In particular, the variation in temperature control and stirring motor speed caused many problems. Also, TCO2, Alkalinity and pH measurements were not made routinely until after the Phaeodactylum bloom during which spontaneous calcification must have occurred (see Section 8.3.3 ). The high-CO2 air-supply system evolved gradually, as did the standard measurement routine aimed at minimising possible errors. Trace metals and vitamins were only supplied to the Emiliana Huxleyi, Skeletonema and fourth Dunaliella cultures .
There are many factors influencing the gas transfer velocities which could be varied in the tank: physical factors - water temperature, air temperature, water stirring rate and paddle size, air stirring rate, - chemical factors - salinity, alkalinity, water pCO2 (and hence pH and TCO2 at constant alkalinity), added bovine carbonic anhydrase, - and biological factors - added enzyme inhibitors, nutrients, added zinc species of algae, presence of bacteria, surface algal films, light intensity.
Clearly there are very many possible combinations which could be investigated. As the physical factors influencing air-sea gas exchange have already been investigated in many other studies, most of these using large and more realistic wind-wave tanks or even lakes, initially it seemed sensible to hold the physical factors constant in order to focus on the effect of the enzyme and the algae. Therefore, during the algal blooms the temperature was maintained at 15C and the water stirring speed at 13 rpm (with a small paddle). This stirring speed was chosen because it gave a low transfer velocity (about 3 cm hr-1 for CO2), and the chemical enhancement which is the focus of this study is only expected to be significant at low windspeeds. A series of "blank" calibration measurements were also made for a range of stirring speeds a range of temperatures, a range of water pCO2, and a range of added enzyme concentrations, varying one factor at a time. Acidified seawater was used as a "control" in which there would be no significant chemical enhancement.
Seawater just off the east Norfolk coast tends to have a slightly higher alkalinity than average seawater, due to mixing with fresh water from the river Yare, whose alkalinity is sometimes greater than 3meq/l (my measurements). It is also well mixed with bottom sediment, and so should contain abundant micronutrients. The pCO2 rises substantially as the water is warmed to 15C from the cold winter North Sea. I could not measure pCO2 in situ, although the temperature and approximate pH were recorded.
Subsequently the tank was only rinsed several times with water between different cultures or seawater samples, scrubbing off adhering algae and also wiping dry before the final rinse. Inevitably a few bacteria were present, but the stock cultures were sufficiently dense that the desired species of algae had a good head-start. The presence of some bacteria may also be beneficial, recycling essential amino acids and other metabolites required by the algae (see Section 8.3.7 ).
It was assumed that the original seawater would contain enough trace metals and vitamins, as it was taken from well mixed coastal waters. The diatom and dinoflagellate cultures grew rapidly with no sign of any nutrient limitation. However the 2nd and 3rd Dunaliella cultures did not grow nearly so well as the first Dunaliella culture, for which the water had not been filtered. This implied some limitation, perhaps lack of vitamins which might have been recycled by bacteria in the unfiltered water. Therefore trace metals and vitamins (as in table 5-1 , measured by volumetric dilution of stock solutions) were also added to the fourth Dunaliella culture, which grew much better, as discussed in Section 8.3.7 . Lack of trace metals is unlikely as the tank contains a steel pipe, screws, and alloy air-stirrer blades.
Zinc by itself was added on a few occasions (noted in results), to ensure that this was not limiting the possible production of carbonic anhydrase. Again, the concentration added was similar to that of f / 2 medium.
The enzyme inhibitor Acetazolamide (typically 50-100mg) was dissolved initially in 50ml 0.1M KOH which was then added to the tank, followed by 50ml of 0.1M HCl which had been titrated against the KOH to ensure equal molarity. Note the Acetazolamide is toxic and thus had to be weighed in the fume cupboard, to only +/-10mg accuracy. A similar procedure was applied on one occasion for another inhibitor, Ethoxyzolamide.
So this problem was circumvented by first acidifying the water, sufficiently to raise the pCO2 to several thousand ppm, removing most of this by rapid gas exchange with air in the tank, and then adding the equivalent amount of alkali to restore the original alkalinity, at which point the pCO2 would fall to a much lower level. The acid and alkali were of course titrated one against the other to ensure that the additions cancelled. As the final pCO2 is quite sensitive to the initial alkalinity of the water, to achieve a particular desired pCO2 this procedure had to be repeated several times, with diminishing quantities of acid and alkali.
One possible drawback of this method is that the pH changes might irreversibly affect any dissolved organic carbon in the water, perhaps changing the gas exchange properties. However, it was not necessary to lower the pH below the bicarbonate-buffered range (i.e. below pH 6), so the change is not too dramatic.
An alternative method used for the first set of low-pCO2 blank measurements, involved heating the water to drive off the CO2 (which is less soluble in hot water) and also to raise the gas exchange rate, and then cooling back to normal temperature after equilibration with CO2-scrubbed air. However, the acid method is more straightforward.
The culture was thus diluted 110 fold (for 1 flask). From this point it took about 2 weeks to reach a similar density as the original culture, i.e. an average doubling time of about 2 days, although the carbon budget plots in chapter 8 suggested linear rather than exponential growth.
Water was extracted from the tank with a 100ml syringe, and a known volume (depending on algal density) was then slowly forced through a Whatman glass fibre filter, the water being returned to the tank. The filter papers were folded, wrapped in aluminium foil, and frozen until analysis. Four samples were taken on each occasion. One day prior to analysis each filter paper was placed in a vial containing 10ml 90% acetone, into which the chlorophyll was extracted
The samples from algal cultures were analysed with a spectrophotometer. The absorbence was measured at 630, 645, 665 and 750 nm wavelengths, and the 665 and 750 nm measurements were repeated after acidification by 2 drops 8% HCl (which knocks the Mg atom from the centre of chlorophyll-a leaving phaeophytin). The instrument calibration was checked with a chlorophyll standard.
The chlorophyll concentration in the extract was calculated according to the formulae of
SCOR: [Chl a] = 11.6 (A665 - A750) -0.14 (A630 - A750) -1.31 (A645 - A750)
and Lorenzen: [Chl a] = 26.7 [(A665 - A750) - (acidified A665 - acidified A750)]
The mean and standard deviation were calculated for each set of four samples. The results are shown on the carbon budget plots in chapter 8, which show both the SCOR and Lorenzen figures (typically these are very close). The standard deviations ranged from 1% to 16%. The relationship between the measured chlorophyll, and biological carbon calculated by mass balance (see Section 6.7 ) is shown in figure 8.1.
The North Sea spring bloom samples were more dilute and had to be analysed on a fluorimeter (Turner model 112, excitation filter BG12, emission filter RG630). Fluorescence was measured before and after acidification as above, the acidified reading being subtracted from the unacidified reading. The ratio of these readings gives an indication of the phaeophytin concentration. The fluorimeter was calibrated using a set of 5 dilutions of a chlorophyll standard (checked with the spectrophotometer as above), spanning the sample range. The mean and standard deviation were calculated for each set of four samples, and the standard deviation ranged from 3% to 47%, the latter from the 30th April sample where chlorophyll was still low.
The LiCOR analyser compares the amount of infra-red radiation absorbed by an air-sample, with that of a CO2 reference gas, as illustrated by figure 5-2 . The absorption corresponds to transitions between vibrational and rotational energy levels of the CO2 molecule, the same process as causes the "greenhouse effect" in the atmosphere. A filter selects the wavelength range 4.26 ±
0.15 mm before detection. The same light source and detector are used for both sample and reference, a rotating chopper disc directing the light to pass through each in turn. The cell volume is about 12cm3 and the inside of the cell is gold-coated to maximise internal reflection. The trapped gas surrounding the chopper motor and detector is purged of CO2 and water vapour by a bottle containing soda lime and magnesium perchlorate, which was changed regularly according to manufacturers directions. Gas flows continuously through the cells, and the response time is very quick, about 0.1 seconds. The output range for the difference between sample pCO2 and reference pCO2 is actually just over +/-5000ppm, although the factory calibration is specified as only for +/- 3000ppm. The specified accuracy is 1ppm for concentrations similar to air, and the repeatability about 0.2ppm.
The analogue electrical output of the analyser is proportional to the difference between the detector response for the sample cell and that for the reference cell. This response is not linear, but has been calibrated as a third order polynomial. The analyser contains sophisticated electronic circuitry which converts the analogue output into a digital reading of pCO2on a display, which can be passed on to a computer. A correction for sample cell temperature is made automatically, but not for atmospheric pressure, which will be considered later.
The LiCOR could also make an automatic correction for the dilution of the dry air by water vapour, to provide the pCO2 as a mole fraction of dry air rather than the actual pCO2 of the sample. However, in these gas exchange experiments the air-water gas flux is calculated from the pCO2 flowing in and out of the headspace, multiplied by the flow rate, so clearly the actual pCO2 rather than the dry-air pCO2 is required and the correction was not applied. It was necessary to wait for the H2O reading to stabilise before noting the pCO2 reading, thus ensuring that the inlet filter had equilibrated with the H2O in the sample air stream and hence was not adding or removing water vapour.
It should be stressed that when the gas exchange rate is calculated from the steady state flows through a tank headspace, errors in the calibration of both the zero and the span cancel entirely, so long as the response to pCO2 remains linear. For the same reason, an atmospheric pressure variation also cancels (see also Section 4.3 ).
Rather than losing several weeks by sending the analyser back to the USA for servicing, it was decided to wash out the sample cell by rinsing and drying it several times. To rinse the cell about a litre of warm MilliQ water was slowly siphoned through. Initially this also contained a mild detergent. Between rinses the cell was dried with a fast flow of nitrogen gas. Eventually the contamination was reduced sufficiently to bring it back within the range of the zero and span controls for CO2. For water vapour the zero was sometimes still slightly out of range - in this case the analyser had to be told that the water vapour concentration in the reference gas was slightly negative, thus it would calculate the correct value for the sample gas. This should not have significantly affected the water vapour correction to the pCO2.
It was clear that this effect was not caused by laboratory air leaking into the flow, because the direction of the memory effect depended on the previous sample. Also, this effect could not be due to slow equilibration in the filter, because if the flow was stopped altogether, the gradual pCO2 change continued despite the filter being some 30cm upstream, whereas on restarting the flow the pCO2 in the intermediate tubing was found to be unchanged.
This behaviour might be explained if a small reservoir of trapped air somewhere within the LiCOR slowly equilibrated with the air in the sample cell. A substance which could absorb or adsorb CO2 would have a similar effect. Exchange with the reservoir would then have greater effect on the sample cell when the air flow was slow, as observed.
This memory effect was frustrating because it forced me to wait many minutes before obtaining a stable reading from a slow gas stream such as that out of a tank headspace. Despite this delay, the memory inevitably introduced a slight error. This problem has been seen by others using these LiCOR analysers (David Cooper, personal communication), but would probably not be a problem during routine pCO2 measurements at sea, for example. This is because there is not a large rapid change of pCO2 in the sample cell (the range of pCO2 measured is much smaller and changes gradually), and also because the gas streams would typically flow much faster during continuous field measurement.
This is justified because the atmospheric pressure cancels in the calculation of the transfer velocity, the key quantity of interest. This was demonstrated by the equations in
A stopwatch was used to measure the time taken (typically 10 - 30 s) for the soap film to move either 25 or 14 cm3. Readings were repeated until at least three agreed to within 0.1s.
The flow of the reference gas was also monitored to ensure slow flow, such that the LiCOR cell was flushed but the cylinder was not depleted too fast.
The gas flow also had to be comparable to the expected flux across the air-water interface. Calculations showed that continuous operation at such flow rates would soon exhaust CO2 reference cylinders. Therefore the high-CO2 supply would have to be made by diluting pure CO2 into a gas stream. Also, the gas could not be made up with pure nitrogen, as oxygen was required for the growth of the algae. Therefore the basis of the supply had to be air pumped in from outside the building. As the diaphragm pump produced a rapidly oscillating pressure, a 2-litre bottle was inserted after the pump to remove this oscillation. Since the pCO2 of such air varies (mainly due to traffic on the road outside), the incoming air was first scrubbed with sodalime to remove the CO2. This stream was split in two, and CO2 added to one stream only, the other having a pCO2 near to zero.
The quantity of CO2 required from the pure-CO2 cylinder was extremely small, and could not be delivered by a high-pressure regulator, particularly as high-pressure CO2 does not behave as a perfect gas. A double-dilution system was therefore devised, as depicted in figure 5-3 .
The pressures were regulated with the valves, to ensure such that the gases flowed were in the required direction, which is generally from left to right in the figure, except where indicated otherwise. A stream of pure CO2 was vented through a valve (A), upstream of which a smaller flow was diverted through a 3-way needle-valve (B) into a stream of air. Most of this stream was also vented through a valve (D), but another 3-way needle -valve (C) allowed a small flow into the high CO2 branch of the scrubbed air stream. Upstream of this, the flow rates of both the scrubbed air streams were controlled by two more valves (E,F). Thus it was possible to control the pCO2 and flow rates independently.
To achieve the correct direction of flow the relative pressures must be maintained in the order: pure CO2 > air venting through valve D > air leaving valve E (towards the tank). To control the pCO2 produced by this double dilution system required much trial- and error, aided by the pCO2 trace preserved on the computer screen, two pressure gauges, and a ball flowmeter for the venting air.
Typically the pCO2 of this high-CO2 air supply was about twice the atmospheric level, i.e. 700ppm. Sometimes a higher pCO2, up to 3000ppm, was used to raise the pCO2 of the tank water, or to offset vigorous biological uptake.
Because the measurement procedure requires frequent rearrangement of the "plumbing" (see Section 5.8 ), plastic tubing and connectors were used for connections in and out of the tank. Note that these tubes are all very close to atmospheric pressure. Stainless steel tubing and Swagelock fittings were used for the upstream air supply system and reference gases which were at higher pressures.
Assume for this example that Headspaces A and D are the efflux and influx headspaces respectively, and the air leaving A is already flowing through the LiCOR sample cell:
Note that the flowrates are measured at the beginning. This avoids any change due to the small pressure drop across the LiCOR and tubing. The order of the measurements may be changed in order to minimise the change in pCO2 between readings. This helps to reduce the "memory effect" ( Section 5.5.6 ).
Figure 5-4 shows a typical trace from the computer program described earlier ( Section 5.5.4 ), following this normal measurement procedure. The numbers in circles have been superimposed to indicate the stages in the procedure as listed above. The vertical scale (pCO2 range) has been changed several times, in order to show in detail the gradual equilibration within the LiCOR cell, due to the memory effect described in Section 5.5.6 . These vertical scales are shown at the bottom of the figure. In this example the steady-state pCO2 in D is about 400ppm, the steady-state pCO2 in A is about 200ppm and the equilibrium pCO2 is about 300ppm. The N2 cylinder gives 3.5ppm, the scrubbed low pCO2 input is 8ppm, and the high pCO2 input is 600ppm with a slight oscillation of about ±
3ppm.
Note the effects of varying the gas-flow rate: when the reference gas is first flushed rapidly through the reference cell, the trace for the sample gas drops instantly with no memory effect (in this case we assume the gas previously in the reference cell had a pCO2 just slightly lower than the reference gas, thus raising this pCO2 causes the sample gas trace to drop accordingly). However when the flow rate is slowed (to preserve calibration gas) the trace jumps back up and gradually falls again. When the equilibrium pCO2 is being sampled (headspaces B and C) the pumped flow is sufficiently fast to cause a slight drop in pressure and hence pCO2, but when the pump is switched off the reading jumps back up.
Nitrogen gas was set up to bubble through the sample, through the LiCOR, and through a flowmeter. This system is illustrated in figure 5-5 . The flow is kept constant, while the pCO2 reading from the LiCOR is integrated by the computer. The procedure was as follows:
TCO2 = 10-12 *(1/1.035) *10 *integral *flowrate *air-pressure /( 60 * R * air- temperature)
Units and conversion factors in this formula are given in the table below:
5.7 Gas flow system for supplying air with controlled pCO2
The design of the steady-state tank required two streams of continuously flowing air, one with pCO2 greater than that of the water, and one with pCO2 less than that of the water. Both the pCO2 and the flow rate of these two air streams would need to be varied, so as to minimise the error in the transfer velocity, and to control the water pCO2.
5.8 Normal procedure for CO2 gas exchange measurement
The normal procedure for measuring CO2 gas exchange was as follows
5.9 Total CO2
Total CO2 in a water sample is measured by acidifying the water such that all the CO32- and HCO3- is converted to CO2 , which is then extracted in a stream of nitrogen and measured using the LiCOR analyser.
|
TCO2 |
mol kg-1 |
|
Integral |
ppm. s |
|
Flowrate |
ml min-1 |
|
Air Pressure |
kg m-1 s-2 |
|
Air Temperature |
K |
|
R (gas constant) |
kg m2 s-2 K-1 mol-1 |
|
10-12 |
ppm => mol fraction ml => m3 |
|
1/ 60 |
min-1 => s-1 |
|
1/1.035 |
l-1 => kg-1 |
TCO2 measurements were not made every day, but were used for occasional comparison with the TCO2 calculated from pCO2 and Alkalinity (see Section 6.3 and figure 6-1 ). This provided a check on the internal consistency of the carbonate system.
Repeated measurements showed that the error in measured TCO2 was approximately 2%. This is a combination of variation in the flow rate and error in the pCO2 beyond the calibration range of the reference CO2 standards. The flow rate (typically 300ml / min) was adjusted such that the maximum pCO2 was less than 3000ppm, the linear range of the LiCOR specified by the manufacturer. Note that the integration had to be done by computer, as the LiCOR's internal integrator had a slow response and insufficient memory.
Whilst it is possible to make much more accurate TCO2 measurements (see Section 1.3.2 , Section 3.4.2 , and references therein), this method was convenient and sufficient for the purposes of these experiments in which the observed changes in TCO2 were large.
The initial pH was measured immediately after taking a 100ml sample from the tank by syringe, thus minimising gas exchange. The pH electrode was calibrated in pH 7 and pH 9 buffers before and after each sample, and was readable to the nearest 0.025 pH units. The sample was then titrated with 0.01M HCl from a burette. The pH was measured at 1ml intervals between 20ml and 30ml. Extrapolating between points, the volume of acid required to bring the sample to pH 4.5 was used to calculate the Total Alkalinity. The 0.01M HCl was made up by volumetric dilution of commerical AnalaR 1M HCl and then titrated against 0.01M KOH (diluted from commericial AnalaR 1M KOH) to confirm its molarity. The two solutions matched to within 1%.
This simple method is based on the definition of Total Alkalinity but is limited by the accuracy of the pH electrode. A more accurate method would be needed if alkalinity were used to calculate pCO2, as a small change in alkalinity corresponds to a large change in pCO2. However, as pCO2 was measured directly, these alkalinity measurements were only required to check that there was no significant change in Total Alkalinity during the course of an algal bloom in the tank. Air-water CO2 exchange and photosynthetic uptake or respiration of CO2 do not affect the total alkalinity, since the contribution of any extra bicarbonate and carbonate ions created by dissociation of carbonic acid (hydrated CO2) is cancelled by the hydrogen ions also created. Only removal of carbonate ions through precipitation of solid CaCO3 would significantly affect the alkalinity. In most cases no significant change was observed and the average of the measurements was used for calculating the carbonate system speciation, as described in Section 6.3 . Where the alkalinity did change (during the Phaeodactylum bloom, and after any addition of phosphate) this was incorporated into the calculations as described later (e.g. Section 8.3.3 ).
For the purpose of determining the speciation of the carbonate system, the pH was the least accurate of the four measured parameters (pCO2, TCO2, Alk, pH) and was therefore plotted on the graphs for comparison, but not used in the calculations.
However, it was critical not to allow any of that pure SF6 to contaminate the lab, or other labs in the building where parts per trillion of SF6 were measured in background air samples. Therefore the SF6 cylinder was only ever opened outside well away from air ducts. 50 ml of SF6 was let into a gas syringe containing 50ml of seawater (and no air). This was shaken vigorously for a few minutes to ensure equilibration. The SF6 gas was then driven out and a 200 microlitre syringe inserted into the gas syringe to withdraw a small sample of SF6 saturated water, whilst being extremely careful to avoid taking in any bubbles of gas. The syringe water temperature was then noted. The microlitre syringe was brought inside and used to spike the tank water with the required volume, again being careful to avoid forming any bubbles. Note that just 0.2 microlitres (a very tiny bubble) of pure SF6 would be equivalent to the whole quantity injected.
A six way valve was used to isolate the sample in a small sample loop before connecting it in line with the GC column. This ensured that the same sample volume was always used. A pump was used to flush air from a tank headspace through this sample loop and back into the headspace again, as shown in figure 5-6 . Before entering the sample loop, the air was dried by passing through a tube packed with magnesium perchlorate. The pump was operated for at least 30 seconds to ensure equilibration within the pipes, and then switched off for 15 seconds to allow the sample to return to atmospheric pressure, before switching the six-way valve. The sample was then taken up in a stream of nitrogen carrier gas flowing through the GC column.
The set up of the GC was very simple. The column was made from a 5m length of 1/8th inch stainless steel tubing packed with molecular sieve 5A. The SF6 was detected with an electron capture detector maintained at 100C, which was connected to a chart recorder and also to the computer. The attenuation was set to 2, the "range" to 1. The oven temperature was kept just above room temperature at 26C, because at higher temperatures the SF6 peak rose up the tail of the large oxygen peak. To resolve much lower concentrations of SF6 such as ambient background levels, oxygen would have to be removed (e.g. by cryogenic trapping). However for this spiked system the SF6 peak from dry air was clearly distinguishable and appeared after about 12 minutes, on a sloping baseline following the oxygen peak. The peak height measured by hand from the chart recorder trace was compared with the peak area integrated by the computer (both taking into account the sloping baseline), and the correlation was extremely good. Thereafter only the chart recorder was used, to free the computer for CO2 and oxygen measurement.
However, it was generally more convenient to use fractions of the highest calibration gas, diluted with air in a gas syringe, and then injected into the sample loop through the magnesium perchlorate. The air mixed in provided an oxygen peak similar to (although slightly smaller than) that of the oxygen in the sample air. The calibration curve was non-linear as expected for an electron capture detector, but the fit was good as shown in figure 5-7 . Note that the peak height increases rapidly at about 11 ppb (right hand plot). The best fit curves are either quartic or cubic polynomials calculated by the computer. These formulae were then used to estimate the SF6 concentration in tank samples.
The peak height was found to be very sensitive to column temperature variation, even of just one or two degrees. Initially a formula was used to compensate for this, based on a series of calibrations at a range of temperatures from 25-30C. Later, temperature control was improved and reference gas samples were interspersed with samples from the tank to ensure a reliable calibration under identical conditions. To avoid large extrapolation, tank samples and calibration gases were usually alternated, the dilution of the calibration gas being chosen to match approximately the expected concentration of the next tank sample. Typically, three tank sample peaks and 2 calibration peaks might be recorded every hour.
To check the assumption that measurements the following day should give a reasonable approximation to the equilibrium level, we can recall that the time constant for exponential approach to equilibrium as derived in Section 4.5.2 is k / d*. The transfer velocity k was typically in the range 1 to 3 cm hr-1 and the effective depth d* is 10.94 cm2 (see figure 4-5 ), so the e-folding time should be about 3-10 hours.
Since the calibration curves seem quite good and there is not too much scatter between the points in figure 5-8 , it seems more likely that the problem lies in the estimation of the amount of SF6 added in the spike.
Here there are several possible sources of error - for example under or over saturation of the initial seawater in the gas syringe, or uncertainty regarding the temperature of this seawater at the time of saturation (recall this was done outside the building to avoid contamination, but with water which had been stored inside the building at a different temperature). This has to be known in order to calculate the solubility and hence the concentration of SF6 in the injected spike. Sometimes a tiny air bubble may have been trapped in the syringe and injected into the tank, although care was taken to avoid this.
This discrepancy was not resolved, despite attempts to refine the measurement and spiking techniques. Transfer velocities calculated using both endpoints are shown later in table 7-2 and figure 7-4 , and are discussed in Section 7.4.1 . It can be seen that neither set fits well enough to the corresponding oxygen transfer velocity measurements to be useful in determining the "Schmidt number dependence" of the gas exchange regime in the tank, as initially hoped. However, the response to varying temperature provided an alternative means of gauging the "Schmidt number dependence" ( Section 7.4.2 )
It was checked that the response of the electrode was a linear function of oxygen concentration - thereafter no calibration was necessary since neither the zero nor the span coefficients would affect the gradient of the logarithmic plots which is used to calculate the transfer velocity.
The computer could be left recording the oxygen measurements overnight, until sufficient time had elapsed that the system had effectively returned to equilibrium (the timescale is similar to that for SF6 as calculated above). The file containing thousands of measurements was then imported into a spreadsheet which was used to calculate the quantity
ln [O2(time t) - O2 (end)] and plot this as a function of time. Eight such plots are shown in figure 5-9 . Note that the scatter in the plots seems to increase with time -this is just a function of the logarithmic vertical scale since as the log values decrease, they correspond to smaller and smaller measured differences between O2(time t) and O2 (end).
A linear least-squares regression was fitted to the data by the computer (excluding the end portion), and from the gradient of this best fit line the transfer velocity can be calculated using the formula already developed in Section 4.5.3 . As for SF6, the equilibrium O2 concentration is needed for this calculation, but in this case it is easy to tell when the measurements have stabilised by looking at the recorded data (before taking the logarithm), and in any case the equilibrium is always close to the normal level in the atmosphere.
The actual transfer velocities calculated from these plots will be reported in table 7-2 and discussed in Section 7.4.1 , as for SF6.
During the influx and efflux measurements made on 4th and 5th March there was considerable biological activity in the water (see Section 8.3.3 ). This probably affected the equilibrium concentration of O2 in the water sufficiently to explain the slight curve apparent in the plots and the different gradients.
Generally, the oxygen transfer velocity measurements were also not sufficiently accurate to act as a "control" from which to calculate the unenhanced transfer velocity for CO2, which was calculated instead using acidified seawater. However, both the O2 and SF6 transfer velocities did remain within the "normal" range even when the CO2 transfer velocity measured concurrently (see table 7-2 ) was much higher, thus indicating at least that these high CO2 transfer velocities could not be due solely to physical factors.
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