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: ducta@ich.vast.ac.vn

J. G. Guinea

e-mail: duc@mncn.csic.es

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).

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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

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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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