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The corals at Con Dao Archipelago (South Vietnam): Before, during and after the bleaching event in 1998

  • Institute of Oceanography, Vietnam Academy of Science and Technology

Abstract and Figures

Reef Check with more detail records was used to monitor variation of coral community of Con Dao archipelago. By April 1998, exposed reefs showed major hard coral cover loss (up to 100%) due to the typhoon, while repeated surveys of 6 sites not hit by the typhoon revealed that high coral cover was still present. Follow-up surveys in October 1998 coincided with the world-wide bleaching event. The frequency of coral colonies suffering bleaching ranged from 0 to 74.2% (n=11 sites) with a mean value of 37% not including more than 10% considered as mortality up to this time. The soft coral Sinularia was most affected by bleaching with almost 100% colonies bleached and then the fire coral Millepora (83%). The hard corals belonging to strongly impacted group included Porites (57%), Symphyllia (42%), Leptastrea (40%), …and also Acropora. Contrarily, there was no bleaching among 45 Galaxea colonies observed. In July 1999, most reefs showed recovery with some new recruits evident, but coral abundance was less than that before bleaching. The long-term influence was significant for massive corals, especially big Porites colonies.
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Proceedings 9
International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000
The corals at Con Dao Archipelago (South Vietnam):
Before, during and after the bleaching event in 1998
Vo Si Tuan
Reef Check with more detail records was used to monitor variation of coral community of Con Dao archipelago. By
April 1998, exposed reefs showed major hard coral cover loss (up to 100%) due to the typhoon, while repeated
surveys of 6 sites not hit by the typhoon revealed that high coral cover was still present. Follow-up surveys in
October 1998 coincided with the world-wide bleaching event. The frequency of coral colonies suffering bleaching
ranged from 0 to 74.2% (n=11 sites) with a mean value of 37% not including more than 10% considered as mortality
up to this time. The soft coral Sinularia was most affected by bleaching with almost 100% colonies bleached and then
the fire coral Millepora (83%). The hard corals belonging to strongly impacted group included Porites (57%),
Symphyllia (42%), Leptastrea (40%), …and also Acropora. Contrarily, there was no bleaching among 45 Galaxea
colonies observed. In July 1999, most reefs showed recovery with some new recruits evident, but coral abundance
was less than that before bleaching. The long-term influence was significant for massive corals, especially big Porites
Institute of Oceanography Nha Trang Vietnam ,E-mail:
Keywords Corals, Bleaching, Recovery, Reef Check,
The Con Dao Archipelago including 14 islands lies
offshore some 220 km to the south of Vung Tau City and
60 km to the mouth of Mekong river on mainland
Vietnam within the coordinates 8
37’ – 8
48’N and 106
32’ – 106
45’E. Almost all marine areas are managed by
Con Dao National Park which is the best in marine
conservation in Vietnam now. Coral reefs are considered
as the most abundant ecosystem in the shallow waters of
Con Dao islands. They distribute widely from littoral to
the depth of 17-20 m with the area estimated being 1000
ha approximately. Their structure is mainly of typical and
non-typical fringing reefs. During last years, coral reefs
here suffer strong impacts due to natural catastrophes
composing of typhoon Linda in November 1997 and
bleaching event in 1998. This study aims to assess vari-
ation of corals during 1994 - 2000 and to suggest solu-
tions for effective management of marine living re-
sources at this islands. Impact of bleaching was empha-
sized in more detail.
Materials And Methods
The first detail surveys on coral reefs at Con Dao
waters were carried out in 1994 and 1995 with 23 sites
studied by vertical and contour transects. Results of point
observations at each meter done along two 50 m contour
transects of the sites repeated later were used for
assessment of coral reef condition during this time. The
repeated surveys in 1998 – 2000 by Reef Check technique
with more detail records on substrate lifeform according
to LIT technique (English et al. 1997) were implemented
at 6 - 8 sites During this time, point observations were
done by recording the composition of coral genera,
lifeform of coral colonies and substratum types. This
application is considered reasonable for surveys done by
combination between professional scientists and volun-
teers from local stakeholders (National Park staff at Con
Dao). This choice was based on the concrete conditions of
the surveys. The staff of the Institute of Oceanography
who were trained on GCRMN method carried out LIT
technique in other areas already. The application at Con
Dao, however, was not available because of lack of time
and funding. The staff of Con Dao National Park who are
not professional in marine science can not apply this
method. Reef Check is easy to apply by NTIO scientists
who are very experienced in detail ecological assessment.
As designation, Reef Check is able to be carried out by
non-scientists (Hodgson 1999) and are very useful for the
staff of CDNP. Implementation of Reef Check, however,
did not provide enough data for detail analysis. This
report is the product of the above- mentioned point of
Corals before bleaching
According to data collected at 23 study sites in 1994
–1995, coral cover reached 42.6% in average. Among
them nearly 70% reefs were with the categories of high
and excellent cover (51-75% and >75%) including the
study sites of Bay Canh island, Bong Lan, Ong Cuong,
Chim Chim, Hon Tre Nho. Typhoon Linda seriously da-
maged or destroyed many of the coral reefs in Con Dao.
The results obtained in May 1998 showed that many sites
in southeastern side (Hon Cau, Bong Lan, Bai Duong)
were badly damaged, with live coral cover near zero.
Some other sites were destroyed at lower levels (values of
coral cover remained about 10%) at sites such as Dat Doc
and Hon Troc. This trend can be recognized through the
data showed at Table 1.
Table 1. Coral cover (%) at some sites of Con Dao before bleaching (Dead coral in 1994 –1995 according to English et all, 1997)
Study sites
1994 –1995 April 1998
Hard corals Soft corals Dead corals Hard corals Soft corals Dead corals
Ben Dam 44.7 0 11.4 31.5625 0.3125 49.375
Bong Lan 55.4 2.3 29.2 10.72917 0.625 50.3125
Chim Chim 36.0 23.0 5.2 47.5 19.375 0
Dat Doc 37.5 0 20.0 19.6875 1.25 58.2*
Hon Cau 52.0 0 42.0 0.625 0 74.375
Da Trang 37.7 0 47.9
Ong Cuong 73.7 5.0 0
Hon Troc 51.5 8.3 6.8 8.755 4.15 65.6*
Bai Duong 43.4 8.3 31.6 2.1875 0.625 85.3125
Hon Tai 52.3 0 26.8
Note: * Branches of dead corals were covered by seaweeds after the typhoon six months earlier, which was considered as DC of Reef Check
During bleaching
According to the information from local fishermen,
bleaching lasted for a long time from June to October and
cause serious impacts to all coral reefs of Con Dao
islands. Data collected in October 1998 showed the
frequency of coral colonies susceptible to bleaching
ranged from 0 to 74.2% depending on the study sites
(n=11) with the mean value of 37%. The situation was
more serious because a number of corals died before
observation and 0 % value of bleaching was only at one
transect with too low coral cover. There were some 14%
corals died at this time (Table 2).
Bleaching impacts also varied a lot with coral genera
(Table 3). Soft coral Sinularia was the most susceptible to
bleaching with approximately 100% among total colonies
observed and then fire coral Millepora (83%). The hard
corals belonging to strong impacted group composed of
Porites (57%), Symphyllia (42%), Leptastrea (40%),
Favia (36%), Goniastrea (30%), Lobophyllia (25%),
Montipora (22%), Acropora (19%). The impact to genus
Acropora, however, could be more serious because of
high cover of dead Acropora corals during the surveys.
The genera that have a better tolerance (about 10% among
total colonies observed) included Fungia, Favites,
Platygyra, Diploastrea. Specially, there was not any
bleaching among 45 Galaxea colonies recorded. An
impressive impact was observed at Porites massive corals,
some of them reached 10 m diameter equivalent to 700 -
1000 years old and were killed by bleaching now. Their
colonies were completely white and then covered by
filamentous algae. Bleaching also occurred at juvenile
corals which were recovered after the typhoon and it may
affect recruitment.
Table 2. The cover of corals (%) and their percentage of bleaching
Study sites Transect Hard corals Soft corals Dead corals % bleaching % died newly
Ben Dam
shallow 26.3 0 8.1 26.19 23.8
deep 31.9 0 45.0 37.25 0
Bong Lan
shallow 7.5 0 3.1 16.66 8.3
deep 19.4 0.6 0 74.19 0
Chim Chim
deep 38.1 8.1 6.3 68.85 16.4
Dat Doc shallow 1.9 0 21.3 0 0
deep 11.3 0 8.8 88.89 83.3
Hon Cau
shallow 0 0 0 0
deep 8.8 0 0 35.71 0
Da Trang
shallow 41.9 1.3 3.7 11.9 9.0
deep 41.2 0 1.9 18.2 3.0
37.79 14.4
After bleaching
After bleaching, the average cover of corals increase
significantly in 1999. The low cover category, however,
still occupied a high rate as during bleaching. Almost all
reefs of Hon Cau,
Bong Lan, Dat Doc affected by typhoon
impacts and lost recruitment ability as mentioned and
leaving the cover values observed in October 1998.
Meanwhile, destroyed reefs at Hon Tai showed excellent
recruitment. Corals here recovered and reached the
abundance as before bleaching.
The surveys in June 2000 confirmed coral recruitment
after strong bleaching at Ben Dam. The reefs at Da Trang,
Hon Tai remained in good conditions. In constraint, the
reefs damaged by typhoon recovered slowly. In general,
the conditions of coral reefs at Con Dao in 2000 were still
worse than that before typhoon Linda and the bleaching
event. The details of coral cover changes at each site
during 1999 - 2000 were described in Table 4.
Table 3. The impacts by bleaching were different from coral genera
No. of colonies susceptible to
Total colonies observed % susceptible to bleaching
Sinularia 16 16 100
Millepora 15 18 83
Porites 36 63 57
Symphyllia 8 19 42
Leptastrea 4 10 40
Favia 5 14 36
Lobophyllia 3 12 25
Montipora 11 50 22
Goniastrea 3 10 20
Acropora 19 98 19
Diploastrea 2 14 14
Platygyra 2 15 13
Favites 1 10 10
Fungia 1 12 8.3
Pachyseris 1 13 7.7
Galaxea 0 45 0
Table 4. Coral cover showing recovery ability at Con Dao islands
Study sites Transect
1999 2000
Hard coral Soft corals Hard coral Soft corals
Ben Dam
Shallow 20.6 0 31.3 0
Deep 45.0 0 45.0 0.6
Bong Lan
Shallow 0 0 13.8 0
Deep 5.0 0.6 3.1 0
Chim Chim
Shallow 23.1 1.9 16.3 1.9
Deep 38.8 6.3 27.5 5.0
Dat Doc
Shallow 5.0 0 3.8 0.6
Deep 11.3 0 19.4 0
Hon Cau
Shallow 11.9 2.5 0.6 0
Deep 1.9 0 0.6 0
Da Trang
Shallow 47.5 0 48.8 0
Deep 51.9 0.6 50.6 1.3
Ong Cuong
Shallow 24.4 0 19.4 0
Deep 8.8 1.3 7.5 1.3
Hon Tai
Shallow 45.0 0 50.7 0
Deep 58.8 7.5 - -
It is very clear that, the natural catastrophes occurring
in 1997 and 1998 caused strong impact to coral reefs at
Con Dao islands. In general, there were signs of recovery
at the reefs influenced by the typhoon such as Hon Cau
and Bong Lan islands after typhoon Linda in November
1997. The changes were not very clear in term of total
cover because of short time for the growth of remaining
colonies. Meanwhile, juvenile corals of different species
were common found to settle on the broken dead coral
and rock surface. This is a very positive sign. It indicated
that the coral reefs of Con Dao have potential to recover
eventually. This recovery will take place through the
growth of remaining colonies, reproduction and larvae
settlement as well as through the survival of very tiny bit
so living tissue on what in May 1998, appeared to be
totally dead piles of rubble. This latter mode of recovery
will be particularly valuable on the shallow reef flats.
Many of the new recruits were obviously the result of
larvae settlement due to their growth position, so this is
additional encouraging evidence that coral in Con Dao are
still reproducing in this manner. i.e. that there is sufficient
reproductive stock remaining on the battered reefs.
After bleaching, there was the differences on recovery
among sites. Some of them such as Hon Cau, Bong Lan,
Dat Doc remain the cover values observed in October
1998. Almost all juvenile corals at these sites were killed
by bleaching and recruitment by remaining colonies
occurred badly. Meanwhile, excellent recruitment by fast
growth of survival tissue and remaining parts of colonies
of branching corals was observed on destroyed reefs at
Hon Tai. It seems coral recovery is depended on once
ability of the larval settlement and recruitment types.
The results of surveys in June 2000 showed coral
recruitment after strong bleaching at Ben Dam with the
dominance of recovery manner by settlement of new
juvenile corals. The recorded juveniles belonged to the
genera : Acropora, Pectinia, Stylophora, Lobophyllia .
This manner were observed at Hon Cau, Bong Lan, Ong
Cuong, Dat Doc with less frequency. The recovery here
were mainly by growth of some survival colonies on reef
flats. It is a reason why total cover on the reefs did not
increase significantly. In contrast, reefs at Da Trang, Hon
Tai remained in good conditions after excellent
recruitment by growth of remaining colonies. The results
of these surveys have been useful for solutions to recover
coral reefs by artificial support. Recovery plans will be
different among the sites depending on impact level and
recruitment ability.
One more question needed to be solved was what was
the impact level of bleaching event at Con Dao islands in
comparison with other areas in Vietnam. The data
collected at Cu Lao Cau island (south central coastal
waters) allowed to have some discussion on relation
between coral bleaching and water temperature. There
were two surveys here in 1998. The survey in May
recorded some corals susceptible to bleaching but not
serious. Water temperature, however, was very high with
record of 31
C at all study sites. Repeated survey in
September observed no bleaching but so many corals
died and covered by filamentous algae (Table 5). It is
thought that these corals had been killed by bleaching
during period from late May to August. According to
discussion on relationship between temperature and
widespread bleaching (Winter et al. 1998), high tempe-
rature here was considered as a reason of bleaching at
least in May.
Table 5. Some data on corals at Cu Lao Cau island in September 1998
Sites Transect Recent dominant genera % coral colonies died newly (mainly Acropora)
Cu Lao Cau 1
shallow Acropora & Montipora 15.79
Deep Soft corals 0.00
Cu Lao Cau 1
shallow Soft coral , Milepora 13.64
illepora 0.00
Cu Lao Cau 3
shallow Acropora, Montipora & soft
Deep Soft coral 6.58
Average % of died corals which were considered to be
susceptible severely to bleaching
The question was why the bleaching event at Cu Lao
Cau had not lasted as long as that in Con Dao. Although
no data on temperature in September, cold waters brought
by upwelling was considered as the positive influence to
save so many corals. According to Vo Van Lanh and La
Van Bai (1997) upwelling occurs annually during June to
September in this region with the surface temperature
less than 22
C in the strong upwelling center. Short
period of high temperature supported recovery of a
number of corals after bleaching except Acropora which
was less tolerant. This was suitable to level of
susceptibility of coral genera at Con Dao islands and at
Great Barrier Reef (Marshall and Baird 2000), and
Okinawa (Yamazato 1999).
The results showed some ideas on different
susceptibility among genera. The most susceptible were
genera Aropora, Millepora and in Okinawa (Yamazata,
1999) and in Great Barrier Reef (Marshall and Baird,
2000). Impacts to soft corals Sinularia at Con Dao was
more severe than in Great Barrier Reef. The surveys at
Con Dao recognized the Galaxea as the most tolerance
with no bleaching occurring as in Okinawa. Meanwhile,
this genus has low susceptibility to bleaching in Great
Barrier Reef. The data obtained by surveys did not show
relation between bleaching susceptibility and lifeform of
coral colonies.
Acknowledgement The surveys at Con Dao received the
support from
DANIDA, WWF, National Center of Natural
Science and Technology of Vietnam, HKUST, ADB 5712-
REG project, Institute of Oceanography and Con Dao
National Park. The author would like to express great
thanks to all of them. Thank you very much for the
support and ideas of Dr. Gregor Hogdson.
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and Technologies for Management of EEZs and
Coastal Zone. Worldwide ICG publishing Ltd: 238 pp.
Marshall PA, Baird AH (2000) Bleaching of corals on the
Great Barrier Reef: different susceptibilities among
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ture and salinity in the strong upwelling center of
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... Although a survey during the major mass bleaching event of 1998 found that some corals inside the upwelling area were susceptible to bleaching, the impact was not serious and not as long-lasting as that in Con Dao Islands. There, 37% of coral colonies, on average, were susceptible to bleaching and 14% of corals died in October 1998 (see Vo, 2000). It is clear that Ninh Hai coastal waters with the short time of declined water temperature in summer, due to upwelling may provide a "refuge" for dispersal and recruitment of corals in coastal waters of Vietnam and potentially for other reefs in the South China Sea, against reef degradation caused by extensive coral bleaching. ...
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Reef geomorphology, species composition and community structure of reef-building corals of Ninh Hai (south central Vietnam) were investigated from 2003–2011, contributing towards development of an integrated, representative national and regional network of Marine Protected Areas. Ninh Hai hosts extensive and diverse fringing coral reefs covering more than 2,300ha, the result of favourable physico-chemical conditions of sea temperature, water clarity, and sediment levels. These well-developed fringing reefs are rare or absent in other parts of Vietnam, and hence provide a high degree of complementarity to the developing national MPA network. The fringing reefs of Ninh Hai are in relatively good condition (average live coral cover > 25%), comprised of some 310 species from 60 genera of reef-building coral, including 11 species and one genus (Scapophyllia) previously unknown from the western South China Sea. Coral community structure shows considerable differences with other reefs in Vietnam. With the adjacent dry coastal forest ecosystem, these reefs are now protected within the Nui Chua National Park, one of very few examples of integrated conservation management of a terrestrial–coastal marine ecosystem in Vietnam or indeed Southeast Asia. The regular presence of cool water upwelling during the summer months may provide a "refuge" against future reef degradation from extensive coral death from 'bleaching' during episode of elevated sea temperatures. These reefs may thus aid in replenishment of other reefs, via dispersal and recruitment of corals locally and regionally.
We investigated possible synergic effects on coral reefs of the local land reclamation activities in the Himmafushi Island (North Malè atoll, Maldives) and the global bleaching event that affected the Maldives in 2016. A BACI (Before-After Control-Impact) sampling design was adopted to contrast effects of dredging activities before and after the occurrence of both dredging and bleaching. The Reef Check protocol, a standardised and worldwide survey method, was applied to collect data through underwater visual surveys on corals, macro-zoobenthos, and fish communities. The bleaching in 2016 hit all the reefs investigated, but only in the reefs around Himmafushi (i.e., the impact sites) the live hard coral reduced significantly its cover and the sand deposited on reefs showed a fourfold increase. Substrate indicators (i.e., coral community and abiotic components) turned out to be more effective than macro-zoobenthos and fish in this short-term environmental impact study.
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High sea surface temperatures caused global coral bleaching during 2015–2016. During this thermal stress event, we quantified within- and among-species variability in bleaching severity for critical habitat-forming Acropora corals. The objective of this study was to understand the drivers of spatial and species-specific variation in the bleaching susceptibility of these corals, and to evaluate whether bleaching susceptibility under extreme thermal stress was consistent with that observed during less severe bleaching events. We surveyed and mapped Acropora corals at 10 sites (N = 596) around the Lizard Island group on the northern Great Barrier Reef. For each colony, bleaching severity was quantified using a new image analysis technique, and we assessed whether small-scale environmental variables (depth, microhabitat, competition intensity) and species traits (colony morphology, colony size, known symbiont clade association) explained variation in bleaching. Results showed that during severe thermal stress, bleaching of branching corals was linked to microhabitat features, and was more severe at reef edge compared with lagoonal sites. Bleaching severity worsened over a very short time-frame (~1 week), but did not differ systematically with water depth, competition intensity, or colony size. At our study location, within- and among-species variation in bleaching severity was relatively low compared to the level of variation reported in the literature. More broadly, our results indicate that variability in bleaching susceptibility during extreme thermal stress is not consistent with that observed during previous bleaching events that have ranged in severity among globally dispersed sites, with fewer species escaping bleaching during severe thermal stress. In addition, shaded microhabitats can provide a refuge from bleaching which provides further evidence of the importance of topographic complexity for maintaining the biodiversity and ecosystem functioning of coral reefs.
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 Much recent attention has been given to coral reef bleaching because of its widespread occurrence, damage to reefs, and possible connection to global change. There is still debate about the relationship between temperature and widespread bleaching. We compared coral reef bleaching at La Parguera, Puerto Rico to a 30-y (1966–1995) record of sea surface temperature (SST) at the same location. The last eight years of the La Parguera SST record have all had greater than average maximum temperatures; over the past 30 y maximum summer temperature has increased 0.7 °C. Coral reef bleaching has been particularly frequent since the middle 1980s. The years 1969, 1987, 1990, and 1995 were especially noteworthy for the severity of bleaching in Puerto Rico. Seven different annual temperature indices were devised to determine the extent to which they could predict severe coral bleaching episodes. Three of these, maximum daily SST, days >29.5 °C, and days >30 °C predict correctly the four years with severe bleaching. A log-log linear relationship was found between SST and the number of days in a given year above that SST at which severe coral beaching was observed. However, the intra-annual relationship between temperature and the incidence of bleaching suggests that no one simple predictor of the onset of coral bleaching within a year may be applicable.
The author has witnessed two coral bleaching events taken place at Sesoko and Okinawa islands in 1980 and 1998. Although the final reports of the 1998 event are not accessible, the author tries to make some comparisons among the events according to his own report on the 1980 event (Yamazato 1981) and his personal experiences of the 1998 event. Coral bleaching in 1980 around Sesoko Island, Okinawa, Japan appears to have been a local event, while that of 1998 was much more extensive affecting all islands of the Ryukyu Islands and Southern Kyushu. Abnormally high surface temperatures were observed on the reef flat during daytime low tide in the summer of 1980, while it was observed continuously throughout the summer of 1998, affecting not only coral reef waters but also coastal waters both day and night. In 1980, over 40% of coral colonies bleached once and about 10% died, the rest of the bleached corals recovered by the following January. In the 1998 event, according to visual observations of the author made on the same reef section, almost 90% of corals and soft corals bleached and died, with only a few colonies of a few species that recovered. In addition to these personal observations, some results of new surveys are presented and comparisons between the two events are made.
Large-scale coral bleaching episodes are potentially major disturbances to coral reef systems, yet a definitive picture of variation in assemblage response and species susceptibilities is still being compiled. Here, we provide a detailed analysis of the bleaching response of 4160 coral colonies, representing 45 genera and 15 families, from two depths at four sites on reefs fringing inshore islands on the Great Barrier Reef. Six weeks after the onset of large-scale bleaching in 1998, between 11 and 83% of colonies along replicate transects were affected by bleaching, and mortality was 1 to 16%. There were significant differences in bleaching response between sites, depths and taxa. Cyphastrea, Turbinaria and Galaxea were relatively unaffected by bleaching, while most acroporids and pocilloporids were highly susceptible. The hydrocorals (Millepora spp.) were the most susceptible taxa, with 85% mortality. Spatial variation in assemblage response was linked to the taxonomic composition of reef sites and their bleaching history. We suggest, therefore, that much of the spatial variation in bleaching response was due to assemblage composition and thermal acclimation.
Coral reef monitoring and management using reef check. The Review of Strategies and Technologies for Management of EEZs and Coastal Zone. Worldwide ICG publishing Ltd
  • G Hogdson
Hogdson G (1999) Coral reef monitoring and management using reef check. The Review of Strategies and Technologies for Management of EEZs and Coastal Zone. Worldwide ICG publishing Ltd: 238 pp.
Survey manual for tropical marine resources. Australian Institute of Marine Science
  • S English
  • C Wilkinson
  • V Baker
English S, Wilkinson C, Baker V (1997) Survey manual for tropical marine resources. Australian Institute of Marine Science. Townsville, Australia.
Variation of temperature and salinity in the strong upwelling center of southern Vietnam. The Contribution on Coastal Strong Upwelling in Southern Central Vietnam
  • La Van Vo Van Lanh
  • Bai
Vo Van Lanh, La Van Bai (1997) Variation of temperature and salinity in the strong upwelling center of southern Vietnam. The Contribution on Coastal Strong Upwelling in Southern Central Vietnam. Science and Techniques Publishing House. Ha Noi (in Vietnamese).