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state of the carbonate system in solution. As temp-
eratures and ionic strength vary little within and
between experiments, single values for the first
and second ionisation coefficient of carbonic acid
can be used for all the data presented (Patterson
et al. 1984). Using k
1
and k
2
values of 6.5 and
10.32, the ratio of CO
2
þH
2
CO
3
:HCO
3
2
:CO
22
will vary across the total range of pHs recorded as
shown in Figure 2.
Figure 3a-d shows the diurnal cycles of all four
flumes after approximate equilibration with respect
to the precipitation of calcium carbonate. In each
case the graph shows mean and 1s variability for
data gathered over a period of 1 month for each 10
minute period in the 24 hour cycle (taken from
data in Fig. 1).
Figure 4a-d shows the diurnal cycle in water
conductivity measured synchronously with the pH
data shown in Figure 3. A slight variability (c.
5 mS) can be observed in Flumes 3 and 4 (sterilized),
which probably relates to a slight variability in
Fig. 2. Henderson-Hasselbach behaviour of carbonate
species across pH range recorded in flume systems.
CO
22
:HCO
3
2
¼ 10
(pH210.32)
.
CO
2
þH
2
CO
3
:HCO
3
2
¼
1/(10
(pH26.5)
).
Fig. 3. Data for Experiment 1. Mean and 1s data for stacked diurnal macroenvironmental pH variability over a 1 month
period of the experiment shown in Figure 1. (a) Flume 1; (b) Flume 2; (c) Flume 3; (d) Flume 4.
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