Geology Reference
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case, a key inhibitor of the algal growth is the direct
contact with copper substrate rather than the amount
of toxic copper ions in bulk water.
The longer the exposition time of a pair of
tablets, the smaller is the difference in weight of
growing tufa between the copper and limestone
tablet. This implies that processes immediately fol-
lowing immersion of the tablets are of crucial
importance. Microscopic observations shed some
light on those processes. Tufa deposited directly
on copper substrata is always built of calcite crystals
without any traces of algae, such as moulds of fila-
mentous algae or diatom frustules, which is a
result of the copper toxicity to algae discussed
above (Figs 13a, 14a & 23c). Tufa with biofabrics
if appears, it does it only 10 - 20 mm or more over
the copper surface. Conversely, tufa with biogenic
components can directly overlie the surface of a
limestone tablet. Thus, just after immersion of a
pair of tablets different processes take place on the
limestone and copper ones. Algae can start to
settle the limestone tablet, which promotes rapid
tufa growth, whereas algal colonization and
growth on a copper tablet is always delayed until
the protective calcite layer or patches crystallize
inorganically.
The amount of tufa which grew on limestone
tablets exposed for more than one year almost
always exceeds the total amount of tufa originating
on tablets seasonally exposed in the same place
(Fig. 25). It points out that after placing, every
tablet experienced some delay in deposition. Every
tablet is subjected to the delay only once, however
the effects of particular delays sum up when calcu-
lating the total amount of tufa on four seasonally
exposed tablets. A similar phenomenon was noted
by Pentecost & Coletta (2007) at La Zitelle spring
in Italy. They called it 'lag period' and explained
it by a slower initial rate of growth due to difference
in crystal sizes between marble substrate and
overgrowing travertine, which affected nucleation
process. Nucleation of calcite crystals can also
play a role in the observed delay, but its effect prob-
ably overlaps with the effect of the rate of coloniza-
tion by algae which actively or passively intensify
tufa growth. Any bare surface immersed in stream
water experiences algal colonization, due to immi-
gration from surrounding algal colonies, mainly
those located upstream. It was studied especially
with regard to recolonization after severe floods
that scoured older algal community (e.g. Fisher
et al. 1982). The rate of succession depends on
several factors, such as temperature, light intensity,
roughness of surface and adaptation of particular
species. It takes a few weeks before a bare surface
is covered by a stable epiphytic community (Korte
& Blinn 1983; Peterson & Hoagland 1990; Kralj
et al. 2006). As a consequence, only inorganic pre-
cipitation can operate between the immersion of a
tablet and the moment, when a tablet is colonized
by at least a pioneer community. Bacteria can
quickly settle on bare substrata (Goulder 1988).
As for algae, diatoms are the most frequent
pioneer colonizers; they are followed by cyanobac-
teria and other algae (Fisher et al. 1982; Rushford
et al. 1986; Winsborough et al. 1994). Colonization
by Vaucheria lasts a four-week period (Wilde
1982). A similar succession has been recorded in
the material obtained during the present experiment.
In some of the tablets the first layer is composed of
sparry calcite crystals with only scarce traces of
algae with the diatoms or traces of their stalks
being more common in this layer (Fig. 13a, b).
Further up from the substrate, encrusted moulds of
filamentous algae abound.
The prolonged succession of algae, along with
rapid inorganically driven crystal growth, probably
accounts for inorganic texture obtained during
some earlier experiments. For example Liu et al.
(1995, fig. 5) observed that a tablet was covered
with calcite crystals without any traces of algae.
However, the immersion period was only 5 days,
which, in the light of the present discussion, was
insufficient for algae to colonize the substrate.
During the present experiment, most of the
copper tablets were covered with calcium carbonate
though four tablets remained uncovered. Similarly,
in earlier experiments reported in literature some
copper surfaces were covered with calcium carbon-
ate and other were not. Thus, conditions favouring
the overgrowth of copper tablets by tufa need to
be discussed. It is significant that during the exper-
iment only the tablets at the Z´zriv´ site exposed
during the winter season and at the Karw´w dam
point
Fig. 25. Comparison between tufa increment on
limestone tablets exposed during the whole experiment
period and total increment on tablets exposed seasonally.
exposed
during
the
winter - spring
season
were
not
covered
with
calcite
at
all.
Earlier
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