Changes in the Composition and Structure of Coral Communities on some reefs Nha Trang Bay, South China Sea

Yu. Ya. Latypov

A.V. Zhirmunsky Institute of Marine Biology, Far Eastern Branch, Russian Academy of Sciences,

ul. Pal’chevskogo 17,Vladivostok,690041 Russia

e-mail: ltpv@stl.ru

Abstract. In October 2003 and January 2005, comparative observations were made on

the reefs of Mieu and Mun islands near the city and port of Nha Trang, which we first

investigated in 1981. Appreciable changes due to anthropogenic impact have occurred on the

reefs that are the nearest to the city. There was a reduction in substrate cover by reef-building

corals, a substitution of dominant scleractinian species, and a decrease in the numbers and

diversity of common species of corallobionts. The index of species diversity for Scleractinian

also decreased. The seaweeds Chnoospora and Halimeda spread into all zones of the reefs.

Changes in coral communities on more distant and protected reefs were not so marked.

Keywords: Reefs, Communities, Corals, Change, Anthropogenic impact.

The city of Nha Trang and its vicinity located on the shores of Nha Trang Bay are

intensely developing. They provide a barrier to the destructive energy of waves, thereby

protecting the coastline from erosion. The coral reef is, in essence, a “living barrier,” its base

perishing and being renewed Residential and communications construction, establishment of

new hotels and underwater swimming centers, intensified mariculture operations, and increased

pressure of tourism have led to enhanced terrigenous runoff in Nha Trang Bay [1-3]. The

infrastructure of the municipal sewage and waste disposal plants and protective measures do not

satisfy modern requirements dictated by economic growth. Deposition of terrigenous material is

the major reason for pollution and damage to coral reefs. Inputs of terrigenous material are

increasing due to the destruction of vegetation cover (agriculture and forest cutting), mining and

building operations, wastewater discharge, and application of fertilizers. This leads to a reduction

in the light arriving at corals, their being buried under sediment, and abrasion of the polyp tissue

through friction. Eutrophic waters affect the metabolism in zooxanthelae and augment the

development of phytoplankton, leading to reduced light levels and to the development of benthic

filter-feeding organisms. Under these circumstances, corals can become inferior to macroalgae

and invertebrates in competition for nutrients [4, 5].

The deterioration of coral reefs is producing concern on the part of Vietnamese scientists

and their government. Efforts have been concentrated on the study of the reasons for reef

ecosystem variations and on the elaboration of a management strategy for conservation and

restoration of coral communities. The present publication is based on data from many years of

observations on the status of selected coral reefs in Nha Trang Bay. We aim to attract the

attention and funding urgently required for immediate conservation measures on the reefs of

Vietnam, which are an organic part of the Indo–West Pacific tropical center of coral diversity

and origin [6- 9].

MATERIALS AND METHODS

We investigated the reefs of Mieu and Mun islands in 1981 (Fig. 1) using the widely

accepted technique of quadrats, transects, and scuba [10]. From September to October 2003 and

in January 2005, 200-m transects with each meter distance marked were set on each reef. Along

the transects, the degree of substrate cover by corals and macrophytes was determined; the

numbers of massive, branched, and encrusting forms of colonies were calculated; and qualitative

and quantitative sampling of common species of corallobionts was carried out. Structural and

unstructured reefs were identified according to presence or absence of geomorphological

zonation and reefogenous deposits [8, 11]. There was a structural reef with a distinct zonation

and reefogenous deposits in the southeastern part of Mieu Island. Unstructured reefs around Mun

Island formed a weakly developed crust on the substrate, hardly changing its profile. The degree

of substrate cover by corals and macrophytes and the species richness of scleractinian and

common species of macrobenthos were assessed during visual observations.

RESULTS

According to the research in 1981, almost all of the zones that are common to most

typical structural reefs, viz., a small lagoon (coastal channel), internal and external reef flat, a

reef slope and a fore reef platform were recorded in the reef of Mieu Island in the Nha Trang

Bay [12, 13] . Fifty to sixty scleractinian species were distributed in the reef, which formed vast

areal populations and biostromes 4–5 m in width and 1 m in height. The level of substrate

coverage by corals was 45–60%; it reached 75–100% in patches of the populations of Acropora

and Porites species. Acropora formosa, A. cytherea, Montipora digitata, M. foliosa, and Porites

nigrescens dominated in substrate coverage, and Acropora (11–17 species), Porites (4–7

species), and Fungia (3–5 species) dominated in species diversity.

The studies that were performed in 2010 and 2013 revealed that the reef of the Mieu

Island in the zone of the reef slope was characterized by heavy silting of the substrate and

macrobenthos, including corals. The flow rate of sedimentation reached 35.28–48.6 g/m2 per day

in the waters that surround the island [14]. A decrease in the number and size of scleractinian

colonies and an increase in the number of algae of the Halimeda and Chnoospora genera was

recorded. The substrate coverage by corals rarely exceeded 30–45%. From the middle of the reef

slope, patches of populations of the brown alga Chnoospora implexa occurred between corals

and their branches. This species, as well as Halimeda opuntia and H. discoidea, spread in all

areas of the reef substrate and rapidly occupied the space between the coral branches, covering

up to 60–75% of the substrate surface. The associated macrobenthos also changed; 20–25 years

ago the sea urchin Diadema setosum (at least 5 ind./m2), holothurians Holothuria edulis and H.

atra (1–2 ind./m2), sea stars Linckia laevigata, Culcita novaeguineae, and Acanthaster planci

(0.1–0.2 ind./m2), bivalves Atrina vexillum (up to 0.2 ind./m2) and Tridacna crocea (0.5 ind./m2),

gastropods Lambis chiragra, L. scorpius, L. lambis, Trochus niloticus, Cypraea tigris, and

Mauritia Arabica (0.2–0.5 ind./m2), and other invertebrates lived here. In 2010 and 2013, only a

few individuals of T. niloticus and A. vexillum, were recorded, as well as extremely rare diadems

and holothurians, while the sea star A. planci occurred often.

The erosion occurring along the coastline of the city, the port of Nha Trang, and nearby

islands due to construction and excavation works; as well, intensification of mariculture greatly

increased sedimentation flows into the Nha Trang Bay, causing eutrophication of the water

column that surrounds Mieu Island and silting of the substrate around it [14-17]. Saturation of

the water by various nutrients is significantly influenced by unregulated mariculture farms, with

more than 40 raft facilities that surround the Mieu Island area. Fecal and residential discharges

from rafts immediately enter the surrounding water. According to the workers, each cell on

average contained from 200 to 500 kg of fish, shrimp, or other breeding objects. Even under at

the minimum loading of a cage, every month each raft was treated with 7000 multivitamin

tablets and antibiotics, i.e., throughout the year each raft discharged 84000 tablets into the water.

This chemical load contributes to the development of algae and to suppression of coral growth

[14, 18, 19]. As a consequence of these changes, reduced substrate coverage by corals and the

biodiversity index occurred, as well as a decrease in the concomitant common species of

macrobenthos and an increase in the substrate coverage by macrophytes. The replacement of

Acropora that dominated previously on the reefs of the Mieu Island with mono species

populations of finely branched Montipora porites that are more resistant to silting is likely to be

regarded as a result of the high content of suspended matter, which exceeded that in the

remaining areas of the Nha Trang Bay by 1.3 times.

On the reefs of Mun Island, in 1981, scleractinian did not form thick reefogenous

deposits and they covered the substrate with a thin crust. However, the species richness and the

degree of coral cover were fairly high. In the near-shore part on large rock blocks, there were

widespread separate colonies of various growth forms with a predominance of A. cytherea, A.

palifera, A. humilis, Stylophora pistillata, Favia speciosa and the hydroids Millepora dichotoma

and M. platyphylla. At 3–4 m offshore (2- to 3-m depth) were coral stands with an obvious

predominance of large plate forms of the corals A. cytherea and A. hyacintus and the thin-

branched hydroid M. dichotoma. The degree of substrate cover reached 75% in some areas.

Between Acropora and Millepora settlements, there were numerous and various colonies of

other scleractinians that are common to the reef slope: Porites, Goniastrea, Platygyra,

Diploastrea, Favia, Favites, Echinopora, Turbinaria, and other fungiids.

However, the number and size of rock blocks decreased with distance from the shore and

with increase in depth. As a result of the reduction in area of hard substrate, numbers and

diversity of scleractinian colonies decreased. The degree of coral cover dropped to 15–20%; the

most frequent were colonies of Acropora, Porites, Turbinaria, and Millepora.

At the base of the blocks, there were many small colonies typical of the lower part of the

reef slope: Symphyllia, Lobophyllia, Euphyllia, Micedium, Pectinia, and individual colonies of

alcyonarian and gorgonarian. Where a hard substrate was present, coral settlements spread for

100–110 m offshore. Between them, there were many various echinoderms, with predominance

of the sea urchin D. setosum in aggregations up to 15 ind./m2. Investigations of reefs in the

southern part of Mun Island in October 2003 and January 2005 revealed no significant changes

in the composition and structure of coral communities. As before, lamellar forms of Acropora

and branched Millepora constitute the bulk of coral settlements. The degree of substrate cover by

live corals decreased as a result of coral bleaching and the death of different parts and even

whole colonies of Acropora, Pocillopora, Seriatopora, and Millepora, as well as some of Fungia

and Sandalolitha. Nevertheless, the diversity of corals, especially at a 5-m depth (not reached by

swimmers with a snorkel and mask) remains fairly high.

Over half an hour of visual observation, I found 108 species belonging to at least 40

scleractinian genera. Ten species of Acropora occurred on this part of the reef, indicating

satisfactory conditions in this habitat. Here, settlements of fungiids, up to 45 ind./m2 (Fungia

fungites, F. concina, Sandalolitha robusta, Polyphyllia talpina, Herpolitha limax, and others),

occurred as bands (1.5–2.5 m across and up to 4–5 m long) oriented perpendicularly to the shore,

with continuous covering of the substrate. Some specimens of fungiids reach a size of 0.5 m. In

addition, there were many young fungiids (1–5 cm across) that reattached to the substrate. This

part of the reef is probably at a late stage of succession, with monospecific coral settlements

spreading over large areas [8,17-20].

DISCUSSION

Erosion processes along the coastline of the city and port of Nha Trang and the

intensification of mariculture operations in many bays of the nearby islands are augmenting

sediment fluxes and eutrophication of waters in Nha Trang Bay [21-23]. An increase in the

number of macroparticles of various origin causes an increase in the turbidity of the sediment,

reduces the photosynthetic capacity of reef-building corals and other benthic organisms, and

influences some physical and biological processes in these organisms [24-26]. A decrease in

species diversity of corals, the degree of coral cover, and growth rates with an increase in

sediment amounts has been demonstrated by numerous studies [26-30]. On the other hand, this

can be accompanied by an increase in the degree of substrate cover by macrophytes [31-33].

Untreated sewage waters and wastes from mariculture farms as dissolved and undissolved

particles bring subsidiary nutrients as well as noxious substances into the water column and

bottom sediments [34].

The different changes in coral communities on the reefs of the Mieu and Mun Islands

provide an obvious example of differentiated anthropogenic impact. The reefs of Mieu Island are

located in close proximity to the city and port of Nha Trang. Mun Island lies farther from the

city; its reefs are in the protected reserve zone, and its inhabitants are the small staff of the

cordon and reserve administration. Water transparency and water exchange over coral

settlements of Mieu Island are 1.48 times lower, and sediment flux per day is 1.3 times higher,

than in the case of Mun Island (Fig. 2).

Ever-increasing anthropogenic pressure is leading to the eutrophication of the waters

washing Mieu Island and to increased silt content in the sediment. As a result, the degree of

substrate cover by corals decreases, and that by macrophytes increases (Fig. 3). There is a

reduction in the biodiversity of reef-building corals and attendant common species of

macrobenthos (Fig. 4). Because of the high content of particulate suspended material in this

locality (1.3 times higher than in other areas), the Acropora species that earlier dominated the

reefs of Mieu Island have been replaced by thin branched Montipora, which appear to be more

tolerant of siltation. The changes on the reefs of Mun Island are similar to those occurring in

many other reefs of the world, primarily the bleaching and death of some coral species.

On the other hand, the succession processes typical of a developing reef are in progress

here. No marked changes were found in the composition and structure of the Mun reef

community; there are new populations of fungiids with young corals that have formed as a result

of larval settlement. Physical and biological effects on the composition and structure of coral

communities have been discussed extensively, and I will not cite these publications herein.

Numerous studies have shown that the state of coral reefs is markedly deteriorating on a global

scale. Evidently, it is necessary to decide what we are trying to conserve: either coral diversity

on a particular reef, or its fish resources, or the ecosystem as a whole.

There is a classic ecological process of coral reef decline in Nha Trang Bay caused

exclusively by human impacts: increased rates of sedimentation and eutrophication since the

beginning of 2000s resulted in dramatic decrease of live coral cover and increase of macroalgae

abundance on reefs close to rivers’ estuaries, Nha Trang port, dumping sites for dredged

materials and marine cage culture zones. An abundance of harmful corallivorous crown-of-thorn

starfish increased to the level of active outbreak and has become the major natural threat for

coral reefs in Nha Trang Bay.

Changes can occur on the level of the individual, population, ecosystem, and landscape.

Effects on these levels occur for short or long periods of time, and short-term impacts can

camouflage long-term effects. Thus, continuous monitoring of instantaneous, short- and long-

term natural and anthropogenic effects is needed to assess the stability of coral reef communities

and to reveal the tendencies of and reasons for changes occurring on the reefs.

ACKNOWLEDGMENTS

The author thanks Prof. Nguyen Tak An, Director of the Institute of Oceanography (Nha Trang),

as well as the staff researchers of this institute, for assistance in carrying out this work. This

study was supported by a grant from the Far East Division of the Russian Academy of Sciences

(section III, B) and the Institute of Oceanography.

REFERENCES

[1]. Kühlmann, D.H.H., The Sensitivity of Coral Reefs to Environmental Pollution, Ambio, 1988,

vol. 17, pp. 13–21.

[2]. Latypov, Yu. Ya., Composition and Distribution of Scleractinians on Reefs of the Phukhanh

Province (South Vietnam), Biol. Morya, 1982, no. 6, pp. 5–12.

[3]. Vo, S.T., De Vantier, L., Long, N.V., et al., Coral Reefs of the Hon Mun Marine Protected

Area, Nha Trang Bay, Vietnam, 2002: Species Composition, Community Structure, Status and

Management Recommendation, Proc. Sci. Conf. “Bien Dong-2002,” Nhatrang, Vietnam, 2002,

pp. 650–690.

[4]. An, N.T., Son, V.D., Thu, P.M., and Huan, N.H., Tracing Sediment Transport and Bed

Regime In Nha Trang Bay, Coll. Mar. Res. Works, 2000, vol. 10, pp. 63–69.

[5]. Hoa, N.X., Dai, N.H., and Vy, N.X., The Absorption, Accumulation of Nitrate, Phosphate

and the Improving Ability for Eutrophic Environment of Seaweed Ulva (Chlorophyta-Ulvales),

Tuyen Tap Nghien Cuu Bien, 2001, vol. 11, pp. 105–114.

[6]. Latypov, Yu. Ya., Reef-building Corals of Vietnam as Part of the Reef Ecosystem of the

Indo-Pacific, Biol. Morya, 2005, vol. 31, no. 2, pp. 75– 81.

[7]. Ekman, S., Zoogeography of the Sea, London: Sidgwick and Jackson, 1953.

[8]. Dai, C.F., Dynamics of Coral Communities, Biodiversity and the Dynamics of Ecosystems,

DIWPA Ser., 1996, vol. 1, pp. 247–265.

[9]. Veron, J.E.N., Corals in Space and Time: The Biogeography and Evolution of the

Scleractinia, Australian Inst. Mar. Sci., 1995.

[10]. Loya, Y and Slobodkin, L.B., The Coral Reefs of Eilat (Gulf of Eilat, Red Sea), Symp.

Zool. Soc. London, 1971, vol. 28, pp. 117–139.

[11]. Wainwright, S.A., Reef Communities Visited by the Israel South Red Sea Expedition,

1962, Bull. Sea Fish. Stn. Israel, 1965, vol. 38, pp. 40–53.

[12]. Latypov, Yu.Ya., Composition and distribution of scleractinians on reefs of Phu Khanh

Province (South Vietnam), Biol. Morya (Vladivostok, Russ. Fed.), 1982, vol. 8, no. 6, pp. 5–12.

[13]. Latypov, Yu.Ya., Korally skleraktinii V’etnama. Tamnasteridy, astrotseniidy,

potsiloporidy, dendrofilliidy (Sclleractinian Corals of Vietnam. Thamnasteridae, Astrocentridae,

Pocilloporidae, Denrophyliidae), Moscow: Nauka, 1990.

[14]. Latypov, Yu.Ya., Changes in the composition and structure of coral communities of Mju

and Moon Islands, Nha Trang Bay, South China Sea, Russ. J. Mar. Biol., 2006, vol. 32, no. 5,

pp. 269–275.

[15]. Pavlov, D.S., Smurov, A.V., Il’yash, L.V., et al., Present Day State of Coral Reefs of Nha

Trang Bay (Southern Vietnam) and Possible Reasons for the Disturbance of Habitats of

Scleractinian Corals, Russ. J. Mar. Biol., 2004, vol. 30, no. 1, pp. 43–50.

[16]. An, N.T., Son, V.D., Thu, P.M., and Huan, N.H., Tracing sediment transport and bed

regime in NhaTrang Bay, Coll. Mar. Res. Works, 2000, vol. 10, pp. 63–69.

[17]. Huan, N.B. and Hoang Son, T.P., The characteristics of distribution and change of the

seawater temperature and salinity of Cai River and northern part of Nhatrang Bay in dry and

rainy seasons, Coll. Mar. Res. Works, 2000, vol. 10, pp. 63–69.

[18]. Tomascik, T. and Sander, F., Effects of eutrophication on reef-building corals, III:

Reproduction of the reefbuilding coral Porites porites, Mar. Biol., 1987, vol. 94, no. 1, pp. 77–

94.

[19} Latypov Yu.Ya. The Spatial–Temporal Variability and Stability of Vietnamese Reef

Communities. Russian Journal of Marine Biology, 2015, Vol. 41,no. 2, pp. 103–110.

[20]. Hubbard, J.A.B., Scleractinian Coral Behavior in Calibrated Current Experiment: An Index

to Their Distribution Patterns, Proc. Sec. Int. Coral Reef Symp., Brisbane, 1974, vol. 2, pp. 38–

43.

[21]. Pichon, M., Dynamics of Benthic Communities in the Coral Reefs of Tulear (Madagascar);

Succession and Transformation of the Biotopes through Reef Tract Evolution, Proc. Sec. Int.

Coral Reef Symp., Brisbane, 1974, vol. 2, pp. 55–68.

[22]. Sheppard, C.R.C., Coral Population on the Reef Slopes and Their Major Controls, Mar.

Ecol. Prog. Ser., 1982, vol. 7, no. 1, pp. 83–115.

[23]. Huan, N.B. and Hoang Son T.P., The Characteristics of Distribution and Change of the

Seawater Temperature and Salinity of Cai River and Northern Part of Nhatrang Bay in Dry and

Rainy Seasons, Coll. Mar. Res. Works, 2000, vol. 10, pp. 63–69.

[24. Rogers, C.S., Responses of Coral Reefs and Reef Organisms to Sedimentation, Mar. Ecol.

Prog. Ser., 1990, vol. 62, no. 1–2, pp. 185–202.

[25]. Soong, K. and Chen T., Coral Transplantation: Regeneration and Growth of Acropora

Fragments in a Nursery, Restr. Ecol., 2003, vol. 11, no. 1, pp. 1–10.

[26]. Andres, N.G. and Witman, J.D., Trends in Community Structure on a Jamaican Reef, Mar.

Ecol. Prog. Ser., 1995, vol. 118, pp. 305–310.

[27]. Cortes, J. and Risk, M.-J., A Reef under Siltation Stress: Cahuita, Costa Rica, Bull. Mar.

Sci., 1985, vol. 36, pp. 339–356.

[28]. Tomascik, T. and Sander, F., Effects of Eutrophication on Reef Corals. II. Structure of

Scleractinian Coral Communities on Fringing Reefs, Barbados, West Indies, Mar.Biol., 1987,

vol. 94, no. 1, pp. 53–75.

[29]. Tomascik, T. and Sander, F., Effects of Eutrophication on Reef Corals. III. Reproduction of

Reef-Building Coral Porites porites, Mar. Biol., 1987, vol. 94, no. 1, pp. 77–94.

[30]. Sebens, K.P., Biodiversity of Coral Reefs: What Are We Losing and Why? Amer. Zool.,

1994, vol. 34, pp. 115–133.

[31]. Pastorok, R.A. and Bilyard, G.R., Effects of Sewage Pollution on Coral- Reef

Communities, Mar. Ecol. Prog. Ser., 1985, vol. 21, no. 1–2, pp. 175–189.

[32]. Sakai, K. and Nishihira, M., Immediate Effect of Terrestrial Runoff on a Coral Community

near a River Mouth in Okinawa, Galaxea, 1991, vol. 10, pp. 125–134.

[33]. Wilkinson, C.R. and Buddemeier, R.W., Global Climate Change and Coral Reefs:

Implications for People and Reefs, Rept. of the UNEP-IOC- ASPEI-IUCN Global Task Team on

the Implications of Climate Change on Coral Reefs, Gland, Switzerland: IUCN, 1994.

Explanation of figures

Figure 1. Map of the Nha Trang Bay

Figure 2 Figure 3

Figure 2. Sediment flux and water transparency on the various reefs in 2003 and 2005. Islands

are ranged in order of increasing distance from the shore.

Figure 3. The degree of substrate cover by corals and macrophytes on the reef of Mieu Island in

1981 and 2005. Reef zones: a—lagoon, b—reef flat, c—reef slope, d—reef slope base, e—fore

reef platform.

Figure 4. Variation of species diversity of corals on various reefs in 1981 and 2005.

Differences between reefs of Thi and Mun islands, which are nature reserves, and reefs of Co-Co

and Mieu islands affected by intensive anthropogenic pressure are clearly seen.