ORIGINAL ARTICLE
Vulnerability, pressures, and protection of karst caves and theirspeleothems in Ha Long Bay, Vietnam
Trinh Anh Duc • Javier Garcia Guinea
Received: 12 March 2013 / Accepted: 24 October 2013
� Springer-Verlag Berlin Heidelberg 2013
Abstract This paper presents the results of morphologi-
cal and microclimatic surveys and speleothem analyses of
ten karst caves located in different isles in Ha Long Bay, a
famous tourist attraction in Vietnam. Characteristically, the
caves have enormous entrances, roomy interiors, but short
length, typical of karst caves in tropical climates. The cave
microclimate was found to be significantly dependent on
the outside atmosphere and human activities. There was a
clear spatial variation in macro features and fabrics of
speleothems from entrance (porous, microcrystalline) to
rear (solid, macrocrystalline) of the caves. Microstructure
analysis with the use of an environmental scanning electron
microscope shows a gradual decrease of biological activity
and microclimate instability from outside to the innermost
parts of the caves are the causes for this spatial variation.
Past and present deterioration of caves and speleothems
directly due to tourist activities and vandalism has been
observed. On the other hand, there are signs of speleothem
regrowth in the caves where tourism has been stopped. This
study has proved that caves and their speleothems in Ha
Long Bay are highly dynamic and understanding of their
environment requires immediate methodological attention.
Based on the analytical results, it is recommended that
regulation of visitor frequency and removal of lamp-flora
are necessary for sustainable development of the show
caves and their speleothems.
Keywords Microclimate � Speleothems � Tourism �Tropical climate � Lamp-flora
Introduction
Ha Long Bay is distinguished by hundreds of small lime-
stone isles rising steeply or vertically from its shallow
waters. Its dramatic and beautiful landscape is deservedly
famous as one of the world’s outstanding natural sights and
a UNESCO World Heritage Site of international geomor-
phological significance (Waltham 2000). Thanks to its
exquisite beauty, especially, since it was declared to be a
natural heritage area by UNESCO in 1994, the Bay has
attracted visitors from all over the world. The number of
visitors increased from dozens in the early 1990s to 20,000
per day recently. The karst caves of stunning speleothems
found on isles scattered in the Bay are a major tourist
attraction.
To attract more visitors, the Bay Management Board
recently implemented numerous tourist attraction programs
including modifying the show caves (e.g., construction of
paths and artificial speleothems, permanent lighting, and
entrance enlargement). Since karst caves are an extremely
slowly developing environment, for the time being, the
modifications may not show obvious problems, but for the
long term, new infrastructure for tourism and the irre-
versible impacts that go with them would cause unsal-
vageable damages to the caves and speleothems.
Electronic supplementary material The online version of thisarticle (doi:10.1007/s12665-013-2884-z) contains supplementarymaterial, which is available to authorized users.
T. A. Duc
Institute of Chemistry, Vietnam Academy of Science
and Technology, A18, 18 Hoang Quoc Viet Str., Cau Giay,
Hanoi, Vietnam
T. A. Duc (&) � J. G. Guinea
Museo Nacional Ciencias Naturales (MNCN), Jose Gutierrez
Abascal, 2, 28006 Madrid, Spain
e-mail: [email protected]
J. G. Guinea
e-mail: [email protected]
123
Environ Earth Sci
DOI 10.1007/s12665-013-2884-z
Experience from all around the world has shown many
such modifications as sources of impact to the sustain-
ability of caves and speleothems (Hamilton-Smith 2002;
Suric et al. 2010). For instance, the modification of cave
entrances affected the natural microclimate in caves (Ho-
yos et al. 1998). The electrification of caves led to the
problem of lamp-flora (Faimon et al. 2003), while the
increase in visitor numbers, with the building of new paths
and the use of new caves for tourism, meant a further
increase in the impact on speleothems (Baker and Genty
1998).
Thus, this work is a first attempt to (a) characterize the
cave microclimate and speleothem petrology in Ha Long
Bay, (b) describe the damage in speleothems due to
anthropogenic activities, and (c) introduce some general
directives for sustainable exploitation of the caves and their
speleothems.
Materials and methods
The study site of Ha Long Bay lies on the northeastern
coast of Vietnam, immediately east of the Red River delta
(Fig. 1a). It is bounded on the north by the mainland hills
either side of Ha Long City (also known as Hon Gai), to the
south by the open waters of the Gulf of Tonkin, to the west
by Cat Ba Island, and to the east by islands of sandstone
(Fig. 1b). The Ha Long limestone rock was formed during
the Carboniferous geologic period (*360 million years
BP) and throughout its development karst experienced
influences of vadose and phreatic diagenesis. As the Bay is
shallow (less than 10 m depth), it is obvious that the con-
tinental karst has been invaded by the sea and modified by
its actions in late Pleistocene (12,000 years BP) and pres-
ent time (Tran et al. 2011).
The study includes surveys of the caves and analyses of
sampled speleothems. With the assistance of the Ha Long
Bay Management Board, surveys were conducted in Sep-
tember 2011, February 2012, October 2012, and November
2012 in ten caves with three different exploitation states
categorized as ‘‘close’’, ‘‘stop’’, and ‘‘show’’ (Table 1). The
‘‘close’’ group includes caves that have not been open for
tourists, very much in their pristine stage. The ‘‘stop’’
group consists of caves that were open for tourists in the
past, but were closed recently due to their low touristic
value. The caves currently open for visits are categorized
as ‘‘show’’ caves. With the Board’s permission, we were
able to visit the ‘‘close’’ caves (Cap La, Dau Giuong, and
Duc Tien) and the ‘‘stop’’ caves (e.g., Dong Tien, Me
Cung). Second visit was conducted in the close caves to
compare hydrological conditions between dry and rainy
seasons. Surveying at each cave including morphological
characteristics (elevation, length, width, height, and
entrance counts) was conducted with simple equipments
such as GPS, metric tape, and cameras to avoid damage.
Microclimate conditions (temperature, humidity, illumi-
nation, and ventilation) at three positions near the entrance,
center, and rear of the cave interior were recorded with the
use of a pocket hygro-thermo-anemo-light meter (Extech
45170, USA). Because the Bay is a UNESCO World
Natural Heritage Site, only representative samples at crit-
ical locations were collected for microstructure analysis.
The purpose of microstructural analysis is to investigate the
spatial variation in speleothem petrology from entrance to
rear of the caves and to examine the role of microclimate
on the speleothem formation.
The environmental scanning electron microscope
(ESEM) was used to analyze textures, composition, crystal
shapes, and sizes of the speleothems. The ESEM XL30
microscope of FEI (Field Emission and Ion Company) is a
low vacuum ESEM (model Quanta) which enables high-
resolution inspection and chemical analysis of non-con-
ductive specimens. This ESEM operating in low vacuum
mode admits hydrated samples to be studied in their ori-
ginal state with the large field detector (LFD), since it is
close to the sample to avoid electron losses. The ESEM
detectors are as follows: the LFD, Everhart–Thornley or
high vacuum secondary electron detector (SED), the IR-
CCD camera, a solid-state back scattered electron detector,
and a new gaseous analytical electron detector (GAD). The
ESEM microscope has a new coupled MONOCL3 Gatan
probe to record cathodoluminescence (CL) spectra and
panchromatic and monochromatic plots with a PA-3 pho-
tomultiplier attached to the ESEM. The photomultiplier
tube covers a spectral range of 185–850 nm. A retractable
parabolic diamond mirror and a photomultiplier tube were
used to collect and amplify luminescence. Position of
samples was 16.2 mm beneath the bottom of the CL mirror
assembly. The excitation for CL measurements was pro-
vided at 25 kV electron beam.
The statistical program MINITAB 14 was employed for
statistical analysis of microclimate data (MINITAB 2004).
Results
Geomorphology and microclimate
Mylroie and Carew (1995) stated that in tropical areas,
(a) cave entrances are often enormous, formed by collapse
rather than speleogenesis per se; (b) certain cave types
exhibit widths compatible with their lengths; and (c) gen-
erally high and stable outside humidity and temperature
levels do not radically differ from cave interiors. These
general statements are true for the caves in Ha Long Bay
(Fig. 2).
Environ Earth Sci
123
In general, most surveyed caves have accessible
entrances at 20–30 m above sea level. Beyond the
entrance, floors are sloping down, reaching sea level at
some point. In floor sections close to sea level, a flat and
alluvial-deposit basement is formed. Large pools contain-
ing salty water (up to 20 %) are found in several caves
(DucTien, Dong Tien, Dau Go). In fact, the existence of
alluvial floors indicates that there is connection between
the outside sea and the cave interior. Relative elevation of
the entrances and the inner compartments makes the tem-
perature inside slightly lower than outside. In other words,
they are ranked as cold caves (the climatic regime of a cave
depends on whether the cavity is directed upward-warm
caves or downward-cold caves from the entrance). In caves
having large or multiple entrances, ventilation is strong. In
cave sections where floors are significantly higher than sea
level, splash pools and rimstones are usually found. Our
visit during the dry season confirms that splash pools are
Fig. 1 a Location of Ha Long Bay, b the studied caves situate in three isle groups marked as DG, BH, and CL, and c typical view from inside a
cave in Ha Long Bay
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123
filled up during rainy periods and dried out within a month
after that. To a large extent, this observation implies that
speleothem growth may be intense only in short periods
during and after rain (Nguyen et al. 2012).
The caves, apart from the close ones, show many
imprints of human activity. As shown in Table 1, two types
namely ‘‘stop’’ and ‘‘show’’ caves are distinguished. The
first group of caves where exploitation has been stopped
has been excluded from the visiting list for several years. In
fact, thanks to their easy access (entrances are wide open
and close to the sea level), they were frequently visited in
the past—not only by tourists but also by fishermen for
sheltering as well as speleothem hunters. Their surface is
covered with dirt brought in by humans and by smoke
stains coming from torches and oil lamps. Traces of van-
dalism are visible in the stop caves (Figs. 3c, 4a). Some
visitors and looters have removed speleothems from the
caves for their own collections or to sell them in the
market. In Vietnam, beautiful speleothems are sold as a
valuable bonsai decoration. Leaving one’s signature in the
cave was a habit of visitors and is still common at
unguarded caves. Today, they look degraded, unattractive
(Figs. 3c, 4a), and unguarded. It should be stated that
nowadays, all tourist activities must be registered and
programmed and they do not include the stop caves.
Therefore, although those stop caves are not explicitly
guarded or under constant surveillance, they are virtually
deserted and free from visitors.
The second group, which includes the show caves, is
characterized with abundant and beautiful speleothems
(Figs. 3a, 4b) and is much less deteriorated than the stop
caves. The reason for less damage in these caves is that
Table 1 List of the visited caves; additional information on the caves can be found in www.halongbay.com.vn
Name Coordinates Microclimate-geomorphology Exploit
state
Speleothems
Thien
Cung
20854042.200N;
107801005.500EDau Go Islands, small entrance at 29 m above sea
level (SL), multiple windows, spacious inside,
medium ventilation, limited natural illumination
Show Plentiful and well-guarded speleothems, few tufa
Dau Go 20854038.300N;
107801009.800EDau Go Islands, large entrance at 27 m above SL
(multiple entrances), spacious inside, strong
ventilation, high illumination
Show Moderate abundance of tufa and well-crystallized
speleothems, traces of collapsed and broken
speleothems on the wall and ceiling, mosses and
fungus coverage near the entrance zone
Bo Nau 20850058.500N;
107805012.400EBo Hon Islands, large entrance just above SL,
small inside, strong ventilation, high
illumination
Stop Moderate abundance of tufa and speleothems,
many collapsed or destroyed, traces of
speleothem regrowth, macro and micro flora
coverage in large section near entrance
Sung Sot 20850039.100N;
107805028.600EBo Hon Islands, small entrance at 25 m above SL,
spacious inside, weak ventilation, limited natural
illumination
Show Plentiful and well-guarded speleothems, few tufa
Dong
Tien
20850045.000N;
107806002.700EBo Hon Islands, large entrance submerged during
high water, wet cave floor (equal to sea level),
strong ventilation (multiple entrances), medium
illumination
Stop Mixture of tufa and speleothems, largely
destroyed due to frequently visited, trace of
regrowth, few aragonite stalagmites on the wet
and modified floor
Tam
Cung
20852019.300N;
107806043.000EMay Den Island, large entrance at 25 m above SL,
medium ventilation, 3 well-connected chambers
to form the cave
Show Plentiful speleothems, few tufa, some damage and
modified (tourism impact)
Me
Cung
20852016.700N;
107807001.400ELarge, multiple entrances at 25 m above SL,
moderately spacious, strong ventilation, medium
illumination, traces of shells encrusted on the
cave floor near entrance
Stop Mixture of tufa and speleothems, largely
destroyed due to frequently visited, trace of
regrowing
Cap La 20851053.800N;
107813028.400ECap La Islands, small vertical entrance at 20 m
above SL, limited ventilation, no illumination
Close Plentiful and intact speleothems in various forms
(drip stones, flowstones, cave crystals), few tufa
in the entrance zone
Dau
Giuong
20852018.400N;
107816041.300EDau Giuong Isle, small entrance at 25 m above
SL, moderately spacious, medium ventilation,
no illumination
Close Few stalactites, plenty well-crystallized
flowstones, splash pools, moderate tufa in the
entrance chamber, mosses and fungus coverage
near the entrance
Duc
Tien
20850020.400N;
107816047.900EVan Gio Island, large entrance at 20 m above SL,
spacious inside, ground salty water pools,
muddy inside, strong ventilation, medium
illumination
Close Moderate abundance of tufa in large chambers
near the entrance covered with macro plants,
mosses and fungus, well-crystallized
speleothems deep inside
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123
they were discovered recently and their entrances are not
easily accessible, which helps keep the looters away.
Today, the show caves are protected by the Ha Long Bay
Management Board, which admits only visitors with tick-
ets. Until recently both the stop and show caves were
affected by the large numbers of visitors. Currently, there is
no restriction to limit the number of visitors or regulate the
visiting frequency. During national holidays and festive
periods, thousands of tourists visit these caves. Transport-
ing visitors to caves, lighting, and the visitors themselves
release a considerable amount of thermal energy in the
caves. Numerous visitors bring a large quantity of dirt,
especially during the rainy season, which accumulate in
some parts of the caves. To facilitate the tours, the
entrance, floor, and many sections of the caves were
reshaped for easy access (Figs. 3a, b, 4b). Artificial sta-
lagmites made of concrete are placed along the walking
paths. Water from outside was pumped into artificial water
pools constructed inside. To make some caves more
attractive, water fountains were also built (Fig. 3a). In
addition to this, there is a recommendation that the cave
entrances should be widened and outside air should be
blown into the caves. Thus, the cave microclimate has
become more dependent on the atmospheric environment
than before these modifications were made. In the past, the
visitors used torches, candles, and oil lamps to illuminate
caves during their visit. Such primitive lightings produced
excessive quantities of soot (Fig. 3c). From the early
2000s, the tourist paths through the show caves have been
illuminated by electric lights (Figs. 3a, 4b). The show cave
chambers are illuminated from early morning until late
evening. With the introduction of electric lighting, flora
like green algae, mosses, and fungi are seen growing on
speleothem surfaces near the illumination sources.
Fig. 2 a Typical morphology
and vegetation of the isles in Ha
Long Bay and b a schematic
diagram of a hypothetical cave,
showing expected microclimatic
patterns and the likely
characteristics of speleothems
forming the range from
microcrystalline tufa deposits at
the cave entrance to normal
speleothems at the back
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123
The group of close caves is located to the northeast of
Ha Long Bay and is the farthest from Ha Long City among
the surveyed cave groups (Fig. 1b). Under current condi-
tions, it is not easy to access the caves, as their entrances
are nearly vertical above sea level with no stairway
(Table 1). Clearly, remote location and difficult access
have helped keep the caves fairly undisturbed until today.
Inside the caves, speleothems were abundant, intact, and in
different forms (Fig. 4c). Especially, rimstones filled with
cave pearls were found in various places, another indicator
of their quiet history. Personal communication with the
Management Board signals that no exploitation is planned
for these caves in the near future.
As illustrated in Fig. 2a, the cave length could not be
excessive because of the small size of islets (hundreds of
meters). The longest cave was estimated as not longer than
200 m. Compared to the height and width (up to 50 m in
some cases), these caves are largely different from many
other temperate caves where the length is tens to hundreds
of times longer than the width and height (Kogovsek and
Sebela 2004; Gazquez et al. 2012; Frisia et al. 2002). In
caves that have large and/or multiple entrances (Table 1;
Figs. 1c, 3b), there should be extensive exchange of air
mass and energy between cave interior and outside
environment.
First evidence of the outside influence on the cave
environment is that there are certain differences between
entrance, center, and rear sections. For all three groups of
caves, we found a decrease of temperature, light intensity,
and air flow and an increase of humidity from entrance to
rear (Fig. 5). We recorded consistently cooler temperatures
at the rear in most cases (Fig. 5a). While this may be a
consequence of different monitoring periods and seasonal
variations, it is more likely due to the differences in the
caves’ physical configuration, such as passage geometry
and size and number of entrances, which are known to
Fig. 3 a Typical
anthropogenic activities in a Ha
Long cave (man-made water
pools and fountains,
illumination) and their impacts
on the speleothems, b entrance
modification of the Dong Tien
cave for easy access, and
c ceiling of the Bo cave, a
‘‘stop’’ exploitation cave with
clear traces of speleothem
destruction and smoke stains.
There are newly growing soda
straws at previously destroyed
stalactite places
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123
largely determine microclimate regimes of individual caves
(de Freitas et al. 1982). In detail, the temperature at the
cave entrance, depending on the recorded times, is equal to
or a little higher than the atmospheric daily temperature
recorded in the Phu Lien metrological station near Ha Long
Bay (Appendix). In the rear sections, temperature is about
the same as the atmospheric daily temperature. Our survey
tactic is to go from cave to cave during daytime. Thus, the
difference of microclimate records between caves, espe-
cially at the entrances, is partly a result of the diurnal
variation of outside atmosphere. Indeed, the records show
that during the same day of survey, temperature in caves
monitored at noon and early afternoon times is higher than
in the ones monitored in early morning or late afternoon
(Appendix). In addition, temperature in the rear sections is
more stable (approximately 1 �C less) than at the entran-
ces. The progressive stability toward the rear is under-
standable since outside environment, characterized by
diurnal variation, has less impact to the rear section than
the entrance. Especially, there is almost no difference in
temperature recorded in the rear sections of the close caves.
The temperature stability is interpreted as, among the three
cave groups, least influenced by outside conditions for the
close caves.
Relative humidity changes exhibit a similar pattern to
temperature variation, except that there is a clear increase
in mean values toward the rear (Fig. 5b). The overall sit-
uation is thus typified by (a) the greatest range and lowest
relative humidity values at the entrance, (b) progressive
increase of minimum and mean values toward the cave
interior, and (c) nearly constant levels deeper inside the
caves. In the entrances, the relative humidity shows an
average of about 80 %, while in the inner cave, the mean is
nearly 90 % (Fig. 5b). The humidity data imply that
evaporation rates decrease toward the interior of the caves.
One significant observation is that humidity values in the
cave entrances are always high, even above 90 % on
numerous occasions (Appendix). As in other tropical karsts
elsewhere, such high humidity level favors the speleothem
growth not only in the inner part of the caves but also at the
entrance and transitional zones (Taborosi et al. 2005).
Another correlation we noticed is that, compared to other
two groups, humidity is a little bit higher in the show caves,
possibly because water fountains and pools installed in
many sections of the caves provide additional source of
water vapor.
Light intensity is largely different among the cave
groups (Fig. 5c). The show group equipped with lighting
systems has a nearly constant illumination level throughout
the interior. The artificial light allows human eyes to see
quite well. The stop group, thanks to their large entrance, is
also well illuminated during daytime. Light intensity lev-
els, obviously, diminish toward the cave interior. The close
group is the least illuminated among the three. Apparently,
human activities (e.g., lighting, entrance widening) play an
important role in increasing illumination inside those
caves.
Fig. 4 Typical scenes in caves of different exploitation states: a a
stop cave full of damaged speleothems, b a show cave characterized
with abundant and beautiful speleothems and constantly illuminated
with artificial light, and c a close cave with intact speleothems of
different forms
Environ Earth Sci
123
Similar to illumination, airflow in the show and stop
caves is stronger than in the close ones (Fig. 5d). While
substantial airflow in the stop caves is due to their large and
multiple entrances, ventilation in the show caves is also
contributed by human activities (e.g., entrance widening
and tourist visiting). The feeling of air movement in those
strong and mid ventilation caves can be detected deep
inside the cave interior. We also noticed that in caves
where airflow was substantial, humidity and temperature
did not vary much (Appendix).
In general, difference in mean values of microclimate
records (Fig. 5) suggested that (a) there is a difference in
microclimate between caves of different exploitation
states and (b) there is a zonation of microclimate inside
the caves of Ha Long Bay. To assess if exploitation has
led to a significant change of microclimate, we employed
cluster analysis. This statistical technique helps to group
the caves in such a way that caves in the same group are
more similar in microclimate conditions to each other
than to those in other groups. The MINITAB program
was employed for this statistical analysis. The analytical
settings are complete linkage method and Euclidean
distance measurement. The analytical result shown in
Fig. 6 clearly supports our hypothesis that caves of the
same exploitation state are statistically similar. All close
caves (Cap La, Dau Giuong, and Duc Tien) are found
clustered into one group. The show caves (except Dau
Go) are clustered into another group. The remaining
cluster includes stop caves and Dau Go. It is not sur-
prised that Dau Go cave is statistically grouped with the
stop caves since, in fact, it is an easily accessible cave
and has a long history of visits, similar to other stop
caves. Perhaps a reason to keep this cave open is
because of its close proximity to Thien Cung cave.
Together, they form a comprehensive attraction spot.
The prospect of microclimate variation along the cave
passage was statistically tested with the use of the paired
t test for mean difference. The microclimate records of
different areas (entrance, center, and rear) were subject to
the test. The null hypothesis is that microclimate records
are identical (mean difference = 0) between two areas.
We chose the 95 % confidence level which means that if
the P value were less than 0.05, we would be able to
reject the null hypothesis at the a = 0.05 level of sig-
nificance. In other words, the microclimate conditions in
two areas would be significantly different at 95 % of
confidence. On the contrary, the microclimate conditions
in two areas would not be significantly different if the
P value were found greater than or equal to 0.05. The
paired t test results listed in Table 2 show that only
between the center and rear areas of the show cave
group, microclimates are not significantly different (their
P value is 0.058). All other results (P values less than
0.05) confirm that microclimate is significantly different
between areas. This implies the influence of outside
atmosphere on the cave microclimate.
15
20
25
30
35
Group
Entrance Central Rear
60
70
80
90
100
R. H
um
idit
y (%
)
Group
Entrance Central Rear
0
200
400
600
800
1000
1200
1400
Illu
min
atio
n (
Lu
x)
Group
Entrance Central Rear
Show Stop Close
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Air
flo
w (
m/s
)
Group
Entrance Central Rear
Show Stop Close Show Stop Close
Show Stop Close
a
Tem
p. (
ToC
)
c
b
d
Fig. 5 Microclimate records (average, min, and max) in different zones of caves of different exploitation states (show, stop, and close)
Environ Earth Sci
123
Speleothem morphology and microstructure
The surveys demonstrate that speleothems in the Ha Long
Bay caves are abundant and appear in various forms:
dripstone, flowstone, and cave crystals (Figs. 3a, 4, 7).
Speleothems range from highly porous and largely bio-
genic accretions of calcareous tufa growing at the caves’
entrance to the dense crystalline speleothems in the caves’
interior. The speleothems display a wide range of distinct
fabrics as they span the microclimatically most variable
and most stable parts of the cave.
The surface of carbonate deposits in areas exposed to
daylight is coated with a unique organic layer with dis-
tinctive pale to dark green, brownish, gray, and black color.
Speleothems observed in the most exposed portions of the
caves are covered by moist and velvety coatings of algae,
lichens, and especially mosses (Fig. 7a). In some cases,
these surface layers can be dry and exhibit desiccation
cracks, wrinkles, and flaking. Also evident can be roots of
higher plants, which grow nearby or emerge from the
speleothems. Further into the twilight zone, the obvious
growths of mosses, algae, lichens, and higher plants are
replaced by epilithic microbial biofilms, resulting in wet
and pasty, powdery, or earthy coatings (Fig. 7b). These
organic layers can vary significantly in composition,
causing diverse coloration: white, gray, yellowish, light to
dark green and brown, purplish, and black (Appendix). The
biofilms are particularly pronounced on the sides facing the
light, and support prolific colonies of prokaryotes and mi-
crophytes. The sides of the speleothems facing the darkness
of cave interior generally lack such biologic consistency
and their surfaces are flat and smooth, rough or botryoidal,
or exhibit jagged coralloid textures. In the innermost twi-
light zone and deeper into the caves, biofilms gradually
diminish, receding to the most damp areas near the spele-
othems’ growing tips and eventually die. As abiotic sur-
faces take over, speleothems gradually gain crystalline
luster. They are colored white, yellowish, or brown, and
their most common textures are coralloid, slightly rough,
and smooth surfaces (Figs. 4c, 7c).
In the stop caves, speleothems in the daylight and twi-
light zones are largely lost. Three factors contributed to this
savage: (a) the speleothems in these zones were porous and
easily broken with force (b), low ceilings usually found in
these zones allow looters to reach the stalactites easily
(Figs. 3c, 4a), and (c) the stop caves were never highly
protected. Interestingly, we observed juvenile speleothems
blossoming from damaged speleothem places (Fig. 3c).
Perhaps, as the caves are deserted today, the quiet and
stable environment inside the caves favors new speleothem
growth. Closer observation shows that many juvenile sta-
lagmites have needle aragonite crystal structure, while the
newly formed stalactites have calcite crystals. Explanations
for the aragonite preference of stalagmites are numerous.
For instance, as suggested by Gazquez et al. (2012), there is
an aragonite preference to calcite at the beginning of spe-
leothems forming. They explained that initially, the envi-
ronment under which speleothems developed was
characterized by restricted movement of the air masses.
These conditions led to relatively high humidity, high CO2
partial pressure, and low evaporation rates. As a result, the
degree of oversaturation of the solution and the nucleation
rates were low. Under conditions of near-equilibrium in
terms of saturation, precipitation of aragonite is favored
over that of calcite (Frisia et al. 2002). Another explanation
is that there is possible inclusion of seawater vapor into
drip water near the floor of studied caves. Since the Mg
inhibited nucleation of calcite (Burton and Walter 1987;
Frisia et al. 2002), aragonite precipitation from water
media contaminated with seawater Mg is preferred to cal-
cite precipitation.
Caves
Sim
ilari
ty
Dong
Tien
Me
Cung
Bo Nau
Dau G
o
Duc T
ien
Dau G
iuong
Cap L
a
Tam C
ung
Sung
Sot
Thien
Cung
0.00
33.33
66.67
100.00
Fig. 6 Dendrogram resulted from cluster analysis of cave microcli-
mate; close caves (Cap La, Dau Giuong, and Duc Tien) are
statistically clustered together; show caves (Thien Cung, Sung Sot,
and Tam Cung) except Dau Go are tied in another cluster; the stop
caves are also statistically clustered into other branch
Table 2 Paired t test for microclimate condition between different
zones of cave interior
Entrance–Center Entrance–Rear Center–Rear
Show caves
T value 3.25 3.24 1.93
P value 0.002 0.002 0.058
Stop caves
T value 2.69 2.70 2.51
P value 0.013 0.013 0.020
Close caves
T value 2.53 2.51 2.23
P value 0.015 0.016 0.031
Environ Earth Sci
123
After 10 years of electric lighting used to illuminate the
dark sections of the show caves, at some places the sur-
face of speleothems is already covered with green algae,
while in other places mosses and fungus are growing near
the lights. According to Faimon et al. (2003), the flora
growing under artificial light on substrates where photo-
trophic organisms do not appear under natural conditions
is named lamp-flora. From both a tourist attraction and
cave sustainability perspective, lamp-flora development is
harmful, as speleothems and cave walls lose their charm
and become weary. Further investigation of the weary
speleothems suggests that lamp-flora thrives in the areas
facing white and bright-yellow lights (incandescent light-
ing) (Figs. 3a, 4b). With other color illuminations, the
growth is less obvious. Although the lamp-flora problem
was acknowledged by the Management Board, no light
control measures have been proposed or tested to halt its
growth.
The ESEM analysis clearly shows that microfabric of
the speleothems is distinctively different in different cave
zones (daylight, twilight, and dark) (Fig. 8). Containing
both calcite and aragonite, speleothems exhibit complex
fabrics. Deposits collected in the daylight zone have
microcrystalline fabrics of calcareous tufa, comparable to
traditional tufa deposits (Ford and Pedley 1996; Fig. 7a).
They characterize the relatively open and microclimati-
cally variable parts of the caves, where encrusted macro-
phyte fabrics dominate the most exposed areas, while
encrusted microbial fabrics and laminated microcrystalline
fabrics (Fig. 7a, b) typify the light-available zones.
Alongside encrusted morphologies, speleothems at the
entrance and in daylight and twilight zones are often
interspersed with organic-rich material, microbial struc-
tures, detrital grains and biofilms, and plant fragments
(Fig. 8a). In some instances, large fragments of macro
plants are found encrusted in the fabric (Fig. 8b). Further
away from the entrance, speleothems are less porous and
more orderly, exhibiting greater proportions of macro
crystals while retaining some microcrystalline material.
Beyond the twilight zone, speleothems are densely layered
and without significant microcrystalline or organic material
and reflect typical speleothems (Fig. 8c).
With permission from the Ha Long Management Board,
we were able to collect material scraped from a flowstone
surface suspected to be covered with lamp-flora (Fig. 3a).
The ESEM image of the sample (Fig. 8d) clearly shows a
biofilm thriving on the sample surface. The images also
indicate that microbial growth interferes with the crystal-
lization process creating porous and crumble microcrys-
talline fabrics.
Fig. 7 Representative samples taken in a daylight zone, b twilight zone, and c dark/no-light zone
Environ Earth Sci
123
Discussion
Cave microclimate and the formation of speleothems
Climatic data from tropical caves are limited, as the
majority of relevant research was carried out in temperate
regions. In the most comprehensive review of cave mi-
croclimatology, only a few references are related to tropi-
cal caves (see Wefer 1991; Taborosi et al. 2005). The
distinctiveness of tropical cave microclimates as opposed
to those of temperate caves as concluded by Gamble et al.
(2000) is as following: (a) external atmospheric variations
project into the caves and diminish toward the rear, and
(b) the deep cave microclimatic zone is missing due to
short length of caves relative to their entrances, heights,
and widths. By identifying a number of features that may
be unique to tropical caves, they showed that existing cave
microclimate models may not be applicable in the tropics.
The microclimatic observations in Ha Long Bay generally
conform to the conclusions of Gamble et al. (2000). We
describe the microclimatic environment of the cave in
terms of four factors, temperature, humidity, light avail-
ability, and airflow/ventilation that can be expected to
affect the carbonate deposition. The first two are intrinsic
parameters, which directly affect the chemistry of precip-
itation (Dreybrodt 1999; White 1997). Light levels have no
immediate impact, but can indirectly affect the process by
defining the local biologic environment. Airflow can
physically influence speleothem formation as well as alter
the degassing and evaporation processes (Hill and Forti
1997). Overall, field surveys and data analysis lead to a
conclusion that environment inside the caves is signifi-
cantly influenced by outside conditions. Microclimate
parameters such as humidity and temperature fluctuate with
the atmospheric counterparts near the entrance, and the
atmospheric dependence gradually decreases toward the
rear of caves (Fig. 5; Table 2).
While documenting drip water geochemistry remains an
objective for future research, it is believed that due to the
small sizes of the studied caves and their similar locations
in Ha Long Bay isles, drip water feeding speleothems is not
qualitatively different. Therefore, we assume that mor-
phologic and petrologic differences among the studied
speleothems are predominantly a function of each speleo-
them’s microclimate and ensuing biologic environment,
determined by their specific position in the caves.
From the entrance toward the rear of the caves, speleo-
thems show progressively lower porosity and heterogeneity
and greater crystal size and level of organization (greater
crystal size implies higher level of organization). Near the
entrance, the increased evaporation causes rapid precipita-
tion of calcite from karst water, resulting in poorly arranged
Fig. 8 ESEM images: a spores
and filaments encrusted in
crystal structure, b large plant
fragment inside crystal
structure, c compact calcite
fabrics, d crumbly, porous
crystal structure covered with
biofilm
Environ Earth Sci
123
and randomly oriented microcrystalline aggregates. This is a
well-known phenomenon influencing the fabric of calcare-
ous tufa deposits worldwide (Ford 1989; Viles and Goudie
1990; Ford and Pedley 1992, 1996). Nonetheless, the irreg-
ular crumbly speleothems we observed in the most exposed
parts of the caves are produced partly by this process, and
essentially comprise a unique category of tufa. In addition to
increased evaporation, the precipitation of these deposits is
affected by the pronounced diurnal, seasonal, and annual
variations in temperature and humidity as well as other
indirectly linked parameters such as changes in canopy
abundance and shading effects. Due to these oscillations,
carbonate deposition is inconsistent and results in the
observed heterogeneities in morphology and fabrics. In
caves with large entrances (e.g., Duc Tien, Bo Nau, Dong
Tien, Dau Go), the transitional zones (twilight zones as
sketched in Fig. 2b) between the entrance and cave interiors
are so broad that they exhibit distinct microclimates of their
own: enclosed enough to sustain abundant speleothem
growth, yet open enough to exhibit considerable light pen-
etration and marked diurnal fluctuations in temperature and
humidity. As the humidity stabilizes with distance away
from the cave entrance (Fig. 5b), the proportion of macro-
crystalline to microcrystalline CaCO3 increases and hetero-
geneities in the fabrics diminish. Finally, as consistently high
humidity levels are reached deeper inside the cave, the
effects of rapid and fluctuating evaporation are nearly
eliminated, and the resultant speleothems are composed of
orderly crystalline fabrics precipitated by CO2 degassing.
Within such a micro climatically stable environment, vari-
ations in water availability and geochemistry are expected to
overcome microclimatic and biologic factors and have pri-
mary control of the speleothem morphology and fabrics.
Statistical analysis presented in the result section indi-
cates that the hypothesis of human impact on the cave
microclimate could not be rejected. Our surveys have poin-
ted out that most tourism-related activities have led to an
increase of light, ventilation, heat, and humidity, or in other
words, destabilized microclimate in the caves. Certainly, an
increased instability of microclimate would result in a higher
level of heterogeneity of speleothems. In fact, human
activities do not only destabilize microclimate, but also
contaminate the cave environment with alien materials (e.g.,
dirt, sea water, and plant fragments) which would interfere
with the growth process and crystal fabrics of speleothems.
For instance, as we have already observed, precipitation of
CaCO3 in an environment contaminated with seawater tends
to be aragonite, which is more crumbly than calcite.
Cave biology and the formation of speleothems
It is now generally accepted that in addition to the inorganic
processes, many carbonate precipitates are caused by
biological activity or are almost entirely biogenic (Viles
1988). This relationship is clearly visible in the caves we
studied, where the patterns of microclimatic change from
the entrance to cave interiors are closely reflected by cave
life.
Near the entrance and in the best-illuminated parts of the
cave, many speleothem deposits are colonized by various
flora species including vascular plants, and are conse-
quently dominated by encrusted macrophyte structures. As
light is reduced deeper into the cave, macrophytes disap-
pear but photosynthetic microbes continue to thrive in
complex epilithic biofilms. This is macroscopically
apparent from the greenish hues on cave walls and espe-
cially on stalactites which, being end points of vadose flow
paths, are wet and preferentially colonized by organisms.
This is also evident in speleothem fabrics, which com-
monly contain calcified filaments and microorganisms.
The succession of biological involvement in speleothem
growth is a reflection of the microclimatic gradient
(Fig. 2b). Microclimatic variations, and especially different
illumination levels, define the ability of specific microor-
ganisms to colonize a particular substrate, and thus deter-
mine the nature of biofilms that develop on a given surface
(Taborosi and Hirakawa 2004). This affects carbonate
precipitation and influences the incipient deposits. In turn,
the morphology and microstructure of speleothems are a
function of factors defined by two parallel pathways:
(a) microclimatic variations directly influence carbonate
precipitation and also determine the composition and
dynamics of biological communities which then (b) influ-
ence carbonate precipitation in their own way.
It is intuitive that sustainable exploitation of caves and
speleothems is achieved by introducing as few external
materials as possible. The installation of lighting is the
most drastic change to occur in a light-deprived cave
environment. An imminent problem is that the available
photons emitted by lights nurture lamp-flora development.
As stated in the previous section, lamp-flora does not
develop in all illuminated sections. The reason for this is
probably due to the different types of illumination and the
distance between light sources and speleothems. In some
cases, light is dispersed in the space and does not illumi-
nate solid surfaces, but serves only to ensure visitor safety.
Protection of the caves and speleothems
Cave surveys and petrographic analyses of speleothems
revealed four major concerns that need attention in future
cave and speleothem protection: (a) growing lamp-flora
encrusted with carbonate is detrimental to speleothems;
(b) high humidity due to seawater influx in the caves
induces aragonite growth in speleothems, which makes
them more fragile than sturdier speleothems made of
Environ Earth Sci
123
calcite; (c) growth of algae and moss in the daylight and
twilight zones play a significant role in speleothem
petrology; (d) anthropogenic activities augment light,
ventilation, heat, and humidity in caves, which are gener-
ally destructive to speleothems.
We are recommending two approaches that will poten-
tially minimize harmful anthropogenic influence in the
future management of these caves: (a) control of lamp-flora
growth, and (b) control the number of visitors in caves.
Control of lamp-flora growth
So far, removal of lamp-flora from speleothems in caves of
Ha Long Bay has not been considered. There are different
approaches to control lamp-flora growth on speleothems:
(a) physical which includes reduction in the intensity and
duration of light, and (b) chemical, which includes appli-
cation of various biocidal chemicals, such as formalin,
bromine, cupric solutions, and solutions based on active
chlorine (Mulec and Kosi 2009). More attention should be
devoted to a suitable lighting installation, focusing on the
minimum lighting duration, the minimum intensity, and
selection of sites which are appropriate for illumination. A
very promising recent approach is the installation of light
emitting diodes (LEDs)—an electroluminescent semicon-
ductor light source—because they have low energy con-
sumption, are long lasting, and can be selected to have a
desirable emission spectrum (Toomey et al. 2009) that is
minimally useful to flora. This new type of illumination is
already in use in several caves around the world. The use of
fiber optics in cave lighting is another alternative to reduce
lamp-flora growth (Mulec 2012). An automatic light
switching system in the show areas would save energy and
reduce illumination time.
Overall, active chlorine—a bleaching solution com-
prising of hypochlorite (OCl-) and/or radical chlorine
(Cl•)—used to be an efficient method to remove lamp-flora.
However, some side effects of using this highly oxidizing
chemical have been reported. For instance, besides chlo-
rine’s unpleasant smell, chlorine compounds also react
with many different substances in nature, which results in
the formation of various toxic products. In addition, chlo-
rine compounds in water lower its pH and thus lead to the
corrosion of carbonate speleothems. Therefore, aggressive
chemicals such as chlorine are not appropriate for a sen-
sitive tropical cave environment. Faimon et al. (2003)
proposed a novel procedure developed for the purpose of
removing lamp-flora. Instead of using chlorine solution, an
environmentally friendly and odor free 15 % solution of
hydrogen peroxide was prepared with a carbonate buffer
(pH 7.0–7.5). This solution could be effectively applied
once per month to halt lamp-flora growth.
Control of cave visitors
It would be wise to regulate the number of cave visitors,
particularly during and after rainy periods. Our surveys
indicate that the sparse canopy and host rock above the
caves can retain water for a relatively short time (Figs. 1c,
2a). The caves dry quickly after the rain. Thus, carbonate
deposition can only be active for a short period during and
after rain. If cave visits are limited during such sensitive
periods, anthropogenic impacts (increase in CO2 level, dirt,
and contaminants) on speleothem growth would be
minimized.
A cave visit should be organized into groups of a certain
number of people, depending on the size of caves. The
interval between group visits should be regulated and
observed to control visitation frequency. To accommodate
this visit regulation, waiting lounges serving as buffer
zones during crowded times can be set up outside the
caves.
Protection of caves: cave protection act
The need for cave protection derives from awareness of the
damage caused by humans in caves and the negative atti-
tude towards it. The legal protection of karst caves repre-
sents the formal and, at least in essence, unambiguous
protection of caves at the national level. It is disappointing
that not only in Ha Long Bay but also in other karst
underground systems in Vietnam, cave protection practices
are mainly for tourism purposes, not for the cave and
speleothem sustainability (MOSTE 1996; MOJ 2005).
Caves are not protected by federal law in Vietnam. Any
protection measure applied to a particular cave is formu-
lated by local authority which depends on the administra-
tive capability of the community and is approved by
corresponding departments at the provincial level. In this
paper, based on the case study of Ha Long Bay caves, we
propose five administrative acts that would help protection
of the caves: (a) speleothem protection, (b) visiting regu-
lation, (c) renovation of cave infrastructure, (d) establish-
ment of cave closure criteria, and (e) program of cave
monitoring.
Speleothem protection act: Currently, protection of
speleothems exists only in show caves where guards are
present. In addition, sale of speleothems of unknown origin
is legal in Vietnam. The new law should prohibit speleo-
them collecting and sale in the Ha Long Bay area.
Cave visiting act: All cave visits should be supervised. It
should be forbidden to dig, collect or damage any petro-
graphic, mineralogical, and paleontological samples in
caves. It should be forbidden to pollute the cave walls,
ceilings or cave floor in any way. It should be forbidden to
throw rocks or other objects into the caves. It should be
Environ Earth Sci
123
forbidden to do any interventions that could affect the cave
entrances and the cave surroundings.
Cave construction and renovation act: If cave infra-
structure is going to be modified, a clause for cave and
speleothem restoration must be established. The restoration
principles are (a) returning caves and speleothems to a state
that is as close as possible to their natural state, (b) reme-
diating undesirable impacts in caves, and (c) improving
their appearance and their natural process of renewal. In
addition, there should be a clause of restoration applied to
caves that are no longer used for tourism to return the caves
to their natural state.
Cave closure act: There should be a clause to define the
status of close caves and caves with controlled access. The
closure of caves is an extreme intervention at a point where
other protective measures no longer suffice.
Cave monitoring act: The monitoring of individual
parameters, particularly in show caves should be manda-
tory. They include irradiation, heat input, humidity, CO2
concentration, dust content, and air movement. Routine
hydrological, biological, and chemical surveys should be
integrated into the environmental impact assessment (EIA)
program of exploited caves. Proposals for improvement of
the existing situation in accordance with the carrying
capacity of the caves in question should also be a part of
EIA.
Conclusion
This paper analyzed and summarized the morphological
and microclimate characteristics of karst caves in Ha
Long Bay. It also provides some insights into the
microstructure of speleothems thriving along the cave
passage. The first conclusion from this study is that cave
microclimate and speleothem petrology and abundance
gradually change from the entrance to the cave interior.
Secondly, the study shows that biology should be con-
sidered as important factor for the sustainable develop-
ment of caves and speleothems. Apparently, human
activities have made significant impact to morphology,
microclimate, and speleothems in caves of Ha Long Bay.
Based on the surveys inside the caves and microscopic
analysis of the sampled speleothems, two proposals are
recommended to alleviate human impact. Proposals to
control lamp-flora growth with the use of ‘‘environmen-
tally friendly’’ biocides and setting up buffer zones for
visitors during congested times are straightforward and
easily applicable. Based on the current Vietnamese laws
and regulations on environmental protection, this paper
proposes five cave protection acts as directive for sus-
tainable exploitation of caves and speleothems in Ha
Long Bay.
Acknowledgments This research is within the framework of the
research agreement between the Vietnam Academy of Science and
Technology (VAST), Vietnam, and the Consejo Superior de Investi-
gaciones Cientificas (CSIC), Spain. The paper was written with
support from protocol LOTUS No. 44/2012/HÐ-NÐT, MOST,
Vietnam. We are grateful to the Ha Long Management Board for
granting cave visit permission. Appreciation is sent to Dr. Zoran
Kilibarda and another anonymous reviewer for their invaluable advice
and comments. This paper was edited by Prof. Alexander Scheeline,
University of Illinois at Urbana-Champaign.
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