BULLETIN OF MARINE SCIENCE. 59(1): 175-187. 1996 PAPER

SPATIAL PATTERNS IN BIODIVERSITY OF A FRINGING REEF COMMUNITY ALONG OPUNOHU BAY, MOOREA, FRENCH POLYNESIA

Mehdi Adjeroud and Bernard Salvat

ABSTRACT Opunohu Bay, located on the North coast of Moorea Island (Society Archipelago), is approximately 3 km long and partially edged by a small fringing reef no wider than 100 m. The main objective of this first quantitative survey concerning bays in French Polynesia, was to examine the spatial pattern of diversity of the macro-benthic fringing reef community between the land-end and the ocean-end of the bay. This study considered the conspicuous benthic organisms (, Echinodermata, Mollusca, Porifera and seaweeds) more than 1 cm long and attached to the substratum. A strong gradient in percent cover and species richness was found for and echinoderms, between the land-end where the two taxa are almost absent and the ocean-end where they are dominant. Species richness of macroalgae, molluscs and sponges. and macroalgal coverage are high in the middle part of the bay. At the ocean-end, dominance (in terms of coverage) of the coral genus Porites is responsible for the decrease in diversity (H'c) and evenness (J'c). Factorial correspondance analysis and ascending hierarchical clustering of species composition correspond to the land-end, the mid- dle part and the ocean-end. The absence of corals and echinoderms at the land-end. where only a few tolerant macro-benthic species occur, can be explained by low salinity and high which occur after heavy rains during the wet season.

Compared to other tropical locations, bays are rare in French Polynesia, and few studies have been carried out in this particular environment despite the:ir relatively small size. Enclosed and sheltered bays can be subject to many natural or man-induced stresses associated with changes in run-off (Banner, 1974; Holthus et aI., 1989). In tropical bays, hydrological characteristics, which are mainly con- trolled by geomorphology and weather conditions (such as the intensity and fre- quency of rainfalls), are relatively similar with rivers or sewage outfalls emptying terrestrial sediments and nutrient-rich freshwater (Yamamoto and Yuine, 1985; Holthus et aI., 1989). These characteristic environmental conditions, which are determined by the volume of bay to volume of run-off, have a large influence on living flora and fauna. According to the Miyadi concept of "embayment degree" (Horikoshi, 1988), a sequential distribution of several biotopes of benthic Com- munities can occur in the bay. French Polynesia, which is far from the Austro- Malayan maximum diversity area, has relatively low coral diversity (51 genera and 168 coral species; Richard, 1985), and communities are poorly developed within bays such as Opunohu Bay (Chevalier and Kuhlmann, 1983). A similar lack of corals has also been observed in Kaneohe Bay, Hawaii, by Maragos et al. (1985). But in the western Pacific (Thailand, Japan, or ), bays generally have more diverse coral communities (Morton, 1974; Horikoshi, 1981, 1988). The aim of this first quantitative study of bays in French Polynesia, was to examine the variation of the coral community along a bay, including the more conspicuous macro-benthic taxa (seaweeds, Porifera, Mollusca and Echinoder- mata). Benthic surveys included community structure variables (species compo- sition, species richness, percent cover, diversity, evenness) and inter-station com- parison (factorial correspondance analysis, ascending hierarchical clustering). A 6-months survey of environmental conditions (temperature, salinity and light at-

175 176 BULLETIN OF MARINE SCIENCE, VOL. 59, NO.1, 1996 tenuation) was conducted. The results are compared with the biogeography of other locations.

STUDY AREA

Moorea (1 r30'S, 149°50'W), one of the Society Islands, comprises 134 kIn2 of land, 49 km2 of reefs and and is 61 km in circumference (Fig. I). The island is surrounded by a narrow coral belt no further than 1.5 kin from the coastline to the end of the barrier reef outer slope. Opunohu Bay (Fig. 1) is on the north coast of Moorea, 6 kin from the north-west extremity of the island, With a length of 3,500 m from the barrier reef to the river mouth of the bay, and near width of 600 m, the bay communicates with the ocean via the Tareu pass, 250 m wide and 60 m deep. Within the bay, depths vary from 50 m near the pass to 15 m at 300 m from the river mouth. On each side of the bay, a small fringing reef (maximum 100 m long) begins 700 m from the river mouth, extends within the bay, and joins the lagoon's fringing reef. The bay is completely surrounded by steeply sloping volcanic mountains reaching 900 m high, and by a valley (9.6 kIn2) where a river flows into the land- end of the bay. Some plantations (0.3 km2) are located in the innermost part of the valley, and only a few private constructions are present along the coast of the bay. A few hundred inhabitants are in the watershed of the valley river and along the coastal zone. The are semi-diurnal with an amplitude rarely exceeding 40 em. Annual average rainfall is 325 em. Tropical cyclones are rare in French Polynesia; major ones occured in 1907, 1982, 1983, and 1991. For further details on general features of French Polynesian Islands, see Gabrie and Sa]vat (1985), and for details on the Opunohu Valley, see Resh et al. (1990).

METHODS

Environmental Setting.-15 hydrological surveys of the surface water were conducted at 21 hydro- logical stations (Fig. 1) over a 6-month period from February (wet season) to July ]992 (dry season). We measured salinity and temperature with a portable WTW model LS 196 Salinometer. Light atten- uation, or transmittance, was estimated by Secchi disk readings. Benthic Survey.-On both sides of Opunohu Bay, 10 stations (stations I to 10; Fig. I) were established at which major macro-benthic taxa including Cnidaria (hard and soft corals, Millepora), Echinoder- mata, Mollusca, Porifera, and seaweeds were investigated. A complete list of species was made by diving 50 m to the right and left of the station defined by a radial perpendicular to the shore line and overlying the fringing reef lengthwise. A stratified sampling method was adopted to quantify the macro-benthic communities. Three linear transects of 20 m were laid parallel to the shore line on each station: one on the reef flat, one on the reef crest and one on the reef wall. For stations 1 and 2 where the stratification is absent, three transects were laid at regular intervals in the hard substrate which corresponds to a fossil reef. Each cora] and macroalgal species underlying the line transect was re- corded and its intercept length measured to the nearest cm. The raw data per transect (i.e., per stratum which are the reef flat, the reef crest and the reef wall) were transcribed and converted to percent cover per species. The coverage obtained for each of the three strata, at each station, was weighted by the relative surface of these three strata and these coverage values were used to calculate the species diversity index (Shannon and Weaver index H'c) and evenness (Pie lou index J'c), both relative of the station. For inter-station comparisons, and also to find a continuum of stations along the bay, we made a factorial correspondance analysis. This technique was complemented by a Q mode as- cending hierarchical clustering based on Jaccard's similarity index. Both were made with the species composition (presence-absence matrix). A linear regression allowed us to study correlations between station location (distance from the river mouth) and community structure variables (species richness, percent cover, H'c and J'c).

RESULTS Environmental Setting.-Surface water temperature values fluctuated between 24°3 C and 31°3 C between February and July 1992 (Table 1). The averages for each station were similar, but the minimum and maximum range reached 6° C at the land-end of the bay (stations a to k and river mouth). The lowest temperatures were measured on 9 March, the end of the hot wet season, after a few days of heavy rain. In the lagoon, the lowest temperatures were measured on 12 July during the cold dry season, which occurs from May to September. Salinity values ranged between 2.0 and 36.6%0. During the rainy season the lowest salinities ADJEROUD AND SALVAT: CORAL COMMUNITY BIODIVERSITY IN POLYNESIA ]77 ...... iI. •••• ~O~ •••• •••••• ...... j' .

.' lagoon .•..•. ..' <) ..' : . :.... 149°50W .... Tareu ...... : ...... t pass ...... •....•...

Figure ]. Opunohu Bay, Moorea Island, location of the ]0 benthic survey stations (] to 10), and the 21 hydrological stations (a to t and river). Species richness of corals (C), macroalgae (A), echinoderms (E), molluscs (M) and sponges (S), diversity (H'c) and evenness (J'c) are given. CRIOBE: Centre de Recherches Insulaires et Observatoire de I'Environment. 178 BULLETIN OF MARINE SCIENCE. VOL. 59. NO.1. 1996

Table 1. Hydrological characters of surface water measured at 21 stations along the bay. Difference between maximal and minimal (range) values are given. Salinity in %0, temperature in °C, light atten- uation in meters of Secchi disk readings. Standard deviation in brackets. *: no data.

Stations Salinity Range Temperature Range Light attenuation Range River 21.1 (12.67) 34.9 27.5 (1.64) 5.3 * * a 24.6 (11.14) 34.4 28.6 (1.64) 6.6 0.5 (0.08) 0.2 b 26.7 (11.51) 34.5 28.6 (1.54) 6.7 1.2 (0.36) 0.9 c 30.3 (8.84) 28.5 28.7 (1.62) 6.8 2.4 (1.12) 3.7 d 29.5 (10.48) 32.5 28.5 (1.53) 6.6 2.6 (1.06) 3.5 e 31.5 (7.97) 26.1 28.6 (1.59) 6.5 2.9 (1.28) 4.4 f 32.1 (8.66) 31.4 28.6 (1.45) 6.3 3.1 (1.28) 3.9 g 33.0 (8.35) 31.4 28.7 (1.52) 6.4 3.4 (1.44) 4.4 h 32.2 (8.31) 31.2 28.6 (1.49) 6.7 3.4 (1.35) 4.7 33.3 (6.86) 26.5 28.6 (1.41) 6.1 3.6 (1.38) 4.6 j 33.9 (5.79) 22.6 28.5 (1.33) 5.6 3.9 (1.54) 4.8 k 34.1 (5.75) 22.5 28.5 (1.27) 5.6 4.3 (1.30) 4 I 34.5 (5.42) 21.4 28.5 (1.18) 4.6 4.7 (1.35) 3.7 m 34.9 (4.17) 16.5 28.5 (1.11) 4.2 5.0 (1.39) 4.2 n 35.1 (3.84) 15.1 28.5 (1.08) 4.4 6.1 (1.65) 3.7 0 35.2 (3.28) 13 28.5 (1.05) 4 5.2 (1.56) 5.5 P 36.2 (0.37) 1.4 28.5 (0.61) 1.7 6.9 (0.94) 3.5 q 36.0 (0.51) 1.5 28.5 (0.69) 2 5.3 (1.38) 5.5 r 36.1 (0.47) 1.4 28.5 (0.72) 2 5.7 (1.08) 4 s 36.0 (0.62) 2.2 28.6 (0.79) 2 6.4(1.81) 7.4 t 36.1 (0.44) 1.3 28.5 (0.7) 2 6.0 (1.45) 5.7

occured on 9 March after 3 rainy days (110, 40 and 85 mm·d-1 on 6, 7 and 8 March, respectively). Generally, only the land-end (stations a, band c and river mouth) was affected by freshwater discharge, but sometimes freshwater influence (salinity of about 5%0) reached the middle part of the bay and disappeared after 1 or 2 days. Light attenuation was correlated with rainfall in the wet season and generally after heavy rainfall, when river flow is able to reached 479 l·s-1, light attenuation was high (i.e. Secchi disk readings not exceeding 2 m) at the land- end. Light attenuation remained high at the land-end even when rainfall was absent or during the dry season. Benthic Survey.-Species composition and richness: the total number of species encountered in the 10 stations consisted of 98 macro-benthic species (Table 2). Corals were dominant (47 species), followed by macroalgae (18 species), molluscs (15 species), echinoderms (9 species) and sponges (9 species). The general trend was an increase in species richness going from the land-end (station 1; Fig. 1) to the ocean-end (station 10). The land-end (stations 1 and 2) has virtually no corals, with only two species represented by three colonies of about 4 cm diameter. Coral species richness was maximal (33 species) at the ocean-end (stations 7 and 8). Five species of macroalgae were counted at the land-end, increasing to 10 in the middle part, and 14 at the ocean-end. There were no major differences in the number of molluscs between the land-end and the ocean-end, with the highest number of species (8) at stations 4 and 6. No echinoderms were found at the land-end and richness was maximal at the ocean-end with six to nine species. The number of species of sponges varied between one and six, with a maximum (six) recorded at stations 8 and lOon the western side. Percent cover: no corals occured in any of the three transects at stations 1 and 2 (Fig. 2). The reef flat situated in the middle part of the bay was almost devoid of corals (coverage less than 1%), and reached relatively high values only at the ADJEROUD AND SAL VAT: CORAL COMMUNITY BIODIVERSITY IN POLYNESIA 179 ocean-end (stations 7 to 10 with values between 4.7 and 12.3%). On the reef crest, coral coverage was low for stations 3 and 4, and between 5.8% and 9.8% for stations 5 to 10. Coverage of the reef wall increased along the bay, with 4.6 and 5.4% in stations 3 and 4 respectively, to high values on the western side at the ocean-end (36.5% and 75.6% for station 7 and 9 respectively). Macroalgal coverage was very low at the land-end and in the middle part of the bay, except at station 3, with values of 24.6% and 8.7% on the reef crest and on the reef waH respectively. At the ocean-end, the reef wall had low macroalgal coverage, be- tween 0 and 1.8%, while coverage was variable on the reef flat and the reef crest (between 1.9 and 36.5%). H'c and J'c: the species diversity index (H'c) increased between the land-end (0 at station 1) and the middle part of the bay (3.3 at station 4; Fig. 1). Between station 4 and 6, H'c ranged from 2.9 to 3.3 with a slight decrease observed for stations 7 to 10. The same changes occured for the evenness index J'c. Stations 4 and 6 having the highest values for H'c and J'c. Factorial correspondance analysis: the first two axes divided the 10 stations into four wel1 distinguishable groups (Fig. 3). The first group was composed of stations I and 2, the second of stations 3, 4, 5 and 6, and a group comprised stations 7, 8, 9 and 10. This latter group is separated into two groups by the second axis: the third group comprised stations 8 and 10, and the fourth group contained stations 7 and 9. Ascending hierarchical clustering: stations 8 and 10 are the most similar and were distinct from the others, while stations 1 and 2 were the least similar in the hierarchical cluster (Fig. 4). Station 3 is readily distinguishable, while stations 4 to 10 form a group in which stations 8 and 10 are distinguished by their high similarity index. Correlation: species richness and percent cover for corals and echinoderms were highly correlated (r2 > 0.8) with location from the river mouth (Table 3). Speci(:s richness for molluscs and sponges, H'c and J'c were not correlated with location. For macroalgae, species richness was significantly correlated with location thus, Ho is rejected (P < 0.05), but the correlation coefficient was not high (0.62).

DISCUSSION The main hydrological characteristics of Opunohu Bay can be summarised as follows: in the wet season, the land-end responds very quickly to heavy rainfalL Temperature, salinity and light attenuation increase within a few hours after heavy rainfalL The decrease of salinity seems to be primarly due to the run-off via river discharge, although submarine groundwater discharge may also play a role, as suggested by Johannes and Hearn (1985), and Lewis (1987). In dry conditions, the bay is under oceanic influence, although light attenuation is still high. Run- off caused by heavy rain have been observed by Nishihira (1987) in Okinawa, Japan. Wolanski and Delesalle (1995) showed that in Opunohu Bay, an offshore wind blowing along the axis of the bay produce an outflow at the surface and an inflow at the bottom resulting in a small-scale upwelling of ocean water at the entrance. Thus, freshwater discharge by the river spreads over the surface of the bay and into deeper waters depending on the amount of rainfall. The results of the benthic survey showed a pronounced gradient in coral and echinoderm species richness, and percent cover. The increase of species richness observed at the ocean-end is due to coral species of the genera , Mon- tipora, Pocillopora, Montastrea and the echinoderms Bohadschia argus, Holo- thuria atra and Thelenota ananas. The total number of coral species encountered 180 BULLETIN OF MARINE SCIENCE. VOL. 59. NO. I. 1996

Table 2. Species composition of the 10 stations along Opunohu Bay. Coral and algal cover (if ob- tained by the transects) is given.

Station Species 2 4 6 9 10 Cnidaria Psammocora contigua + 0.36 0.22 0.06 0.23 + 0.7 0.07 Psammocora profundace/la + 0.13 0.12 0.12 + 0.16 0.16 0.05 0.04 Stylocoenie/la armata 0 0.01 0.02 0.04 Poci/lopora damicormis 0.12 + 0.33 0.12 2 + Poci/lopora eydouxi + + + Pocillopora verrucosa + 0.4 + + Acropora nasuta + + Acropora nobilis + Acropora robusta + + + Acropora valida + + + + Astreopora myriophthalma + + + + Montipora foliosa + + + + + Montipora hispida + Montipora tuberculosa + + + + + + + Montipora aequituberculata + + + + + 0.02 + + Montipora verrucosa + + + + + Montipora spumosa + cactus 0.4 + + + om + l.l5 0.04 Pavona maldivensis + + Pavona varians 0.11 0.23 0.03 + 0.16 0.02 0.01 0.03 Gardineroseris planulata + Leptoseris mycetoseroides + + Fungia concinna + + + + + + + Fungia danai + + + Fungia paumotensis 0.07 + + + + + + + Fungia repanda + + + + + Fungia scuta ria + + Herpolita limax + + + + + + Sandalolitha dentata + Porites australiensis 0.5 + Porites /obata + 0.08 0.65 Porites /utea 0.53 0.09 0.54 0.8 2.53 4.75 5.37 5.45 Porites vaughani 0.06 0.01 0.1 + + Porites rus + 0.23 1.45 0.34 4.33 l.l9 6.39 2.84 Montastrea curta 0.03 0.06 + + + Leptastrea purpurea + + Leptastrea transversa + + 0.14 0.42 0.16 0.52 0.2 0.06 0.04 0.68 Cyphastrea microphtha/ma + + + + + 0.4 Cyphastrea serailia + Acanthastrea echinata + 0.02 + + + + Millepora platyphylla 0.29 + Stoichactis kenti + + Rhodactis sp. + + + + Zoanthus sp. 1 + 0.08 + + Zoanthus sp. 2 1.5 0.11 Palythoa sp. + om + 0.02 + + Porifera Paratetilla bacca + + + + + Leucetta sp. + + Leucetta cf. microraphis + + + + + Dysidea sp. + + + Axinella sp. + + + + Haliclona sp. + Spirastrealla aff. decumbens + + ADJEROUD AND SALVAT: CORAL COMMUNITY BIODIVERSITY IN POLYNESIA 181

Table 2. Continued.

Station

Species 2 4 6 7 9 10 Chondrosia sp. 1 + + + + + + + Chondrosia sp. 2 + Mollusca Trochus niloticus + + + + + + + Cerithium echinatum + + + + + + Lambis truncata + Conus imperialis + Conus leopardus + Conus striatus + Conus vexillum + Arca ventricosa + + + + + + + + + + Pedum spondyloideum + + + Pinna muricata + Spondylus varians + + + + + + Chama imbricata + + + + + + Tridacna maxima + + + + + + + Quidnipagus palatam + Asaphis violasceus + Echinodermata Diadema savignyi + Echinothrix calamaris + + + + + + + Echinothrix diadema + + + + + + + + Echinometra mathaei + + + + + + + + Bohadschia argus + + + + + + Holothuria atra + + + + Thelenota ananas + + + + Culcita schmideliana + + + + + + Linckia multifora + Seaweeds Plectonema sp. + + Symploca hydnoides + + + + + + + + Boodlea compos ita + + + 0.37 + 0.13 + Cladophoropsis sp. + Ventricaria ventricosa + + 0.05 0.02 0.02 + + 0.06 0.01 + Halimeda minima + + + + + + + + + Halimeda opuntia 0.14 4.47 2.71 0.11 + 0.37 0.41 1.28 1.52 1.6 Halimeda sp. 0.05 Caulerpa racemosa + + + + Dictyota gr. acutiloba 6.2 + 0.57 + 0.59 17.7 Padina tenuis + + + + + + + 0.03 Turbinaria ornata 0.76 0.13 0.45 5.12 5.82 0.78 4.6 Titanophora sp. + + + + Actinotrichia fragilis + + + + Acanthophora spicifera + + + Galaxaura gr. tenera + + Galaxaura subverticillata + + 0.35 + + Porolithon craspedium + + + 0.39 0.32 0.07 1.17 + 0.59 + in Opunohu Bay (47 species) is of the same order of species richness (49) found on the fringing reef around Moorea by Chevalier and Ki.ihlmann (1983). The increasing percent cover of corals encountered out of the bay is due to the dom- inance of the genus Porites (Fig. 5). Between the land-end and the ocean-end, the reef flat is progressively colonised mainly by P. lutea, and the reef wall by large colonies (reaching 2 m diameter) of P. rus. The dominance of the genus 182 BULLETIN OF MARINE SCIENCE, VOL. 59, NO. I, 1996

...... :\. . ••o~ . •.~~~v ..' ...•...... •

..' ...... '..' : ...... Tareu ., . pass '" . . :...... : /.:

Figure 2. Coral cover (Cf, Cc and Cw) and macroalgal cover (Af, Ac and Aw) measured on the reef flat (Cf and Af), on the reef crest (Cc and Ac) and on the reef waIl (Cw and Aw) of Opunohu Bay. ADJEROUD AND SALVAT: CORAL COMMUNITY BIODIVERSITY IN POLYNESIA 183

2,5 f2 (18.14%)

2

1,5

0,5

f1 (20.39%) 0

-0,5

-1

-1,5 -2 -1 0 2 3 4 5 Figure 3. First two axes of the factorial correspondance analysis based on total species composition. Four species were omited (Spirastrella aff. decumbens, Pinna muricata, Quidnipagus palatam, AsapUs violasceus).

800 600 400 200 o ) similarity 1 (Jaccard's index) 2 3 4 5 6 7 9 8 10 Figure 4. Dendrogram of the ascending hierarchical clustering made on species composition. Simi- larity calculated with the Jaccard's index, group average sorting method. 184 BULLETIN OF MARINE SCIENCE, VOL. 59, NO, I, 1996

Table 3. Correlation between location of benthic survey stations from the river mouth (x variable) and community structure variables (y variable: species richness SR, coral cover, H'c and J'c). *: Ho (slope of line equation = 0, r2 = 0, r = 0) is rejected, P < 0.05. y Line equation " Location from the Corals SR y = 0,28x + 0,18 0.94 * river mouth Coral cover y = 0,08x - 2.03 0.92 * Echinoderms SR y = 0,06x - 0,14 0,82 * Algae SR y = 0,07x + 4,94 0,62 * H'c y = 0,02x + 0,86 0.42 J'c y = 0,003x + 0,34 0,24 sponges SR y = 0.02x + 1.34 0,24 molluscs SR y = 0,02x + 3,52 0,23

Porites, characteristic of Polynesian fringing reefs, is also responsible for the decrease of H' c and l'c observed at stations 7 to 10. At Ko Phuket, Thailand, Brown et aI. (1990) found that P. [utea, which dominates reef flat communities in the Gulf of Thailand (Oitlev, 1978; Sakai et aI., 1986) and Malaysian reef flats (Goh Ah Hong and Sasekumar, 1981) was responsible for the decrease in species diversity. In Kaneohe Bay, the slight decline of species diversity between 1973 and 1983 was due to the greater dominance of P. compressa (Holthus et aI., 1989). Porites appears to grow well in turbid and low salinity environment, and becomes dominant by outlasting other corals (Sakai et aI., 1986). Done and Potts (1992) and Potts et aI. (1985) showed that dominance of Porites colonies in Pandora Reef, , was attributed to their high larval recruitment, their propensity to fragment and the fragments or colonies surviving as independent colonies. Porites was the first coral observed at the mouth of the Rewa River, Fiji, by Squires (1962), and P. [utea is the dominant species in area where the "embayment de- gree" is strongest (Horikoshi, 1988). In our study, Leptastrea transversa and Psammocora profundacella, which are the only corals found at the land-end of the bay, seem to be the most resistant to low salinity and high light attenuation, while Porites appears only in the middle part and becomes dominant along the bay. Chevalier and KUhlmann (1983), Crossland (1928) and Salvat et aI. (1979) arrive at similar observations. There was not the same kind of zonation for macroalgae, molluscs and sponges,

VJ 100 ~ 'C 0 ~ ...... -m:;i;" ..~: " 4-< . ~. 0 ®: @ ~ '" ". ,.:-:.. ::::~ ::~. Q) <-: #..:.' ~ ~:::: (.) 50 m ~. < f" f:*~ ". I .~.. .••..• ~~:: ~l e:::: aI:: .~~:::. ~ :::::: ~~ ~ .... :. :::;~ " W S I !f ''1;' ~1 :..• 0 ~~: ~: "0 &: iI" , 1& .: :-;-:- k I Ifm ,. ~~: t: "-:. *:,. it liT ~ a 1*11 1 2 3 4 5 6 7 8 9 10 stations Figure 5, Dominance of the genus Porites in the 10 stations. Dominance is expressed as the percent of the genus relative to the total coral coverage. ADJEROUD AND SALVAT: CORAL COMMUNITY BIODIVERSITY IN POLYNESIA ]85

which have not the same physiological constraints as corals. Species richness and percent cover for these groups was highly variable with no general trend between the land-end and the ocean-end. The most ubiquitous macroalgal species were those belonging to the genera Halimeda, Ventriearia, Padina, Turbinaria and Porolithon; Area ventrieosa, Troehus nilotieus and Tridaena maxima for mol- luscs; and Chondrosia sp. for sponges. The gradient observed in this study seems to be closely connected with envi- ronmental conditions. Water temperature appears to have no effect on the gradient of macro-benthic organisms. Goodbody (1961) argued that temperature is not re- sponsible for the mortality after a heavy rain in Kingston Harbor, Jamaica. Likewise Rochford (1958), who classified Australian estuarine systems, concluded that within each estuarine system, temperature is much less important than salinity, bottom sediments and circulation in controlling the distribution of marine and freshwater species. In contrast, salinity, turbidity and sedimentation seem to have important effects (Wolff, 1983), especially on coral and echinoderm communities. As ob- served by Yamamoto and Yuine (1985) and Nishihira (1987) around Okinawa, Japan, orange-colored water covered the land-end of the bay on 9 March 1992, salinities were less than 10%0, and Sechhi disk reading was 1 m. Goodbody (1961) and Goreau (1964) also measured salinities less than 10%0 after heavy rain in Kingston Harbor, Jamaica, but the recovery of normal salinity took 2 to 3 months. Sakai and Nishihira (1991) found that corals were affected in terms of abundance, coverage, and diversity (H'c and J'c) by salinity and water transparency. Never- theless, in case of terrestrial run-off, it is sometimes difficult to determine whether the effect was due to sedimentation, freshwater or a synergetic effect (Goodbody, 1961; Goreau, 1964). The mortality of corals that Sakai and Nishihira (1991) ob- served after heavy run-off, was mainly caused by salinity stress. The effects of sedimentation and turbidity on coral communities (Loya, 1976; Acevedo and Mo- relock, 1988), coral populations (Dodge and Vaisnys, 1977), as well as coral or- ganisms (Rogers, 1983, 1990) are well documented. Destruction of corals by heavy run-off during cyclonic conditions has been reported in Tahiti by Crossland (1928) and in Kaneohe Bay by Holthus et al. (1989). Kinsman (1964) and Coffroth (1985) indicated that corals can tolerate dilution to 27%0, but salinity of 23%0 or less, causes death within few days (Kato, 1987; Hoegh-Guldberg and Smith, 1989). Muthiga and Szmant (1987) indicated that changes greater than 10%0 below the acclimation salinity (either up or down) caused decrease respiratory and photosyn- thesis rates proportional to the magnitude of the salinity change. Thus, it appears that the lack of corals at the land-end of Opunohu Bay is mainly due to the salinity and sedimentation/turbidity stress following heavy rain, which can become lethal in case of cyclonic flashfloods. The lack of corals may be also due to other factors associated with terreslrial run-off: high nutrient load of groundwater or sewage causing ; which in tum stimulates algal development (Maragos et aI., 1985; Pastorok and Bilyard, 1985) and reduces the coral larvae settlement (Tomascik, 1991; Hunte and Witten- berg, 1992). In Opunohu Bay, eutrophication must be considered, because hard substrate at the land-end has an algal turf coverage between 37 and 85%. The factorial correspondance analysis associated with the ascending hierachical clustering separated the bay into three parts according to the species composition as well as community structure variables. The land-end of the bay is mainly under terrestrial influences. Using the Myadi concept (Horikoshi, 1988), we interpret this zonation as an increasing embayment effect. The biological organization ap- pears to be mainly controlled by the degree of confinement (Guelorget et aI., 1990), evident not only in tropical , but also in Mediterranean lagoons 186 BULLETIN OF MARINE SCIENCE, VOL. 59. NO. I. 1996

(Perthuisot and Guelorget, 1987). The degree of restriction is measured by bio- logical indicators, notably the benthic macro-fauna, which integrate the short term variations of the environment, as reported by Horikoshi (1981, 1988) in Kabira Cove, Japan, and Palau, Micronesia and by Licuanan and Gomez (1988) in the . However, in these studies, the land-end of the bay, which corresponds to a very sheltered location, is more diverse, even luxuriant, and is colonised by many species of foliaceous and branching corals. Morton (1974) found the most diverse coral community in British Solomon Islands in broad fringing reefs in sheltered embayments. The isolation of French Polynesia from the Austeo-Malay- an maximum diversity area (Crossland, 1928) and the local ecological conditions, which reduce the number of potential colonists, explain the low number of species encountered in Opunohu Bay.

ACKNOWLEDGMENTS

The authors thank C. E. Payri (Universite Francraise du Pacifique, Tahiti) for the identification of macroalgae, R. Galzin (Ecole Pratique des Hautes Etudes) for the logistic support and assistance at the research station (CRlOBE, Moorea), C. Wilkinson (Australian Institute of Marine Science, Towns- ville) and two anonymous reviewers for helpful suggestions of the manuscript.

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DATEACCEPTED:May 18, 1995.

ADDRESS: EPHE - URA CNRS 1453, Universite de Perpignan, 66860 Perpignan, France, fax: (33) 68 50 36 86, tel: (33) 68 66 20 55, email: "adjeroud@univ-perpfr" and Centre de Recherches Insulaires et Observatoire de /'Environnement, EPHE, Moorea, Polynesie franraise.