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same change between mid-summer and mid-winter
conditions with night-time pH increased by c. 0.1
and day-time pH increased, with maximum values
increased by 0.2 and 0.1 for Flumes 1 (Fig. 7)
and 2 (Fig. 8) respectively.
inspection), this result is distinctly counterintuitive.
When it is further considered that the source water in
the two systems is identical and that their chemical
evolutions through the experiment are similar, with
rapid precipitation of CaCO
3(solid)
during the first
1-2 weeks followed by approximate equilibrium
(Fig. 1e), it must be assumed that the buffering
capacity of the two colonized systems should be
approximately the same. Consequently, it must be
assumed that although it may be smaller in terms
of biomass, the Flume 2 biofilm is capable of
achieving higher CO
2
fluxes through photosynthesis
and respiration. The nature of the linked microbial-
carbonate system (equation 1) means that it is likely
that this Flume also experiences enhanced diurnal
flux of calcium between solid and aqueous phases
(Bissett et al. 2008b; Liu et al. 2008; Shiraishi
et al. 2008a). It is therefore significant to note that
Flume 2 experienced enhanced loss of calcium
from solution compared to Flume 1 over the
course of this experiment (see Fig. 1e and Rogerson
et al. 2008). Indeed, not only was the difference in
Ca
2þ
Discussion
Photosynthetic influence on
macroenvironment water chemistry
The pH of alkaline streams is regulated by the
dynamic equilibrium between the rates of photosyn-
thesis and respiration and the degree of buffering
provided by ambient alkalinity (Liu et al. 2008).
Due to the pH range found for tufa streams, and in
our flume systems, most of this alkalinity will be
as bicarbonate (HCO
3(aq
2
) ions. Consequently,
phototrophic and heterotrophic activity within
these systems are intimately linked with the carbon-
ate system, forming a 'bioreactor' (Visscher & Stolz
2005) potentially driving either precipitation or dis-
solution. In our systems, which are dominated by
cyanobacteria (Pedley et al. 2009), the dominant
metabolic effect can be expressed following
Visscher & Stolz (2005) by:
(aq)
loss between Flume 2 and the sterilized
systems (Flumes 3 and 4) double that of Flume 1,
but it continued to be lost throughout the experiment
and never reached true equilibrium, suggesting that
the diurnal calcium cycle was asymmetric in this
system with a small mass of calcium added to the
calcite inventory during the average cycle. It may
be coincidence that this asymmetry occurs only in
the system with the higher amplitude diurnal
cycle, however the inference that photosynthetically
driven precipitation is enhanced under fast flow con-
ditions compares favourably with recent findings in
German carbonate creeks (Bissett et al. 2008a).
Increasingly, flow rate is becoming an important,
perhaps the most important, parameter to consider
with regard to precipitation kinetics (see Pedley
et al. 2010 and Hammer et al. 2010).
The co-occurrence of high amplitude diurnal pH
variability and enhanced precipitation of calcite in
Flume 3 provides indirect evidence that photosyn-
thesis may be promoting precipitation in these
systems. Figure 3, which shows mean pH in
Flumes 1-4 over 1 month under approximately
equilibrium conditions, provide a means of asses-
sing exchange of ions between solid and aqueous
phases more directly. From this data compilation,
it becomes clear that diurnal change in macroenvir-
onmental pH occurs as two opposed asymptotes
resembling titration curves. Indeed, these opposed
curves almost certainly do reflect the response of
the HCO
3(aq
2
buffered system to a change in the rela-
tive rates of microbial carbon import and export,
reflected in movement of the chemical system
described in equation 1 to the right when photosyn-
thesis becomes active at the start of the day and vice
versa. The consequence of this observation is that
Respiration
2HCO
3(aq)
þCa
2þ
(aq)
! CH
2
O
(cyanobacterial)
þO
2(gas)
þCaCO
3(solid)
!
Photosynthesis
(1)
22
do
not appear in this equation. This is because the
pH (generally c. 8) ensures that .90% of the
dissolved inorganic carbon will be present as
HCO
3(aq
2
, and consequently it is primarily this
inventory from which carbon for both precipitation
and microbial metabolic processes
It should be noted that free CO
2(gas)
and CO
3(aq)
is
taken
(Visscher & Stolz 2005).
Figure 1 shows an increase in day to night pH
variability in the colonized flumes (1 and 2), over
the first 10 days. This is driven by the precipitation
of calcite decreasing Ca
2þ
(aq)
and HCO
3(aq
2
concen-
trations and consequently reducing the buffering
capacity of the system. In this early phase of
the experiment, most of the metabolic activity of
the biofilm is compensated by the buffering of the
ambient water. Although it is counter-intuitive, it
is therefore during this period that the highest
diurnal changes in calcium flux via equation 1 will
be occurring. In both data sets, the amplitude
in Flume 2 is characteristically higher than in
Flume 1. Given that Flume 1 contains substantially
more bryophyte coverage and significantly greater
biofilm development
(established
by
visual
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