Geology Reference
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(Whitton 1975). The H ´ j dam site may serve as
an example.
ever detected during the described experiments
was at the L´ˇky site in front of the waterfall
(L ´ˇky E point) between July - October 2003. The
layer of compact tufa up to 17.1 mm thick, display-
ing crystalline texture, was deposited there (Fig. 9b).
The mean rate of tufa growth equals there 10.7 mg/
cm 2 /day or 0.142 mm/day. The real rate just after
placing the tablet was probably lower, bearing in
mind the 'lag period' resulting from nucleation
process (Pentecost & Coletta 2007). Nonetheless,
such a high rate of inorganically driven crystal
growth significantly impedes growth of algal flora
(Golubi ´ 1969; Kano & Fuji 2000). Any alga
which appears on the crystal surface is instantly
covered with calcite, either being trapped by ex-
panded older crystals or providing an ideal place
for nucleation of the new ones (Figs 11b - f &
12a, b). Hence, attempts of colonization are abor-
tive; their traces are visible as moulds of filamentous
algae, completely entombed within sparry calcite
crystals. Impossibility of colonization due to high
rate of carbonate mineral growth results in a scarcity
of algal structures in travertines. There it works in
combination with other factors as for instance temp-
erature and toxic elements dissolved in water.
Conversely, under the condition of low crystal
growth rate, tufa built of clotted micrite or hemi-
sperically layered micrite textures was deposited.
For example, at the Karw ´ w cascade between
December 2002 - March 2003, when the growth
rate equalled 0.18 mg/cm 2 /day, tufa built of clot-
ted micrite was formed. Similar tufa originated at
the H ´ j dam point during spring and summer 2003.
The occurrence of tufa with fibrous texture is
intimately connected with high rate of tufa growth.
The samples from the Karw ´ w cascade point and
those from the L ´ ˇ ky top and E points exhibit
such a texture. The tufa growth rate of these
samples was 1.93, 3.87 and 6.43 mg/cm 2 /day res-
pectively. The samples with fibrous texture formed
in settings with relatively high-energy, but not
under extreme energy conditions. At the L´ ˇ ky top
point they were formed in a fast-flowing stream
while at Karw ´ w on the face of a small cascade.
Formation of this texture at the L ´ ˇ ky E point,
where water falls freely along the vertical wall,
needs further explanation. Energy there seems to
be unfavourable for algae to colonize. However,
the separation of flow at the head of the waterfall
caused a shadow zone between the wall and water
in the place where the tablets were installed.
Although water constantly flushed the tablets, con-
ditions were not so severe to completely preclude
algal colonization.
Tufa with fibrous texture originates due to exter-
nal calcification on filamentous algae. This process
isolates the organisms from their environment, and
hence impedes the access of nutrient and light and,
Light intensity. The light intensity is decisive for
biological activity in streams. Particular taxonomic
groups demand different light intensity, which is
an important factor controlling the distribution of
algae in streams (Canfield & Hoyer 1988; Allan
1995, 88 - 89). Therefore, the amount of available
light exerts a fundamental control on the tufa
texture. Bearing in mind annual changes of
day-time length at the European latitudes, the short-
est period of insolation is in winter. However, all the
studied sites are shaded by the vegetation which
consists mainly of deciduous species. This effect
decreases solar irradiance in summer. Thus, the
overlapping of these two effects causes that the
best light condition is in spring just before leaves
fully develop (McIntire 1968; Pentecost 2003).
Vaucheria is especially vulnerable to low inten-
sity of light (Ensminger et al. 2005). This clearly
explains the superabundance of tufa displaying
fibrous texture and built of this alga in spring at
the Karw ´w and L ´ˇky sites. At L ´ˇky this type
of tufa developed also in summer and autumn, but
only on the tablets installed at the south-facing
wall of the L ´ˇky waterfall (L´ˇky W point), that
is in the well irradiated location. At other points
and at Karw´w it ceased to grow.
Green algae and cyanobacteria are more tolerant
to shading (Merz 1992; Ensminger et al. 2005). This
results in the development of tufa with cyanobacter-
ial moulds at sites exposed to twilight, located in
narrow and relatively deep gorges, as for example
at the H´j site and to predominance of this type of
tufa during summer season in streams shaded by
leaf canopies, as at the Karw´w site and at the
L ´ˇky top and E points.
Formation of tufa with diatom moulds seems to
be independent from light conditions as a result of
the tolerance of diatoms to low light intensity
(McIntire 1968). Moreover, diatoms probably can
cope well with changing irradiation (Allan 1995,
68). Therefore, their distribution is independent of
light conditions and they dominate in tufa deposited
in low-light conditions, inhospitable to other algae.
Rate of calcite growth. The rate of calcite growth
obviously depends on the chemistry of parent sol-
ution, as discussed above. However, the results of
the experiment described here prove that calcite
growth rate, in turn, exerts a significant influence
on the texture of deposits by controlling diversity
and density of algal communities. A similar idea
was previously put forward by Pedley (1992, 2000).
The high rate of tufa growth favours the crystal-
line texture. The L ´ˇky site provides a good
example. The extremely high rate of tufa growth
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