Phosphate supply explains variation in nucleic acid allocation but not C : P stoichiometry in the western North Atlantic

. Marine microbial communities mediate many bio-geochemical transformations in the ocean. Consequently, processes such as primary production and carbon (C) export are linked to nutrient regeneration and are inﬂuenced by the resource demand and elemental composition of marine microbial biomass. Laboratory studies have demonstrated that differential partitioning of element resources to various cellular components can directly inﬂuence overall cellular elemental ratios, especially with respect to growth machinery (i.e., ribosomal RNA) and phosphorus (P) allocation. To investigate whether allocation to RNA is related to biomass P content and overall C : P biomass composition in the open ocean, we characterized patterns of P allocation and C : P elemental ratios along an environmental gradient of phosphate supply in the North Atlantic subtropical gyre (NASG) from 35.67 ◦ N, 64.17 ◦ W to 22.676 ◦ N, 65.526 ◦ W.


Introduction
Redfield observations of the stoichiometric similarly between dissolved nutrients in the deep ocean and surface ocean plankton represent one of the cornerstones of marine biogeochemistry (Redfield, 1934(Redfield, , 1958)).Coupling among macronutrients across dissolved and particulate fractions carries consequences for the flux of energy and elements in marine systems (Arrigo, 1999;Deutsch and Weber, 2012;Hessen et al., 2004).For example, carbon (C): phosphorus (P) ratios link nutrient cycling with CO 2 fixation and the biological C pump (Omta et al., 2006;Tyrrell, 1999).It is now clear that broad-scale spatial and temporal variability in elemental ratios and departures from Redfield proportions are common (Martiny et al., 2013;Michaels et al., 2001).Furthermore, incorporating stoichiometric flexibility into ecological models has improved their ability to capture certain biogeochemical dynamics (Christian, 2005;Deutsch and Weber, 2012;Flynn, 2010).Understanding both the patterns and mechanisms of stoichiometric variability is central to interpreting biogeochemical data and predicting ecological consequences.
Published by Copernicus Publications on behalf of the European Geosciences Union.
Several mechanisms have been identified that may contribute to variation in stoichiometric ratios at various scales.Marine plankton have taxon-specific limitations on biomass stoichiometry and can have different stoichiometric composition under the same environmental conditions (Grob et al., 2013;Quigg et al., 2003Quigg et al., , 2011;;Zimmerman et al., 2013).Consequently, community composition affects community stoichiometry, and may contribute to significant differences between oceanic regions (Martiny et al., 2013;Weber and Deutsch, 2010).Several studies have also demonstrated significant differences in elemental stoichiometry as populations or communities respond to prevailing environmental conditions, most often characterized by nutrient supply ratios (Bratbak, 1985;Rhee, 1978;Tezuka, 1990;Vrede et al., 2002).Taxonomic constraints and nutrient supply are not mutually exclusive mechanisms and likely interact along environmental gradients to influence stoichiometry at ecosystem scales.
Underlying these mechanisms is a dynamic allocation of cellular resources according to ecological growth strategy (Elser et al., 2003;Franklin et al., 2011;Klausmeier et al., 2004;Vrede et al., 2004).Biochemical allocation influences the elemental composition of a cell via inherent elemental differences in biomolecules, including proteins, carbohydrates, nucleic acids, lipids, and polyphosphate (Geider and La Roche, 2002;Sterner and Elser, 2002).Nucleic acids in particular have been highlighted as quantitatively important to cellular P quotas due to their relatively high P content and the necessity of ribosomal RNA (rRNA) for growth (Elser et al., 2000(Elser et al., , 2003;;Sterner and Elser, 2002).In the field of ecological stoichiometry, relationships among RNA content, biomass P, and growth are formalized as the growth rate hypothesis (GRH).The influence of RNA P on total cellular P content is expected to be strongest under conditions of P limitation (Elser et al., 2003;Makino et al., 2003), implying that differential nucleic acid allocation may significantly contribute to variation in community C : P stoichiometry in P-limited ecosystems.To our knowledge, the relationships between RNA allocation and P content, and the resulting impact on biomass C : P ratios have been studied exclusively in culture-based experiments, and no published studies have examined these relationships in the open ocean.
The Sargasso Sea in the North Atlantic subtropical gyre (NASG) is characterized by low concentrations of dissolved inorganic P and rapid turnover rates, which indicate conditions of P stress (Ammerman et al., 2003;Cavender-Bares et al., 2001;Cotner et al., 1997;Mather et al., 2008;Wu et al., 2000).Evidence suggests that chronic P depletion has driven organisms from the western NASG to evolve sophisticated genetic mechanisms for responding to P fluctuations (Coleman and Chisholm, 2010;Martiny et al., 2006).Plankton can use both dissolved inorganic and organic forms to satisfy cellular P quotas (Casey et al., 2009;Lomas et al., 2010), but inorganic forms (operationally defined as soluble reactive phosphorus, SRP) are more directly bioavailable and preferred by osmotrophs (Dyhrman et al., 2007;Moore et al., 2005).Accordingly, taxon-specific rates of inorganic P uptake by native plankton responded positively to increasing SRP concentration (Casey et al., 2009).Biochemical allocation may be similarly responsive to SRP supply in the western NASG, but this prediction remains to be tested.
The objective of this study was to analyze the relationship between biochemical allocation strategy and nutrient supply in open ocean communities to evaluate a potential driver of ecosystem-scale variability in the particulate element ratios of surface waters.To address this objective, we analyzed biological pools of P and biomass C : P ratios of bulk seawater along a latitudinal gradient in the Sargasso Sea representing a range of SRP supply rates.The surface waters north of the Bermuda Atlantic Time-series Study (BATS; 31.67 • N, 64.17 • W) station generally experience more frequent exchange with nutrient-rich deep water due to convective mixing, resulting in higher SRP fluxes at the northern latitudes than in the more permanently stratified water of the southern latitudes (Cavender-Bares et al., 2001).We hypothesized that in this P-depleted system, SRP supply influences biochemical allocation, with increased allocation toward P-rich RNA for growth at higher SRP flux rates.In turn, we hypothesized that these changes in allocation may reduce biomass C : P ratios.We therefore expected higher SRP fluxes to correlate with higher total RNA, RNA : DNA ratios, proportion of P allocated to RNA and total P biomass but lower C : P ratios.Support for our hypothesis would imply that the RNA allocation strategy is a principal biological mechanism for linking nutrient supply to variation in biomass stoichiometry at the ecosystem scale.

Sample collection
Samples were collected from a Niskin bottle on a rosette equipped with a CTD instrument (conductivity, temperature and depth; Sea-bird Electronics, Inc., Bellevue, WA, USA) during cruise AE1226/BV47 aboard the R/V Atlantic Explorer in the Sargasso Sea (Fig. 1).Sampling occurred from 27 September to 6 October 2012 on a transect from 35.67 • N, 64.176 • W to 22.67 • N, 65.52 • W, past the BATS site at 31.67 • N, 64.17 • W (Fig. 1).
Samples were collected for determination of particulate (nominally > 0.3 µm) organic carbon (POC), particulate phosphorus (PPhos), nucleic acids (RNA and DNA), soluble reactive phosphorus (SRP), and total dissolved phosphorus (TDP).Surface seawater (≤ 5 m depth) from replicate Niskin bottles was collected directly into polycarbonate Nalgene bottles (Thermo Scientific Nalgene, Rochester, NY, USA), which had previously been washed with Micro-90 detergent (International Products Corp., Burlington, NJ, USA) and 10 % HCl (with repeated rinsing after each wash), and were also collected as described and processed identically to the samples.Separate samples for SRP and TDP analyses were collected from different Niskin bottles on the same hydrocasts, representing depths from 0 to 250 m.SRP and TDP samples were stored unfiltered in acid-cleaned, highdensity polyethylene (HDPE) bottles at −20 • C until analysis (Lomas et al., 2010).Additional hydrographic and biogeochemical measurements were collected at each sampling station (st.) and are accessible through the BATS web page (http://bats.bios.edu/).Sample collection and analysis details for these additional parameters can be found in the BATS Method Manual (Knap et al., 1997) or online (http://bats.bios.edu/).To complement our data, we analyzed POC, PPhos, and SRP concentrations from three additional cruises in this region but spanning multiple seasons (cruise X0705 in June 2007, BVal39 in October 2007, andX0804 in May 2008).SRP flux and surface C : P ratios were calculated and analyzed as described below.

Dissolved phosphorus pools
SRP concentrations were measured using the magnesiuminduced co-precipitation method (MAGIC, Karl and Tien, 1992) with modifications as described by Lomas et al. (2010).TDP concentrations were determined using a modified version of the high temperature/acid persulfate oxidation method (Lomas et al., 2010;Ridal and Moore, 1990).Dissolved organic phosphorus (DOP) concentrations were calculated as the difference between TDP and SRP concentrations.The upward flux of SRP was calculated as the product of the coefficient of vertical diffusivity (0.000035 m 2 s −1 for BATS, Ledwell et al., 2008) and the concentration gradient from 80-160 m at each sampling station.SRP was drawn down to the method detection limit above 80 m, thus SRP flux could not be calculated reliably over shallower depths.For stations where the measured concentration gradient was ambiguous (stations 4, 7, and 10), SRP flux was linearly interpolated from the two immediately adjacent values.

Particulate elements
POC was determined using a CHN analyzer (Thermo Finnigan EA 1112) after samples were treated with HCl (0.2 M) to remove inorganic material and dried overnight at 65 • C. Sample C mass was calculated from chromatogram area using atropine standards and corrected for filter blanks.PPhos was determined using an ash-hydrolysis method with MgSO 4 (0.017 M) treatment as previously described (Lomas et al., 2010;Solorzano and Sharp, 1980).Sample P was calculated from a linear regression of absorbance vs. known concentrations of potassium phosphate standards and corrected for filter blanks.PPhos includes both organic and inorganic phosphorus, as no effort was made to separate the two fractions.POC and PPhos bulk concentrations and nutrient molar ratios are reported.

Nucleic acids
Seawater RNA and DNA concentrations were determined using high-sensitivity, macromolecule-specific Quant-iT fluorophores (Molecular Probes, Inc., Eugene, OR, USA) following a crude lysis as previously described (Zimmerman et al., 2013).Briefly, nucleic acids and proteins were released from filters by mechanical lysis (MP FastPrep-24 bead beater, MP Biomedicals, Solon, OH, USA) in a solution of Tris buffer (5 mM) and RNA preservative (saturated ammonium sulfate solution).Sample supernatant was used to prepare assays in 96-well microplates with fluorescent dye, buffer, and prediluted standards provided with each kit (E. coli rRNA or λ ds-DNA).Fluorescence was measured on a SpectraMax M2 microplate reader (Molecular Devices, LLC, Sunnyvale, CA).Standards, buffers, and reagents were stored and used according to the manufacturer's suggestions.Spiked control samples from the ship's underway system were included to account for potential signal quench.Macromolecule concentrations were calculated based on standard curve regressions of fluorescence vs. known standard concentrations.The amount of P present in RNA (RNA-P) and DNA (DNA-P) was calculated assuming nucleic acids are 9 % P on average (Sterner and Elser, 2002)  Filled circles represent stations where the SRP flux was linearly interpolated from the two immediately adjacent values because the concentration gradient was not measurable.

Statistical analysis
All statistical analyses were conducted using the "stats" package in R (R Core Team, 2012).Values are expressed as means ± standard error (SE), unless otherwise indicated.Differences in allocation and stoichiometry among stations along the transect were examined using analysis of variance (ANOVA).Data that did not meet assumptions of normality and homoscedasticity were evaluated with the nonparametric Kruskal-Wallis ANOVA.Spearman's rank correlations were used to assess latitudinal trends, as well as associations between measured variables.This correlation analysis reduces the influence of outlying data points that were observed in our data set (e.g., st.16 SRP flux).We used Wilcoxon signed rank tests to test whether C : P ratios differed from Redfield proportions (C : P = 106) at each station (n = 4 per station) and averaged across all stations (n = 11).We also used Wilcoxon signed rank tests to test for differences between RNA and DNA allocation along the transect.We considered all statistical analyses to be significant for P < 0.05.

Transect description
We analyzed biological allocation of P resources and particulate C : P ratios of bulk seawater at 11 stations along a N-S transect spanning 13 • of latitude (22.67-35.67• N; 1445 km) in the oligotrophic Sargasso Sea (Fig. 1; Table 1).Surface (≤ 5 m) water temperature was negatively correlated with latitude (Spearman rank correlation, ρ = −0.92,P < 0.001; Fig. A1), and decreased from 28.8  Consequently, variation in temperature is inherent to the latitudinal trends described below.

Latitudinal trends in phosphorus pools
DOP was the largest P pool measured, ranging from 18.7 to 53.8 nmol L −1 (Fig. 2a), but was not significantly correlated with latitude.The highest DOP concentrations were found at either end of the transect, with the lowest concentration occurring at the BATS station.All of the surface SRP concentrations were low (< 5 nmol L −1 ; Fig. 2a), and six stations were below the nominal detection limit of the MAGIC-SRP method (∼ 1 nmol L −1 ; Lomas et al., 2010).

Latitudinal trends in nucleic acids
Concentrations of bulk particulate RNA and DNA in surface waters varied ∼ threefold across stations (P < 0.001 for both, ANOVA; Fig. 3a), ranging 0.22-0.63µg L −1 for RNA and 0.38-1.15µg L −1 for DNA.Similar to SRP flux, both nucleic acids showed relatively uniform concentrations in the lower latitudes and an increase at the north end of the transect (Spearman rank correlations, RNA : ρ = 0.59, P = 0.027, DNA: ρ = 0.51, P = 0.057).Concentrations of DNA were consistently higher than RNA (P < 0.001, Wilcoxon signed rank test), and RNA : DNA ratios along the transect varied significantly from 0.46 to 0.71 (P = 0.046, ANOVA; Fig. 3b).
In contrast to total nucleic acid concentrations, the maximum RNA : DNA ratio was at 30.67 • N (st.3), just south of BATS, but RNA : DNA ratios across stations still showed a significant monotonic increase with latitude (Spearman rank correlation, ρ = 0.65, P = 0.016).Overall, the contribution of P in nucleic acids to total PPhos was low (< 32 % in RNA and DNA combined; Fig. 3c), and the proportion of PPhos in RNA (P RNA , 0.06-0.12)was lower than in DNA (P DNA , 0.11-0.19;P = 0.004, Wilcoxon signed rank test).Allocation of P to both nucleic acids was variable and did not significantly differ among stations, but mean P RNA was positively correlated with latitude across the transect (Spearman rank correlation, ρ = 0.57, P = 0.034).

Discussion
We hypothesized that greater supply of phosphate across the base of the euphotic zone in the northern latitudes of the Sargasso Sea would support more P biomass and facilitate greater biochemical allocation toward P-rich RNA for growth than in the more permanently stratified lower latitudes.Phosphate supply (operationally defined as soluble reactive phosphorus, SRP) varied sevenfold across our latitudinal transect (Fig. 2b, Table 1), but this gradient was accompanied by less variation in biochemical allocation (1.5-fold for RNA : DNA ratio, threefold for nucleic acid concentration) in corresponding surface waters (Fig. 3, Table 1).In support of our hypothesis, we found evidence for a community-level increase in allocation to RNA in response to higher SRP supply rates (Fig. 6); however, a concurrent increase in total PPhos with SRP flux was not supported (Fig. 5).This disconnect may reflect a mismatch in sampling horizons, although previous studies in this region clearly show that phytoplankton in the surface mixed layer obtain nutrients from depth (Fawcett et al., 2011(Fawcett et al., , 2014;;Johnson et al., 2010).Therefore it is likely that phosphate from depth supports surface plankton.Alternatively, the relatively small proportion of total PPhos represented by RNA and weak change in RNA : DNA ratio across sampling stations (Table 1) may have contributed to the disconnect between SRP flux and total PPhos.We additionally predicted that the supply of phosphate from depth is related to community-level C : P stoichiometry in the P-depleted surface waters of the NASG.However, the C : P ratio did not vary systematically with latitude or SRP supply rate (Figs.4a, 7).Accumulated dead plankton material could mask the effect of changes in the RNA content of living plankton biomass on particulate P content and community C : P ratios.However, this does not appear to be the case in the subtropical North Atlantic, as shown by an analysis of the summed contributions of flow-cytometrically sorted populations to total particulate carbon, nitrogen, and phosphorus pools (Martiny et al., 2013).Thus there was little support for a consistent influence of nutrient supply rate or biochemical allocation on whole community C : P stoichiometry despite a regionally coherent response in biochemical allocation to nutrient supply rate.Both POC and DNA concentrations increased with SRP flux along the transect (Figs.6a, A5).These results suggest that greater supply of resources from nutrient-rich deep water facilitated an increase in the total biomass supported in surface water.Likewise, the significant increase in the RNA : DNA ratio, a potential proxy for growth rate (Dortch et al., 1983), with SRP supply (Fig. 6b) suggests that community growth rate could be similarly sensitive to nutrient supply.However, we recognize that the correlation between the RNA : DNA ratio and growth rate tends to be weaker for a mixed community than an individual taxon (Jeffrey et al., 1996;Kemp et al., 1993).Though we are unable to tease apart the individual influences of SRP and other nutrients that may have been supplied concurrently, high C : P ratios (i.e., significantly greater than Redfield) measured along the transect indicated P stress.Given that the western NASG is characterized by conditions of P stress (Ammerman et al., 2003;Cavender-Bares et al., 2001;Cotner et al., 1997;Mather et al., 2008;Wu et al., 2000), we hypothesize that it is likely P regulates growth and biomass accumulation, though other factors may also have an impact.Higher biomass may be the result of increased carrying capacity of the ecosystem (i.e., greater abundance of individual populations) or a shift to larger cells, whose relatively higher cellular quotas could be supported by the increased supply rate (Chisholm, 1992;Edwards et al., 2012;Marañón et al., 2013).Additionally, with the data we collected, we cannot exclude the possibility that this pattern reflects accumulation of particulate matter resulting from a decrease in loss processes (e.g., grazing and viral lysis).

Biogeosciences
Although we focused on SRP supply rates under the assumption that SRP is the form of P preferred by marine microbes (Casey et al., 2009), DOP is also important for sustaining primary productivity in the Sargasso Sea (Lomas et al., 2010).DOP may be especially important as an alternative P source at lower SRP supply rates, though we did not detect any significant correlations between DOP and the biological parameters we measured.There does not appear to be long-range transport of DOP from the eastern North Atlantic upwelling regions (Roussenov et al., 2006;Torres-Valdés et al., 2009).Additionally, uptake of SRP by organisms in this region has been shown to be sensitive to SRP concentration while DOP uptake rates were not sensitive to DOP concentration (Casey et al., 2009).
Overall, P from nucleic acids only represented ∼ 22 % of total PPhos (Fig. 3c).Our values are likely driven by generally low concentrations of total particulate RNA and DNA in our samples (Fig. 3a of values reported for the range of nucleic acid concentrations in the Gulf of Mexico (Jeffrey et al., 1996) and the oligotrophic Mediterranean Sea (Dell'Anno et al., 1999).We recognize that there may be uncertainty in our P RNA and P DNA data because we could not account for potential variation in nucleic acid extraction efficiency due to incomplete cell lysis, and therefore these values should be considered as minimum quotas.Regardless, our calculations of P RNA in natural marine microbial communities are low in comparison to previous studies.Makino et al. (2003) demonstrated that the allocation of P resources to RNA can vary in cultures of E. coli from 40 to 80 %, depending on growth rate.Likewise, the proportion of biomass P represented by RNA varied from 25 to 93 % for communities of lake bacteria under manipulated growth and substrate ratio conditions (Makino and Cotner, 2004).In addition to PPhos bound in nucleic acids, ∼ 23 % of total PPhos may be found in phospholipids (Van Mooy et al., 2006).Our results suggest that there appear to be additional quantitatively important reservoirs of PPhos for plankton communities in the NASG.A likely candidate is polyphosphate, which has recently also been shown to be part of a stress response to P deficiency in diatoms (Dyhrman et al., 2012) and to increase in concentration from the coastal to open ocean (Martin and Van Mooy, 2013).These potentially important alternative reservoirs of PPhos warrant further investigation in order to comprehensively understand dynamic P allocation in natural plankton communities.We speculate that taxonomic diversity within communities of marine plankton likely contributed to variation in C : P stoichiometry along the SRP supply gradient.Previous studies have demonstrated that a broad range of plankton taxa, from heterotrophic bacteria to diatoms and micrograzers, persist in this oligotrophic region (DuRand et al., 2001;Longnecker et al., 2010;Treusch et al., 2012;Worden and Binder, 2003), and that the relative abundances of planktonic groups change with latitude (Cavender-Bares et al., 2001;Martiny et al., 2013).These broad taxonomic groups represent a range of ecological and trophic strategies, which inherently differ in resource requirements and may therefore experience varying degrees of P stress in the same environment (Casey et al., 2009;Lomas et al., 2004).For example, photosynthetic machinery imposes different constraints on cellular resource allocation and stoichiometry for autotrophs versus heterotrophs (Vrede et al., 2004).Cell sizes vary across taxonomic groups and are robustly correlated with nutrient uptake and use properties (Chisholm, 1992;Edwards et al., 2012;Marañón et al., 2013).Consequently, multiple effects of broad taxonomic diversity likely interact at the ecosystem scale to constrain emergent community patterns in biochemical allocation and stoichiometry.
Underlying broad-scale taxonomic diversity is additional fine-scale functional diversity in resource use and physiological plasticity related to P cycling.This diversity potentially further exacerbated the decoupling of SRP supply from community C : P stoichiometry in our study.For example, within the well-studied Prochlorococcus and Synechococcus genera, lineages vary in their P-stress response mechanisms as well as in their ability to use a variety of inorganic and organic P sources (Martiny et al., 2006;Moore et al., 2005).Synechococcus also shows interstrain differences in the ability to store P as polyphosphate under conditions of P starvation (Mazard et al., 2012), with probable impacts on cellular C : P ratios.Therefore, different individuals within the same taxonomic group may experience different degrees of P stress and exhibit different strategies when responding to changes in nutrient supply.Accordingly, it may be necessary to evaluate genotype-and even cell-specific responses to environmental P supply to understand variability in the aggregate community patterns of biochemical allocation and stoichiometry in response to large-scale changes in nutrient conditions in the ocean.
Our results have important implications for ecosystemscale variability in the particulate element ratios of the ocean surface.We have shown that community C : P stoichiometry varied little across a latitudinal gradient of SRP supply (Figs.4a, 7).PPhos concentrations across the range of the transect were likewise decoupled from both SRP supply rate (Fig. 5) and allocation of biomass P to RNA (Fig. A3).By contrast, we detected a coherent community response in nucleic acid allocation to SRP flux (Fig. 6).Collectively, our results imply that neither biochemical allocation (at least to nucleic acids) nor prevailing environmental conditions (i.e., phosphate supply rate) are the primary mechanisms for explaining emergent community variation in C : P stoichiometry at this scale.We speculate that alternative reservoirs of cellular P, such as phospholipids and storage compounds, and taxonomic composition, which appears to be important at broader spatial scales (Martiny et al., 2013;Weber and Deutsch, 2010), may also constrain ecosystem-scale variation in elemental ratios, though these remain to be tested.Detailed characterization of taxon-specific resource strategies in marine plankton will be essential for identifying the level of taxonomic grouping relevant to understanding emergent community variation in C : P stoichiometry.Building such a framework will in turn strengthen predictions of community response to large-scale changes in ocean nutrient conditions.

Fig. 2 .
Fig. 2. Phosphorus pools as a function of latitude across the BV47 cruise transect (see Fig. 1).Shaded region represents the approximate location of BATS.(a) Surface concentrations of dissolved organic phosphorus (DOP, filled circles), bulk particulate phosphorus (PPhos, open circles; shown as means ± SE), and soluble reactive phosphorus (SRP, open squares, "nd" denotes stations that were below detection).(b) Estimated vertical SRP flux (µmol m −2 d −1) across the base of the euphotic zone.Flux was calculated as the product of the vertical concentration gradient (from 80 to 160 m) and diffusivity coefficient (0.000035 m 2 s −1 ;Ledwell et al., 2008).Filled circles represent stations where the SRP flux was linearly interpolated from the two immediately adjacent values because the concentration gradient was not measurable.

Fig. 3 .
Fig. 3. Allocation to nucleic acids as a function of latitude across the BV47 cruise transect (see Fig. 1).Shaded region represents the approximate location of BATS.All points represent means ± SE.(a) Concentrations (µg L −1 ) of particulate RNA (filled circles) and DNA (open circles).Concentrations of DNA were consistently higher than RNA (P < 0.001, Wilcoxon signed rank test).(b) RNA : DNA mass ratios.(c) Phosphorus in nucleic acids as a proportion of total particulate phosphorus (PPhos).The proportion of PPhos in RNA was consistently lower than in DNA (P = 0.004, Wilcoxon signed rank test).

Fig. 7 .
Figure 7 ), which correspond to the low end www Figure A4

Table 1 .
Summary of abiotic and biotic transect parameters a , including surface concentrations and upward flux of SRP and concentrations of TDP, DOP, PPhos, POC, and nucleic acids (RNA and DNA).Particulate C : P molar ratios and proportions of total PPhos in RNA and DNA (P RNA and P DNA ) were also calculated.
• C at 22.67 • N (st.11) to 25.7 • C north of the BATS station (34.67 • N, st.15).Biogeosciences, 11, 1599-1611, 2014 www.biogeosciences.net/11/1599/2014/ Values shown represent the mean of four station replicates, except for TDP, SRP, and DOP (n = 2).Geometric means are reported for the C : P ratios.b SRP flux values for stations 4, 7, and 10 were calculated from linear interpolation of the two immediately adjacent stations.c NA, value was below detection. a