Litchman et al. 2009). Our partialcorrelation analysis, which tested for trait correlations aftercontrolling for the effect of cell volume, yielded equivocalresults. This analysis yielded only positive significantcorrelations between nutrient traits, with the exception ofK P vs. K N for freshwater species, which is also negative inthe raw correlation analysis (Table 3). Furthermore, thesignificant correlations occurred among pairs of traits thatwere strongly correlated in the raw correlation analysis(Table 3). It therefore seems possible that the partialcorrelation analysis did not succeed in fully removing thecorrelated effect of volume on nutrient traits. Measurementerror for either nutrient traits or cell volume, along withplasticity in cell volume, will tend to add noise to the trait–volume relationship. This noise will prevent the partialcorrelation from fully removing the true relationshipbetween a trait and volume. The same considerations applyto the negative correlations between m max and nutrienttraits (Table 3).However, some of the partial correlations may indeedrepresent constraints, or correlated selection pressures. Thenegative correlation between K P and K N among freshwaterspecies is unlikely to be driven by volume, and may contributeto a trade-off in competitive ability for nitrate vs.phosphate among freshwater species. Among the positivepartial correlations, nearly all would constrain the evolutionof high scaled uptake affinity for a single nutrient(for marine species, V N max vs. Q N min , V P max vs. KP ; forfreshwater species, V N max vs. Q N min), or would constrain thesimultaneous evolution of high scaled uptake affinity forboth nutrients (for marine species, V N max vs. KP , V P max vs. KN ;for freshwater species, V N max vs. V P max , V P max vs. Q N min ).Therefore, these relationships may represent eco-evolutionary

564 Edwards et al.Fig. 7. Between-taxon variation in composite traits for marine species. All variables arelog 10 -transformed. For each taxon, mean trait value and mean volume are plotted, 6 1 SE. TheSMA fits from Fig. 2 are plotted as dotted lines. hapt, haptophytes; diat, diatoms; chlor,chlorophytes; dino, dinoflagellates; raph, raphidophytes. (A) P affinity, (B) N affinity, (C) scaledP affinity, (D) scaled N affinity.Table 3. Correlation matrices for between-trait relationships. Entries in the upper right are for marine species, entries in the lowerleft are for freshwater species.V N max K N Q N minV P max K P Q P minm maxRaw Kendall rank correlationsV N 0.28* 0.69** 0.6** 0.71** 0.54* 20.37*maxK N 0.27 0.53* 0.6** 0.52* 0.56* 20.27*Q N 0.66** 0.13 0.66** 0.69** 0.84** 20.61**minV P 0.43* 20.066 0.52* 0.59** 0.66** 20.22maxK P 20.14 20.45* 20.3 0.19 0.64** 20.14Q P 0.7** 20.037 0.55** 0.42** 20.0034 20.35minm max 20.029 20.16 20.22 20.2 20.032 20.2Partial Kendall rank correlations thatcontrol for cell volumeV N 0.086 0.45* 0.34 0.53* 0.072 20.12maxK N 0.18 0.032 0.37* 0.19 0.078 20.091Q N 0.57* 0.065 0.087 0.13 0.53* 20.35*minV P 0.39* 20.1 0.62** 0.31* 0.13 0.06maxK P 0.051 20.38* 20.0095 0.19 0.20 0.12Q P 0.62* 20.18 0.3 0.21 0.059 20.23minm max 0.024 20.12 20.25 20.2 20.027 20.042* p , 0.05, ** p , 0.001.

Reviews in L&O 565freshwater environments, highlighting the interaction ofphylogeny and environmental controls.AcknowledgmentsThis work was in part supported by National ScienceFoundation grant 09-28819 to E.L. and C.A.K., 08-45932 toE.L., and a James S. McDonnell Foundation grant to C.A.K. andE.L. We thank Jonathan J. Cole and two anonymous reviewersfor helpful comments. This is Kellogg Biological Stationcontribution No. 1635.Fig. 8. Distribution of Q N min :Q P minmolar ratios. X-axis is ona log scale. (A) Freshwater species, (B) marine species.trade-offs among nutrient utilization traits. Edwards et al.(2011) used the same dataset to test for trade-offs amongscaled N affinity, scaled P affinity, and cell volume. Theyfound evidence for a general three-way trade-off among thesethree traits, with the relationship between scaled N affinityand scaled P affinity most evident in freshwater species, andthe relationship between cell volume and the two nutrienttraits most evident in marine species.Our extensive compilation of nutrient utilization traits inmarine and freshwater phytoplankton shows that there arefundamental scaling relationships for most traits, and theserelationships are usually shared by marine and freshwaterphytoplankton. However, there are some trait differencesthat possibly reflect different selective pressures betweenmarine and freshwater environments, such as the relativestrength of limitation by different nutrients. Major taxadiffer in their nutrient utilization traits, reflecting contrastingecological strategies that are often driven by differencesin cell volume. Our data also suggest that some taxa, suchas diatoms, may have different strategies in marine vs.ReferencesAKSNES, D. L., AND J. K. EGGE. 1991. A theoretical model fornutrient uptake in phytoplankton. Mar. Ecol. Prog. Ser. 70:65–72, doi:10.3354/meps070065BANSE, K. 1976. Rates of growth, respiration and photosynthesisof unicellular algae as related to cell size—review. J. Phycol.12: 135–140.CHISHOLM, S. W. 1992. Phytoplankton size, p. 213–237. In P. G.Falkowski and A. D. Woodhead [eds.], Primary productivityand biogeochemical cycles in the sea. Plenum.DELONG, J. P., J. G. OKIE, M.E.MOSES, R.M.SIBLY, AND J. H.BROWN. 2010. Shifts in metabolic scaling, production, andefficiency across major evolutionary transitions. Proc. Natl.Acad. Sci. USA 107: 12941–12945, doi:10.1073/pnas.1007783107DILLON, P. J., AND F. H. RIGLER. 1974. Test of a simple nutrientbudget model predicting phosphorus concentration in lake water.J. Fish. Res. Board Can. 31: 1771–1778, doi:10.1139/f74-225EDWARDS, K. F., C. A. KLAUSMEIER, AND E. LITCHMAN. 2011.Evidence for a three-way tradeoff between nitrogen andphosphorus competitive abilities and cell size in phytoplankton.Ecology 92: 1085–1095.ELSER, J. J., AND OTHERS. 2007. Global analysis of nitrogen andphosphorus limitation of primary producers in freshwater,marine and terrestrial ecosystems. Ecol. Lett. 10: 1135–1142,doi:10.1111/j.1461-0248.2007.01113.xEPPLEY, R. W., J. N. ROGERS, AND J. J. MCCARTHY. 1969. Halfsaturationconstants for uptake of nitrate and ammonium bymarine phytoplankton. Limnol. Oceanogr. 14: 912–920,doi:10.4319/lo.1969.14.6.0912GOLDMAN, J. C., J. J. MCCARTHY, AND D. G. PEAVEY. 1979.Growth influence on the chemical composition of phytoplanktonin oceanic waters. Nature 279: 210–215, doi:10.1038/279210a0GROVER, J. P. 1989. Influence of cell shape and size on algalcompetitive ability. J. Phycol. 25: 402–405, doi:10.1111/j.1529-8817.1989.tb00138.x———. 1991. Dynamics of competition among microalgae invariable environments: Experimental tests of alternativemodels. Oikos 62: 231–243, doi:10.2307/3545269HEALEY, F. P. 1980. Slope of the monod equation as an indicatorof advantage in nutrient competition. Microb. Ecol. 5:281–286, doi:10.1007/BF02020335HECKY, R. E., AND P. KILHAM. 1988. Nutrient limitation ofphytoplankton in freshwater and marine environments: Areview of recent evidence on the effects of enrichment. Limnol.Oceanogr. 33: 796–822, doi:10.4319/lo.1988.33.4_part_2.0796IRWIN, A. J., Z. V. FINKEL, O. M. E. SCHOFIELD, AND P. G.FALKOWSKI. 2006. Scaling-up from nutrient physiology to thesize-structure of phytoplankton communities. J. PlanktonRes. 28: 459–471, doi:10.1093/plankt/fbi148KAMENIR, Y., Z. DUBINSKY, AND R. HARRIS. 2010. Taxonomic sizestructure consistency of the English Channel phytoplankton. J. Exp.Mar. Biol. Ecol. 383: 105–110, doi:10.1016/j.jembe.2009.12.009

566 Edwards et al.KILHAM, P., AND R. E. HECKY. 1988. Comparative ecology ofmarine and freshwater phytoplankton. Limnol. Oceanogr. 33:776–795, doi:10.4319/lo.1988.33.4_part_2.0776KLAUSMEIER, C. A., E. LITCHMAN,T.DAUFRESNE, AND S. A. LEVIN.2004. Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton.Nature 429: 171–174, doi:10.1038/nature02454LITCHMAN, E., AND C. A. KLAUSMEIER. 2008. Trait-based communityecology of phytoplankton. Annu. Rev. Ecol. Evol. Syst.39: 615–639, doi:10.1146/annurev.ecolsys.39.110707.173549———, ———, J. R. MILLER, O. M. SCHOFIELD, AND P. G.FALKOWSKI. 2006. Multi-nutrient, multi-group model ofpresent and future oceanic phytoplankton communities.Biogeosciences 3: 585–606, doi:10.5194/bg-3-585-2006———, ———, O. M. SCHOFIELD, AND P. G. FALKOWSKI. 2007. Therole of functional traits and trade-offs in structuring phytoplanktoncommunities: Scaling from cellular to ecosystem level.Ecol. Lett. 10: 1170–1181, doi:10.1111/j.1461-0248.2007.01117.x———, ———, AND K. YOSHIYAMA. 2009. Contrasting sizeevolution in marine and freshwater diatoms. Proc. Natl. Acad.Sci. USA 106: 2665–2670, doi:10.1073/pnas.0810891106MOREL, F. M. M. 1987. Kinetics of nutrient uptake and growth inphytoplankton. J. Phycol. 23: 137–150.PTACNIK, R., S. DIEHL, AND S. BERGER. 2003. Performance ofsinking and nonsinking phytoplankton taxa in a gradient ofmixing depths. Limnol. Oceanogr. 48: 1903–1912, doi:10.4319/lo.2003.48.5.1903REYNOLDS, C. S. 2006. Ecology of phytoplankton. CambridgeUniv. Press.RHEE, G. Y., AND I. J. GOTHAM. 1980. Optimum N : P ratios andcoexistence of planktonic algae. J. Phycol. 16: 486–489,doi:10.1111/j.1529-8817.1980.tb03065.xSHUTER, B. J. 1978. Size-dependence of phosphorus and nitrogensubsistence quotas in unicellular micro-organisms. Limnol.Oceanogr. 23: 1248–1255, doi:10.4319/lo.1978.23.6.1248SMAYDA, T. J. 1997. Harmful algal blooms: Their ecophysiology andgeneral relevance to phytoplankton blooms in the sea. Limnol.Oceanogr. 42: 1137–1153, doi:10.4319/lo.1997.42.5_part_2.1137STERNER, R., AND J. J. ELSER. 2002. Ecological stoichiometry.Princeton Univ. Press.STOLTE, W., AND R. RIEGMAN. 1996. A model approach for sizeselectivecompetition of marine phytoplankton for fluctuatingnitrate and ammonium. J. Phycol. 32: 732–740, doi:10.1111/j.0022-3646.1996.00732.xTAMBI, H., G. A. F. FLATEN, J. K. EGGE, G. BODTKER, A.JACOBSEN, AND T. F. THINGSTAD. 2009. Relationship betweenphosphate affinities and cell size and shape in various bacteriaand phytoplankton. Aquat. Microb. Ecol. 57: 311–320,doi:10.3354/ame01369TANG, E. P. Y. 1995. The allometry of algal growth rates. J.Plankton Res. 17: 1325–1335, doi:10.1093/plankt/17.6.1325TILMAN, D. 1982. Resource competition and community structure.Princeton Univ. Press.WARTON, D. I., AND J. ORMEROD. 2007. smatr: (Standardised)major axis estimation and testing routines. R packageversion 2.1. Available from http://web.maths.unsw.edu.au/,dwarton———, I. J. WRIGHT, D.S.FALSTER, AND M. WESTOBY. 2006.Bivariate line-fitting methods for allometry. Biol. Rev. 81:259–291, doi:10.1017/S1464793106007007Associate editor: Jonathan J. ColeReceived: 04 October 2011Accepted: 03 January 2012Amended: 20 January 2012