Changes in the Composition and Structure of Coral Communitieson Some Reefs Nha Trang Bay

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

Yu.Ya. Latypov

A.V. Zhirmunsky Institute of Marine Biology, Far EasternBranch, Russian Academy of Sciences, ul.Pal’chevskogo 17,Vladivostok,690041 Russia

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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 theshores 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 Residentialand communications construction, establishmentof new hotels and underwater swimming centers,intensified mariculture operations, and increased pressureof tourism have led to enhanced terrigenous runoffin Nha Trang Bay[1-3]. The infrastructure of themunicipal sewage and waste disposal plants and protectivemeasures do not satisfy modern requirements dictatedby economic growth.Deposition of terrigenous material is the major reasonfor pollution and damage to coral reefs. Inputs ofterrigenous material are increasing due to the destructionof vegetation cover (agriculture and forest cutting),mining and building operations, wastewater discharge,and application of fertilizers. This leads to a reductionin the light arriving at corals, their being buried undersediment, 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 developmentof benthic filter-feeding organisms. Under these circumstances,corals can become inferior to macroalgaeand invertebrates in competition for nutrients [4, 5].

The deterioration of coral reefs is producing concernon the part of Vietnamese scientists and their government.Efforts have been concentrated on the study ofthe reasons for reef ecosystem variations and on theelaboration of a management strategy for conservationand restoration of coral communities. The present publicationis based on data from many years of observationson the status of selected coral reefs in Nha TrangBay. We aim to attract the attention and fundingurgently required for immediate conservation measureson the reefs of Vietnam, which are an organic part of theIndo–West Pacific tropical center of coral diversity andorigin [6- 9].

MATERIALS AND METHODS

We investigated the reefs of Mieu and Mun islandsin 1981 (Fig. 1) using the widely accepted technique ofquadrats, transects, and scuba[10]. From September toOctober 2003 and in January 2005, 200-m transectswith each meter distance marked were set on each reef.Along the transects, the degree of substrate cover bycorals and macrophytes was determined; the numbersof massive, branched, and encrusting forms of colonieswere calculated; and qualitative and quantitative samplingof common species of corallobionts was carriedout. Structural and unstructured reefs were identifiedaccording to presence or absence of geomorphologicalzonation and reefogenous deposits [8, 11]. There wasa structural reef with a distinct zonation and reefogenousdeposits in the southeastern part of Mieu Island.Unstructured reefs around Mun Island formed aweakly developed crust on the substrate, hardly changingits profile. The degree of substrate cover by coralsand macrophytes and the species richness of scleractinianand common species of macrobenthos wereassessed during visual observations.

RESULTS

According to the research in 1981, almost all of thezones that are common to most typical structuralreefs, viz., a small lagoon (coastal channel), internaland external reef flat, a reef slope and a fore reef platform were recorded in the reef of Mieu Island inthe Nha Trang Bay[12, 13] . Fifty to sixty scleractinianspecies were distributed in the reef, which formed vastareal populations and biostromes 4–5 m in width and1 m in height. The level of substrate coverage by coralswas 45–60%; it reached 75–100% in patches of thepopulations of Acropora and Porites species. Acroporaformosa, A. cytherea, Montipora digitata, M. foliosa,and Porites nigrescensdominated in substrate coverage, and Acropora (11–17 species), Porites (4–7 species), and Fungia (3–5 species) dominated in speciesdiversity.

The studies that were performed in 2010 and 2013revealed that the reef of the Mieu Island in thezone 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 theisland [14]. A decrease in the number and size of scleractinian colonies and an increase in the number ofalgae of the Halimeda and Chnoospora genera wasrecorded. The substrate coverage by corals rarelyexceeded 30–45%. From the middle of the reef slope,patches of populations of the brown alga Chnoosporaimplexa 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 least5 ind./m2), holothurians Holothuria edulis and H. atra(1–2 ind./m2), sea stars Linckia laevigata, Culcitanovaeguineae, and Acanthaster planci (0.1–0.2 ind./m2), bivalves Atrina vexillum (up to0.2 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. niloticusand A. vexillum, were recorded, as well as extremelyrare diadems and holothurians, while the sea starA. planci occurred often.

The erosion occurring along the coastline of thecity, the port of Nha Trang, and nearby islands due toconstruction and excavation works; as well, intensification of mariculture greatly increased sedimentationflows into the Nha Trang Bay, causing eutrophicationof the water column that surrounds Mieu Islandand 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 dischargesfrom rafts immediately enter the surrounding water.According to the workers, each cell on average contained from 200 to 500 kg of fish, shrimp, or otherbreeding objects. Even under at the minimum loadingof a cage, every month each raft was treated with 7000multivitamin tablets and antibiotics, i.e., throughoutthe year each raft discharged 84000 tablets into thewater. 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 indexoccurred, as well as a decrease in the concomitant common species of macrobenthos and anincrease in the substrate coverage by macrophytes.The replacement of Acroporathat dominated previously on the reefs of the Mieu Island with monospecies populations of finely branched Montiporaporites that are more resistant to silting islikely to be regarded as a result of the high content ofsuspended 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 coveredthe substrate with a thin crust. However,the species richness and the degree of coral cover werefairly high. In the near-shore part on large rock blocks,there were widespread separate colonies of variousgrowth forms with a predominance of A. cytherea,A. palifera, A. humilis, Stylophorapistillata, Favia speciosaand the hydroids Millepora dichotomaandM. platyphylla. At 3–4 m offshore (2- to 3-m depth)were coral stands with an obvious predominance oflarge plate forms of the corals A. cytherea and A. hyacintusand the thin-branched hydroid M. dichotoma.The degree of substrate cover reached 75% in someareas. Between Acropora and Millepora settlements,there were numerous and various colonies of otherscleractinians 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 decreasedwith 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 theblocks, there were many small colonies typical of thelower part of the reef slope: Symphyllia, Lobophyllia,Euphyllia, Micedium, Pectinia, and individual coloniesof alcyonarian and gorgonarian. Where a hard substratewas present, coral settlements spread for 100–110 m offshore. Between them, there were many variousechinoderms, with predominance of the sea urchinD. setosum in aggregations up to 15 ind./m2.Investigations of reefs in the southern part of MunIsland in October 2003 and January 2005 revealed nosignificant changes in the composition and structure ofcoral communities. As before, lamellar forms ofAcropora and branched Millepora constitute the bulkof coral settlements. The degree of substrate cover bylive corals decreased as a result of coral bleaching andthe death of different parts and even whole colonies ofAcropora, Pocillopora, Seriatopora, and Millepora, aswell as some of Fungia and Sandalolitha. Nevertheless,the diversity of corals, especially at a 5-m depth (notreached 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 40scleractinian genera. Ten species of Acropora occurredon this part of the reef, indicating satisfactory conditionsin this habitat. Here, settlements of fungiids, up to45 ind./m2 (Fungia fungites, F. concina, Sandalolitharobusta, Polyphylliatalpina, Herpolithalimax, andothers), occurred as bands (1.5–2.5 m across and up to4–5 m long) oriented perpendicularly to the shore, withcontinuous covering of the substrate. Some specimensof fungiids reach a size of 0.5 m. In addition, there weremany young fungiids (1–5 cm across) that reattached tothe substrate. This part of the reef is probably at a latestage of succession, with monospecific coral settlementsspreading over large areas[8,17-20].

DISCUSSION

Erosion processes along the coastline of the city andport of Nha Trang and the intensification of maricultureoperations in many bays of the nearby islands are augmentingsediment fluxes and eutrophication of waters inNha Trang Bay[21-23]. An increase in the number ofmacroparticles of various origin causes an increase in theturbidity of the sediment, reduces the photosyntheticcapacity of reef-building corals and other benthic organisms,and influences some physical and biological processesin these organisms[24-26]. A decrease in speciesdiversity of corals, the degree of coral cover, and growthrates with an increase in sediment amounts has beendemonstrated by numerous studies [26-30]. Onthe other hand, this can be accompanied by an increasein the degree of substrate cover by macrophytes [31-33]. Untreated sewage waters and wastes from mariculturefarms as dissolved and undissolved particles bringsubsidiary nutrients as well as noxious substances intothe water column and bottom sediments [34].

The different changes in coral communities on thereefs of the Mieu and Mun Islands provide an obviousexample of differentiated anthropogenic impact. Thereefs of Mieu Island are located in close proximity to thecity and port of Nha Trang. Mun Island lies fartherfrom the city; its reefs are in the protected reserve zone,and its inhabitants are the small staff of the cordon andreserve administration. Water transparency and waterexchange over coral settlements of Mieu Island are1.48 times lower, and sediment flux per day is 1.3 timeshigher, than in the case of Mun Island (Fig. 2).

Ever-increasing anthropogenic pressure is leadingto the eutrophication of the waters washing Mieu Islandand to increased silt content in the sediment. As a result,the degree of substrate cover by corals decreases, andthat by macrophytes increases (Fig. 3). There is areduction in the biodiversity of reef-building corals andattendant common species of macrobenthos (Fig. 4).Because of the high content of particulate suspendedmaterial in this locality (1.3 times higher than in otherareas), the Acropora species that earlier dominated thereefs of Mieu Island have been replaced by thinbranchedMontipora, which appear to be more tolerantof siltation.The changes on the reefs of Mun Island are similarto 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 ofa developing reef are in progress here. No markedchanges were found in the composition and structure ofthe Mun reef community; there are new populations offungiids with young corals that have formed as a result of larval settlement.Physical and biological effects on the compositionand structure of coral communities have been discussed extensively, and I will not cite these publicationsherein. Numerous studies have shown that the state ofcoral reefs is markedly deteriorating on a global scale.Evidently, it is necessary to decide what we are tryingto 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 levelsoccur for short or long periods of time, and short-termimpacts can camouflage long-term effects. Thus,continuous monitoring of instantaneous, short- andlong-term natural and anthropogenic effects is neededto assess the stability of coral reef communities and toreveal the tendencies of and reasons for changes occurringon the reefs.

ACKNOWLEDGMENTS

The author thanks Prof. Nguyen TakAn, Director ofthe Institute of Oceanography (Nha Trang), as well asthe staff researchers of this institute, for assistance incarrying out this work. This study was supported by agrant from the Far East Division of the Russian Academyof Sciences (section III, B) and the Institute ofOceanography.

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