The limited spatial and temporal coverage of historical in vitro measurements of O2-based plankton respiration and NCP prohibit their direct use for evaluating global distributions of these processes. However, these data do encompass a span of values representative of the full oceanic range, allowing development of photosynthesis-versus-respiration relationships that can be coupled with satellite data to generate global distributions of NCP. In the current study, this basic approach was followed using three different sets of PvR parameterizations. In the first two parameterizations where data from all latitudes were included, large areas of the global ocean emerged as net heterotrophic (Figures 3b and 3c, white areas). This finding is similar to that of Duarte and Agustí  who found 25 of the 56 biogeochemical provinces described by Longhurst  in organic deficit, and which occupied ∼80% of the surface ocean. In that work, this deficit was proposed to be made up by surpluses in the remaining ecological provinces. However, these conspicuous regions of NCP deficit were near entirely eliminated by employing a single PvR relationship derived from in vitro data collected outside the 10°–40° latitude zone (Figure 3d). NCP values from this third parameterization gave a global annual rate (781 ± 393 Tmol C a−1) and latitudinal distribution consistent with independent estimates based on modeled carbon export. Agreement was also found at the local scale with in situ measurements of NCP (Table 4). Additionally, this satellite-based global NCP estimate is broadly consistent with data on meso- and bathypelagic respiration, which suggest NCP ranging from 630 to 2,800 Tmol C a−1 and having a mode of 1,500 Tmol C a−1 (Table 1).
 The comparisons described above provide good reason to believe that a problem exists regarding in vitromeasurements of NCP in low-productivity, rapidly recycling low latitude systems (i.e., the oligotrophic gyres). However, before discussing this issue further, it is worthwhile considering the implications of broad ocean regions of net heterotrophy. For example, our global PvR relationship yielded NCP deficits for 52% of the global ocean surface area (Figure 3c), while using previously published values (Table 2, second and third rows) result in 77% of the open ocean being net heterotrophic (not shown).
4.1. Implications of Large-Scale, Persistent Net Heterotrophy in Oligotrophic Gyres
 The oceans have a very low carrying capacity for particulate and dissolved organic carbon (POC and DOC) that, without supplementation, cannot support extended periods of net heterotrophy over broad scales. If the NCP deficits implied in Figures 3b and 3c were real, then these deficits must be fueled by imported organic material, transferred either in time or space. If we take 50 mmol C m−2 d−1 as the average carbon deficit for the surface ocean (median value from the studies of Duarte et al. , Gist et al. , González et al. , Robinson et al. , Serret et al. , Williams et al. , and Aranguren-Gassis et al.  and assume a mean mixed layer depth of 50 m (neither value is critical), the required replenishment rate for organic material is 1 mmol C m−3 d−1 (∼350 mmol C m−3 a−1). Using these values and some simple assumptions, scaling analyses can be conducted to assess possible routes for the necessary organic subsidy.
 Transfer of organic material in time has been put forward as an explanation for observed net heterotrophy. In this scenario, oligotrophic systems are viewed as alternating between heterotrophic and autotrophic periods, or exhibiting infrequent bursts of intense autotrophy. This on-and-off switching of net autotrophy is a familiar feature of temperate regions with seasonality [Blight et al., 1995; Serret et al., 1999]. However, the subtropical oligotrophic oceans lack strong seasonal forcing to drive changes in nutrient stress. Despite the suggestion of Serret et al.  that net heterotrophy is not always associated with nutrient stress, Gist et al.  make a convincing case for changes in the balance between photosynthesis and respiration being associated with shifts in nutrient stress. They propose that such shifts arise from changes in the relative depths of the mixed layer and nitracline in the South Atlantic subtropical gyre, but a similar relationship is not observed at the HOT site in the Pacific [Williams et al., 2004]. However, temporary spikes in oxygen concentration are seen in the upper water column at HOT, lending support to the notion of intermittent bursts of net autotrophy [Karl et al., 2003]. This concept of intermittency is hard to sustain as a basis for the protracted heterotrophy, as no such productivity peaks are seen in the extensive 14C measurements made at Station ALOHA [Quay et al., 2010]. Further, Riser and Johnson  concluded that the annual oxygen field can be accounted for without resorting to episodic events.
 Gist et al.  estimated that to sustain the inferred net heterotrophic period of the Atlantic Subtropical Gyres, ∼7.5 mol C m−2 would need to be transferred from the autotrophic to heterotrophic phase. A similar value can be estimated from observations of Serret et al. . In neither case was the mechanism of transport discussed in detail, but if we assume the storage to be distributed through a water column of 100 m, then there would need to be an elevation of DOC + POC of 75 mmol C m−3 as one enters the net heterotrophic period. Even though Gist et al.  demonstrate sufficient excess production during periods of net autotrophy in some places, it is hard to see how the required quantities of organic material can be transferred over time as we simply do not observe elevations in the abiotic organic pool on this scale. Carlson et al.  found seasonal fluctuations in DOC inventory at the BATS site to be <10 mmol C m−3 in the upper water column and <1 mol C m−2 when integrated over the upper 250 m.
 Input of new organic material to the gyres can occur from i) the sides (eddy diffusion), ii) below (by upward mixing of deep water DOM), or iii) above (aeolian deposition and diffusion from the atmosphere). We discuss each of these pathways in turn.
 Lateral transport has been suggested as a plausible mechanism by some investigators [Duarte et al., 1999, 2001; Serret et al., 2002], but the most detailed analysis was given by Hansell et al. . In that work, the authors analyzed the specific case of the North Atlantic subtropical gyre, where reported NCP deficits are 8–38 mol C m−2 a−1 [Duarte et al., 2001; González et al., 2001; Robinson et al., 2002; Serret et al., 2001]. Hansell et al.  found that the combined import of allochthonous organic material is an order of magnitude or more lower (0.7 mol C m−2 a−1) than the required rate and that the region was in approximate metabolic balance.
 A more general argument can be made regarding lateral transport by calculating the concentration gradient required to make up the above estimated 1 mmol C m−3 d−1 deficit. If we consider diffusion through the circumference of a disk, then given a parameterization of horizontal diffusivity and the previously stated net consumption rate, the concentration gradient and concentrations at points along the radius can be calculated. Okubo's [Okubo, 1971] analysis of horizontal diffusivity provides a length-scaled diffusion coefficient of 4.7 × 10−5 r4/3 m2 s−1, where r is the radius of the patch (the gyre in this case) in meters. At 500 km from the center of the disk, the concentration gradient required to drive the inward diffusion would be ∼1 mmol m−3 km−1 and the DOC concentration at that point ∼1,000 mmol m−3 above that at the center of the gyre (see auxiliary material). Horizontal gradients and concentrations of DOC or POC of this magnitude are never approached in the oceans, and the gradients that do exist tend to have the reverse sense. That is, the DOC concentration tends to decrease as one moves from the center of oligotrophic gyres outward [Abell et al., 2000; Hansell et al., 2009].
 Vertical gradients of bulk DOC and POC also have the opposite sense to that required for net import of organic material. However, there is evidence that a component of upwelling DOC is assimilated by bacteria in the epipelagic zone [Cherrier et al., 1999]. If we assume mean concentrations and radiocarbon-based ages of 35 mmol C m−3 and 6,000 years for upwelling DOC and 65 mmol C m−3 and 2,200 years for downwelling DOC, we derive a figure of 10 mmol C m−3 for the quantity of upwelled deepwater DOC potentially available to the epipelagic population (auxiliary material). The above-estimated demand of 40 mmol C m−2 d−1 (∼15,000 mmol m−2 a−1) would require an upwelling rate of ∼4 m per day (1,500 m a−1). Estimates of global oceanic upwelling rates are only a small fraction of this value [Munk, 1966].
 Atmospheric input of organic carbon has also been proposed as a mechanism for meeting the NCP deficit [Dachs et al., 2005]. Organic material is supplied to the oceans from the atmosphere by three main mechanisms: dry deposition, wet deposition, and exchange diffusion. The first two processes are reasonably well constrained [Jurado et al., 2008], but their globally averaged rates (∼0.1 mmol C m−2 d−1) are well below those required to make up the implied organic deficit in the oligotrophic oceans. By comparison, the diffusive input of organic material from the atmosphere has been measured to be many times greater, with rates of 20–30 mmol m−2 d−1 [Dachs et al., 2005; Jurado et al., 2008; Ruiz-Halpern et al., 2010] and approaching the 40 mmol m−2 d−1 deficit calculated above. However, it is seems unlikely that these rates are widespread and persistent enough to meet the organic demand at the global scale. Further, these high diffusive rates reported in the literature come from unique physical settings (e.g., coastal fjord) not representative of the global ocean. In their extensive review of atmospheric fluxes to the global ocean, Kanakidou et al.  report values of less than 0.6 gC m−2 annum−1 (∼0.15 mmol C m−2 d−1) for soluble carbon deposition in the oligotrophic gyres, far below the 50 mmol C m−2 d−1 estimated above to meet the requirements of the purported net heterotrophy. Two additional lines of evidence further support the inability of atmospheric deposition to support the purported heterotrophic deficit. First, it has been noted that the 13δC of mixed layer dissolved inorganic carbon (DIC) in the subtropical gyres requires net autotrophy and is not consistent with import of external biologically produced organic material [Williams et al., 2012]. Second, subtropical oligotrophic gyres are characterized by seasonal decreases in DIC [e.g., Michaels et al., 1994], whereas an external subsidy would necessitate seasonal increases in DIC. Therefore, we are ultimately unable to identify any mechanisms that might support an organic subsidy to the central oligotrophic gyres.
 As discussed in section 1, the concept of persistent, large-scale net heterotrophy is quantitatively difficult to reconcile with our current understanding of ocean carbon cycling. In the present analysis, we show that global assessment of NCP based on field observations, outside the problematic oligotrophic gyres, gives rise to distributions and total annual budgets consistent with independent estimates based on modeled carbon export and deep-water respiration rates. We also note that in situ techniques for assessing NCP, that do not require bottle incubations consistently, indicate net autotrophy in the central ocean gyres. Finally, we have evaluated potential sources for carbon subsidies that could support broad regions of net heterotrophy and have failed to identify any candidate mechanisms. Taken together, it therefore seems that the most likely explanation for the ‘net heterotrophy paradox’ is that a methodological error or bias exists inin vitro measurements that is only detected in oligotrophic systems. Four immediate questions follow from this conclusion. (1) Does the bias/error arise from an underestimation of photosynthesis, overestimation of respiration, or both? (2) What is the primary driver giving rise to the bias/error? (3) What is the underlying physiological mechanism? (4) What are the general consequences of this issue with respect to other rate measurement techniques?
4.2. Bias in In Vitro Measurements Likely Underlies Apparent Net Heterotrophy
 The ocean biogeochemical community has been inclined to attribute the issue of a net heterotrophic bias in the in vitro measurement to an artifactual enhancement of respiration, rather than inhibition of photosynthesis [Morán et al., 2007]. In some respects, this is counter-intuitive and it can be argued that inhibition of photosynthesis is more likely. Stimulation of respiration would require growth of the heterotrophs or an increase in their substrate concentration. The growth rate of these organisms in the ocean is low (<0.2 d−1 [Ducklow, 2000]) and it is questionable whether they could increase their biomass sufficiently within the timescale of incubation. Similarly, no mechanism has been identified that could sustain increased substrate concentration. Substrates may be adsorbed onto surfaces (the walls of containers), but the gradient required for diffusion to sustain transport toward the adsorbing surface requires that concentrations near the surface be lower than in the bulk environment. Further, the timescales of incubation are too short for extensive adsorption (see below). One potential perturbation that could impact respiration is exclusion of large and rare zooplankton that feed on microheterotrophs. Under-representation of these larger predators could conceivably enhance micrograzer respiration during an incubation by decreasing their mortality. We are unable to quantify this so it must be left open.
 On the other hand, suppression of photosynthesis can be rapid or even instantaneous. Quay et al.  measured photosynthesis at the HOT site using both in situ (17O2) and in vitro (18O2 tracer) methodologies and reported that in situ rates exceeded in vitro rates by 25–60% (Figure 6). Applying this error alone to in vitro determined photosynthetic rates is sufficient to transform the negative NCP values reported by Williams et al.  for HOT, into net autotrophic rates comparable to the in situ data of Quay et al. . By comparison, if we take Quay et al.'s value of 14 ± 4 mmol O2 m−2 d−1 as an average in situ argon/oxygen-determined NCP rate and their mean 17O2in situ-determined gross photosynthetic rate of 103 mmol O2 m−2 d−1, we obtain a mean in situ-derived respiration rate of 89 mmol O2 m−2 d−1 (Figure 6). Williams et al.  reported a mean in vitro dark bottle respiration rate over a 13 month period of 86 mmol O2 m−2 d−1 for the same location. This consistency between independent measurements strongly suggests that the bias/error associated with in vitro measurements does not reside in the assessment of respiration rates.
Figure 6. Comparison of in situ rates at Station ALOHA reported by Quay et al.  with in vitro rates of Williams et al. . The errors shown are standard errors, in the case of the Quay et al. study they have been converted from the reported presumed standard deviations by dividing by √20, the number of separate observations.
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 Given the aforementioned consideration, it again appears most likely that the problem with the in vitro technique lies in the accurate representation of photosynthesis. This conclusion should not, in fact, be especially surprising. The respiration rate measured in a ‘dark bottle’ during an in vitro incubation is fueled by production that occurred in situ (i.e., under natural conditions), and predominantly by production that occurred in the very recent past for tightly coupled ecosystems such as those of oligotrophic gyres. In contrast, the photosynthetic rate derived from the ‘light bottle’ treatment reflects production occurring on the day of the incubation in a static environment very much different from in situ conditions. It seems reasonable to think that some aspect of this artificial environment is the root cause for suppression in photosynthesis, but at this point we have yet to definitively identify precisely the mechanism(s) involved. Nevertheless, a number of candidate issues can be evaluated.
 One of the key issues for understanding the problem of the in vitro technique is identifying whether the error/bias is unique to measurements made in oligotrophic systems or is a common problem to all areas that is simply overwhelmed by much higher rates of net autotrophy in other systems. While we cannot rule out this latter possibility, it would mean that in vitro measurements of NCP are underestimated everywhere. Accordingly, a global correction to in vitro-based NCP estimates would drive values at mid- and high-latitudes well above the constraints imposed by the in situ data and model estimates of carbon export, suggesting that perhaps the problem is limited to oligotrophic waters. We have no definitive proof of this, but this conclusion helps narrow the range of possible explanations. For example, fixed-depth incubation of samples collected from an active mixing layer creates an unnatural daily light regime that could significantly impact light-bottle photosynthesis (either through enhancing photoinhibition or causing light limitation, depending on incubation depth). However, it is not clear why such effects would be limited toin vitro measurements in oligotrophic systems. Similarly, high surface temperatures at low latitudes could impact both photosynthesis and bacterial respiration [Rivkin and Legendre, 2001], but we do not observe the apparent net heterotrophy in equatorial waters adjacent to the oligotrophic gyres. One interesting observation by Teira et al., 2001] was that net heterotrophy could be correlated to phytoplankton size and DOC release, but this observation provides no insight on why in vitro incubations yield a deficit while in situ measurements do not.
4.3. The Cause and Mechanism of False Heterotrophy
 It may be useful, in exploring the basis of false heterotrophy, to separate the ultimate cause and the immediate mechanism of the bias. If the biases seen in the in vitro measurements of NCP are restricted to the oligotrophic zones, then this should give insight to the primary cause as it would need to be a unique feature of these areas. Conversely, the mechanism that gives rise to the bias, i.e., the process it invokes can be general – such as some general feature of the in vitro procedure.
 A distinguishing feature of the gyres is that photosynthesis is largely supported by rapidly recycling nutrients. González et al.  and Gist et al.  both noted a relationship between measured net autotrophy and separation of the mixed layer and nitricline: the greater the separation between these horizons, the more negative NCP became. In a tightly coupled recycling system, ammonia turnover times are typically less than a day (i.e., notably shorter than typical incubation times). Data of Alwyn and Rees  for 6 h 15NH3 tracer incubations suggest an ammonia turnover time of 8.5 ± 10 h. Much shorter turnover times have also been observed (3.7 ± 3.1 h (D. Bronk, unpublished data, 2011)). True turnover rates may be even lower than those observed, as the incubation time is prone to control the calculated value. With such rapid recycling, there is clearly a potential for in vitro incubations impacting photosynthesis through disruption of the tight coupling between ammonia generation and utilization.
 If a perturbed nutrient environment is indeed the primary cause of the apparent net heterotrophy problem, then the specific mechanism(s) involved must be systemic in the in vitroapproach, rather than due to sample collection and processing. In general, three things result when a sample is incubated in a bottle: 1) turbulence is quickly and almost completely extinguished, 2) a new surface is presented (the incubation bottle wall), and 3) the sample is isolated from the adjacent environment. Past discussion of so-called “bottle effects” has focused on the second of the three phenomena, although in many respects it is the least likely to give rise to adverse effects. In the following subsections we explore these three issues in greater detail.
 Extinguished turbulence: Once a sample is placed in a container with rigid walls, turbulent diffusion is extinguished within seconds. On the scale of a typical 125 cm3incubation bottle in a well-mixed water column, the turbulent diffusion coefficient will have values of the order 10−6 to 10−7 m2 s−1 (derived from the analysis of Okubo ). These values can be compared to molecular diffusion coefficients for nutrients, which are of the order 10−9 m2 s−1 [Blackburn and Fenchel, 1999]. The loss of turbulence means that mixing rates within the bottle as a whole fall by ∼100-fold or more (from a few minutes to hours to several days). The reduction in mixing means that, on scales >10 mm, homogenization will be incomplete, opening up the possibility for an uneven distribution of nutrients within the incubation bottle. One might envision zones of enhanced and depleted nutrient concentrations around point sources of production and consumption. Given NH3 cycling times of 3 h (see above), material produced (or consumed) by point sources greater than 3 mm apart will not be uniformly mixed between them. Heterotrophic protists are major sources of recycled nitrogen and occur at 0.25 to 1 × 106 cells m−3 [Beers et al., 1975; Jackson, 1980] giving them a mean spacing of 10–15 mm. However, they are motile and the separation distance between the nutrient tails they leave behind is critical. Taking the above abundances and assuming swimming speeds of 0.2 to 1 mm s−1 [Strom and Morello, 1998; Wang et al., 2008], we obtain spatial separations between nutrient trails of 0.1 to 0.4 mm (Jackson  calculated similar separation distances), which would be mixed within a few minutes or less. Even in the case of the mesozooplankton, the separation distance of nutrient trails (3 mm) estimated by Jackson  is within the diffusion time for a 3 h cycling time. Thus, on the size and time scales of in vitro O2 incubations and the environments we are considering, turbulence is not needed to maintain the regenerated nutrient supply to photosynthetic organisms. From purely theoretical studies Munk and Riley  and Lazier and Mann  came to a similar conclusion, that turbulence is not necessary to prevent the formation of microzones.
 A new surface: Once turbulence is extinguished, the main non-biological motion is molecular diffusion. If we take 1 cm as half the distance between the center of the incubation bottle and its wall and assume a diffusion coefficient of 10−9 and 10−10 m2 s−1 for nutrients and DOM, respectively [Blackburn and Fenchel, 1999], then a characteristic timescale for diffusion may be calculated as L2 D−1 (where L is the length scale and D the molecular diffusion coefficient [Lazier and Mann, 1989]). This gives timescales of 105 to 106 seconds (i.e. >24 h to several days), which is comparable to or much longer than an incubation. Thus, within the timescale of the incubation, the wall is a distant object and processes generated by the wall itself (adsorption or release of organic and inorganic nutrients, or toxic metals, etc.) will have insufficient time to significantly impact the bulk sample. Further the growth rate (0.2 day −1 [Ducklow, 2000]) of open ocean bacteria noted earlier is too low to give much effect over the period of incubation.
 Container walls have also been argued to be a site facilitating bacterial growth. However, the organisms must migrate to the surface before they can establish themselves and grow. Although bacteria swim quite rapidly (up to 100 μm s−1), their progress is primarily random. Calculations [Blackburn et al., 1997] and observations [Kiørboe et al., 2003] suggest that net diffusion rates for motile bacteria are in the range of 10−9 to 10−10 m2 s−1, which is less than the molecular diffusion coefficient and far too slow to enable extensive colonization of the internal surface during a 24 h incubation. This conclusion is consistent with the observations of García-Martín et al.  who found no evidence for a wall effects in 24 h in vitro measurements of oxygen consumption rates.
 Isolation: Enclosing a seawater sample in a bottle isolates it from external exchange and essentially excludes organisms that occur at low abundances. In oligotrophic areas, nitrate is continually or intermittently [Johnson et al., 2010] supplied within the mixed layer by turbulent diffusion from below the nitracline. Isolation of the sample will surely disrupt this process, but evaluating its effect requires careful consideration. First, while enclosing seawater in an incubation bottle reduces input of new nitrate, it also prevents any export of newly fixed organic matter. Assuming a steady state (relative to the timescale of incubation), this trapped material then becomes substrate for production during the incubation in proportion to the loss of nitrate due to enclosure. This thinking further assumes that the organic matter is fully labile and recycled near-instantaneously. However, even if this assumption is violated and the nitrogen contained in the export fraction is not recycled over the timescale of the incubation, then the consumers trapped in the bottle will be excreting ammonium from the production consumed 24 h earlier. Thus, it does not seem that cutting off supply of nitrate in a 24 h incubation will have a great effect.
 As noted earlier, another consequence of isolation is the exclusion of rare particles (e.g., large zooplankton). In the oligotrophic ocean, mesozooplankton (e.g., copepods) occur at abundances of 100 animals m−3 or less [Beers et al., 1975; Jackson, 1980] and are essentially omitted from samples of 10−4 m3 volume, such as those used in in vitro incubations. However, they can make a significant contribution to recycled ammonia. For example, Hernandez-Leon et al. estimate that they contribute 15% and 25% of the total ammonia pool in the latitude zones 20–40°N and 20–40°S, respectively. The caveat to this argument is that, if they are omitted, so too will their respiratory contribution such that any reduction in NCP arising from a decrease in photosynthesis via loss of recycled ammonia will be offset by an associated decrease in respiration. Assuming the non-heterotrophic organisms (phytoplankton) make a 50% contribution to the ammonia pool, then the 15–25% reduction in the supply of regenerated NH3 will be accompanied by half that reduction in respiration.
 In summary, we are unable to identify a mechanism associated with ammonia regeneration that would give rise to the substantial (20–40%) reduction in photosynthesis that appears to be occurring in in vitro incubations. We have been able to identify a small effect (∼10%) associated with the failure to capture mesozooplankton of copepod size. We have also mentioned potential effects of large grazer exclusion on microheterotrophs, but a complete evaluation of such complex interaction is beyond the scope of the current study. One potentially important distinction that we have made is that the respiratory rate measured in the ‘dark bottle’ will reflect rates of production occurring in situ prior to sample collection, while photosynthesis in the ‘light bottle’ reflects rates of oxygen evolution in an altered environment. This difference makes the two measurements like comparing ‘apples and oranges’. Perhaps some headway could be made on understanding the nature of the in vitro measurement problem by augmenting the technique to include a third treatment where samples are incubated on the first day in the light and then respiration measured on the second day in the dark. If respiration is predominantly fueled by production on the preceding day (as may be likely in oligotrophic systems), then the third treatment may provide a more valid respiration rate for deriving the photosynthetic rate, or at least an indication that the system has been perturbed from its original state.
4.4. Consequences of Our Interpretation for In Vitro Field Measurements in General?
 Our analysis leads us to the view that the negative NCP rates predicted for, and observed in, oligotrophic waters result primarily from partial inhibition of photosynthesis. Although these effects have been demonstrated using oxygen flux measurements, the photosynthetic metabolisms of carbon and oxygen are tightly coupled over the period of in vitro incubations, so the inhibition should also apply to in vitrocarbon-based measurements of photosynthesis (e.g.,14C) in oligotrophic regions. Quay et al.  found that the scale of inhibition during in vitro incubations is of order 10% to 40%. P. Quay (personal communication, 2012) quantifies the underestimate in in vitro observations for the 14C technique to be ∼25%. We can derive an estimate of the inhibition from parameters in Table 4 (see auxiliary material, section S5), which yield somewhat higher errors of 40% to 60%. The difference may reflect, in major part, the different processes measured by the oxygen and 14C techniques in the light bottle.
 In conclusion, our analysis leads us to the view that in vitro light bottle measurements of photosynthesis give systematically low values in oligotrophic areas. Outside these areas, we simply have no clear evidence that serious errors exist with the in vitro technique. In addition, We have no evidence to suggest that in vitro measurements of respiration give unreliable estimates of in situ rates either in oligotrophic areas or elsewhere. Williams and del Giorgio  argued that, because of its time integrating properties, respiration was a better property than photosynthesis as a measure of carbon flux. The present study further suggests it may also be a more accurate measure.