Water renewal time for classification of atoll lagoons in the Tuamotu Archipelago (French Polynesia)

The feasibility of using hydrodynamic renewal time as the basis for a classification of atoll lagoons is tested for atolls of the Tuamotu Archipelago, French Polynesia. Renewal time depends on the inflow of oceanic water through the rim of the atoll, on a daily time scale, due to wave forcing. Renewal time is computed for a large set of morphologically diverse atolls, according to significant wave height (satellite altimetry data), morphometric indicators (high-resolution satellite images), and in-situ flow measurements. Renewal times with respect to wave height are presented for a variety of atolls. Renewal times range from less than 1 day for very open and shallow atolls, to several tens of days for semi-open moderately deep atolls, and to several years for closed or very large and deep atolls. Comparisons between phytoplanktonic biomass (in the range 0.1 to 1 µg l^-1 for total chlorophyll) and renewal time (0.1 to 130 days) leads to the identification of two groups of atolls. We obtain a significant relationship between biomass and renewal time, but only for atolls with lagoon surface areas greater than 25 km².     

Dinitrogen fixation by exposed communities on the rim of Tikehau atoll (Tuamotu Archipelago, French Polynesia)

Various types of sub-aerially exposed microbial mats, including emergent mats, beach sand, beach rock and Kopara mats, are widespread on the 78 km (25 km²) of rim surrounding the Tikehau atoll lagoon. These mats form laminated accretions or diffuse microbial communities growing under high insolation and temperatures, and are therefore subject to desiccation. Both heterocystous and non-heterocystous cyanobacteria occur in these mats. Using acetylene reduction techniques, nitrogenase activity was observed at all sites over a period of 5 years and was 3–17 times higher during daylight than at night in all communities except for beach rock. ¹⁵N₂ measurements indicated a molar ratio of acetylene reduction to N₂ fixed of 1.6 for all exposed communities. Estimated N₂ fixation ranged from 1.44 to 8.0 mg N m⁻² day⁻¹ in these exposed communities (mean of 4.66 mg N m⁻² day⁻¹) with beachrock showing the highest rates. For the whole reef rim, daily N₂ fixation amounted to 98.42 kg N day⁻¹ which represents 28% of the rate of fixation in the entire lagoon (area 400 km²).     

Catastrophic impact of hurricanes on atoll outer reef slopes in the Tuamotu (French Polynesia)

Underwater effects on coral reefs of the six hurricanes which ravaged French Polynesia between December 82 and April 83 were observed by SCUBA diving around high islands and atolls during September and October 1983. Special attention was paid to Tikehau atoll reef formations (Tuamotu archipelago) where quantitative studies on scleractinians, cryptofauna and fishes were conducted in 1982 immediatly prior to the hurricanes. On outer reef slopes coral destruction, varying from 50 to 100%, was a function of depth. Upper slope coral communities composed of small colonies well adapted to high energy level environments, suffered less than deeper formations. However, there is a narrow erosional trough in this zone at a depth of 6 m that was probably the result of storm-wave action (plunge point). Coral destruction was spectacular at depths greater than 12 m: 60 to 80% between 12 m and 30 m and 100% beyond 35 m, whereas earlier living coral coverage ranged from 60 to 75% in these zones. The outer slope was transformed into a scree zone covered with coarse sand and dead coral rubble. Dives on different sites around steep outer slopes (>45°) of the atolls and more gentle slopes (<25°) of some parts of the high islands permitted the formulation of an explanatory hypothesis: direct coral destruction by hurricane-induced waves occurred between the surface and 18–20 m; on low-angle slopes broken colonies were thrown up on reef flats and beaches; on steep slopes avalanches destroyed much of the living corals and left scree slopes of rubble and sand.     

Pleistocene mollusc assemblage in the slightly raised atoll lagoon of Niau (Tuamotu,French Polynesia)

Surveys conducted at Niau, a slightly raised atoll in the Tuamotu Archipelago, revealed a brackish community with only a few extant species of fishes, crustaceans and molluscs in the residual semi-enclosed lagoon. Dredging of superficial sediments near the Niau lagoon resulted in a rich shell mollusc assemblage with 120 species, dating from the Pleistocene Last Interglacial Maximum (120,000 years BP). The assemblage contains 3 previously described new species, collected nowhere else since, and 12 species locally extinct but extant in the Indo-Pacific. Results are compared with previous studies on Pleistocene reefs of the Indo-Pacific.     

Crab cryptofauna (Brachyura and Anomura) of Tikehau,Tuamotu Archipelago,French Polynesia

A detailed survey of the carcinological cryptofauna in Tikehau atoll in the Tuamotus, was made: (1) to determine whether Tikehau atoll has a diverse fauna; (2) to compare it with a high island Polynesian reef; (3) to identify the distribution pattern of crabs in relation to depth and to different structures of the atoll; and (4) to compare these results with those obtained in Madagascar, La Réunion and Mauritius. Sixty five species were collected at 13 sites. Cluster analysis based on Sanders' index of affinity revealed one main group of species occurring on outer slopes and another occurring on all reef-flat and lagoonal sites. The crustacean cryptofauna occurring in the atoll appears to be impoverished in comparison to that found on Polynesian high island reefs and on Malagasian reefs. However, all outer reef slope sites at the 3 localities studied share some species in common.     

Tropical zooplankton in the highly-enclosed lagoon of Taiaro Atoll (Tuamotu Archipelago, French Polynesia)

 Nocturnal zooplankton assemblages around Taiaro Atoll were sampled over six nights during February 1994. Replicate zooplankton samples were collected at windward and leeward locations in the enclosed lagoon and adjacent ocean with a metered net (85 cm diameter, 500 μm mesh) towed for 15 min at 5 m depth. The zooplankton community in the lagoon was very different from that in the ocean. Oceanic samples contained 50 mostly holoplanktonic taxa (diversity index, H′=2.62; evenness index, J′=0.67). Lagoonal samples contained 19 mostly meroplanktonic taxa (H′=1.54, J′=0.52) with three taxa (decapod larvae; an ostracod, Cypridina sp.; a copepod, Acartia fossae) contributing >90% of the individuals. Unlike the ocean, zooplankton distributions in the lagoon were not homogenous; instead spatial patterns were apparently formed by the interaction between hydrodynamic processes and species-specific behaviour. Accepted: 28 August 1997     

Thirty-year recovery of mollusc communities after nuclear experimentations on Fangataufa atoll (Tuamotu,French Polynesia)

A 30-year study of temporal changes in gastropod community structure on the reefs of a Pacific Ocean atoll (Fangataufa, Tuamotu Archipelago) subjected to atmospheric nuclear tests during the 1960s offered the opportunity for an otherwise impossible field experiment that could help ecologists understand mollusc primary succession. Reef molluscs were partly or entirely wiped out by the heat of the nuclear tests and the reefs were recolonized by ocean larvae. On all reefs, community composition before the tests was very different from what it evolved to afterwards. A new method of analysis was developed to study the temporal variation in community composition before versus after the tests (temporal beta diversity). Analyses showed that community compositions diverged through time among the reefs. Only some species can survive the harsh conditions of supralittoral zones, so the same species recolonized them; environmental filtering controlled the development of the new communities. In the reef flat and edge zones, differences in community composition seem to be the result of neutral stochastic colonization by larvae coming from the open ocean. All reefs developed a community composition quite different from that before the nuclear tests.     

Stable isotopes in fish otoliths discriminate between lagoonal and oceanic residents of Taiaro Atoll (Tuamotu Archipelago, French Polynesia)

 The fully enclosed Taiaro lagoon is hypersaline (42.5 psu) and non-tidal; constant salinity and water level result from strong evaporation balanced by low percolation through the lagoon floor. Seawater can flow over the atoll rim during exceptionally high seas and may then replenish lagoonal communities with propagules of oceanic origin. The distinctive water chemistry of the lagoon suggests a possible way of identifying these immigrants. We established this potential by analysing stable isotopes of carbon and oxygen in the recent growth layers of otoliths of two adult reef fishes, Chaetodon ulietensis and Acanthurus triostegus, collected from both sides of the atoll rim. Fish from the two locations were discriminated by their isotopic signatures, suggesting that analysis of the microchemical signatures deposited during the larval development could be used in future work to determine which individuals and species complete their life-cycles in this unusual lagoon. Accepted: 28 August 1997     

Objectives and background to the 1994 Franco-Australian expedition to Taiaro Atoll (Tuamotu Archipelago, French Polynesia)

 The 9 km² uplifted lagoon of Taiaro Atoll (15°45′S, 144°38′W) is hypersaline due to its isolation from the ocean, yet it contains a high diversity of fish. The question unifying our expedition was to discover whether these assemblages could be self-sustaining despite very limited contact with the ocean. Although we were constrained by time, collections of fish larvae showed that some species can complete their life-cycle within the lagoon, while others differed genetically between the lagoon and the ocean, consistent with restricted gene flow. The lagoon contained few oceanic species of zooplankton, confirming its general isolation, but nevertheless some fish species may depend upon infrequent colonisation from the ocean (when large waves drive water over the normally dry reef crest). Isotopic signatures in fish otoliths suggest the basis for a more definitive and inclusive test of the sources of the lagoonal assemblage. Accepted: 28 August 1997     

Assessment of deep water stocks of pearl oysters at Takapoto Atoll (Tuamotu Archipelago, French Polynesia)

An assessment of natural stocks of the commercially important black pearl oyster, Pinctada margaritifera, was conducted in the lagoon of Takapoto Atoll, French Polynesia. The sampling methodology combined estimates of reef area from remote sensing, depth profiles across the lagoon, and in situ sampling using scuba. Confidence limits around estimates of mean oyster density began to stabilize after ten dives, and did not decrease further after 30. Densities of oysters increased steadily with depth, ranging from a mean of 1 per 100 m² at 0–10 m to 8/100 m² at 30–40 m. More than half of the stock occurred at depths greater than 30 m. The total stock was estimated to be 4.3 ± 0.67 million, equivalent to 834 metric tonnes (including shells). Accepted: 15 June 1999     

Replenishment of fish populations in the enclosed lagoon of Taiaro Atoll: (Tuamotu Archipelago, French Polynesia) evidence from eggs and larvae

 Taiaro Atoll Lagoon is normally isolated from the ocean, but at least 125 marine fish species of 31 families are present there. We sampled fish larvae in Taiaro Lagoon and the nearby ocean in February 1994 with plankton net, neuston net and light trap to investigate which taxa were completing their life cycles in the lagoon. Concentrations of fish eggs and larvae were very high in the lagoon indicating intense spawning, but larvae of only 18 taxa of 10 families were present. Only six, a callionymid, gobiids, a hemiramphid, a microdesmid, and two pomacentrids, were present across a full range of pelagic sizes, and were clearly completing their pelagic stage in the lagoon. Four other taxa, an apogonid, two labrids and a scarid, were common, but the largest individuals were small (<5 mm) postflexion larvae. These may have been completing their pelagic stage in the lagoon. The remaining lagoonal larvae (eight taxa) were rare and at the preflexion stage, so we could only conclude that they hatched from eggs spawned in the lagoon. Nineteen taxa of 15 families found as adults in the lagoon were present outside the lagoon as larvae, but not inside, suggesting that they may not normally complete their life cycles in the lagoon. Horizontal distributions of larvae in the lagoon are apparently due to the interaction of larval vertical distribution behaviour with a wind-driven countercurrent system. Accepted: 16 October 1996     

Holocene trends in distribution and diversity of benthic foraminifera assemblages in atoll lagoons,Belize, Central America

Seven benthic foraminiferal assemblages were identified in vibracores through Holocene lagoons of three Belize atoll lagoons (Glovers Reef, Lighthouse Reef, Turneffe Islands). These include (1) the low-diversity Cribroelphidium assemblage (2) the Cribroelphidium-Elphidium assemblage (3) the Quinqueloculina-Triloculina-Peneroplis assemblage (4) the high-diversity miliolid assemblage (5) the Archaias-miliolid assemblage (6) the low-diversity miliolid assemblage, and (7) the mixed assemblage. Altogether, 109 species and 56 genera were identified. The highest diversities are observed in the largest lagoon (Turneffe Islands), whereas one of the smaller lagoons (Glovers Reef) exhibits the lowest diversities during the Holocene. No significant changes in diversity over time occur, however, a slight trend to higher diversity may be observed through the Holocene, suggesting that the foraminiferal faunas in the atolls are in a diversification stage. Faunal diversity in atoll lagoons appears to be controlled largely by habitat size, habitat heterogeneity, and water circulation. Habitat age and water depth only play minor roles. Substrate texture, water depth, and turbidity influence the predominant modes of life of benthic foraminifera encountered in the lagoons (epifaunal versus infaunal versus symbiont-bearing). Time-averaging effects were not observed, even though lagoonal sedimentation rates fluctuate in individual cores and the three lagoons, and despite the fact that sediments are modified through bioturbation by callianassid shrimps. This finding underlines the potential of benthic foraminifera for paleoecological studies in the fossil record of reefs and carbonate platforms.     

Mass mortality of macrobenthic communities in the lagoon of Hikueru atoll (French Polynesia)

Epibenthic macrofauna communities (corals, molluscs, echinoderms, and macroalgae) were investigated at Hikueru Atoll, Tuamotu Archipelago. The very low diversity and abundance that we observed (1 coral genus, and 2 mollusc, 1 echinoderm, and 17 macroalgal species) is likely to be the result of a mass mortality event that occurred 18 months earlier. Hydrological surveys suggest that this high mortality was the result of a major dystrophic crisis. Other atoll lagoons in the Tuamotu Archipelago have also been affected by high mortalities in the past, but Hikueru appears to have been the most frequently and intensely affected. The present study confirms that mass mortalities associated with phytoplankton blooms may be one of the major disturbances affecting coral reef ecosystems. Accepted: 14 May 2000     

Lipid composition of water and surface sediments in Takapoto atoll lagoon (French Polynesia)

Dissolved, particulate and sedimentary lipid compounds were analyzed in samples collected in May 1988 at three sites in the lagoon of the closed atoll of Takapoto (Tuamotu archipelago, French Polynesia). The study provides background information dealing with water quality and the nature and concentration of lipids. Non-aromatic hydrocarbons and fatty acids were isolated from lipids and analyzed by gas chromatography/mass spectrometry. Non-aromatic hydrocarbon concentrations did not exceed 1000 ng l^-1 in water, and 2300 ng g^-1 in surface sediments and are among the lowest encountered in pristine marine environments. No noticeable petroleum pollution was evidenced in the lagoon. Nevertheless, traces of petroleum-derived compounds were detected at the central site for both surface and deep water. Total fatty acid concentrations varied in the range 6.3–14.4 g l^-1 for the particulate phase and in the range 0.5–3.2 g l^-1 for the dissolved phase. The molecular fingerprints of fatty acids and hydrocarbons evidenced a predominant algal, and to a lesser extent microbial, origin of the organic matter present in water and sediments. Mono- and polyunsaturated fatty acids, which are essential components for animal metabolism, were identified in noticeable amounts in suspended matter (1.8–4.6 g l^-1), and at highly variable levels in the dissolved phase (0.08–1.21 g l^-1).     

Habitat selective factors influencing the distribution of larger benthic foraminiferal assemblages over the Kepulauan Seribu

Symbiont-bearing foraminifera are used to study the effects of habitat deterioration on benthic communities in coral reefs dominated by macroalgae. It is shown that, despite their preference for nutrient deprived conditions, some symbiont-bearing foraminifera occur on reefs heavily affected by nutrient stress and macro-algal dominance, thus highlighting the need for a better understanding of the autecology of species and assemblages in these conditions. Both diversity and habitat fractionation increases as terrestrial and nutrient influence decline. The assemblage structure in the most nearshore reefs are dominated by generalist species, while, additionally, more specialistic species occur at the more offshore reefs. Apart from larger scale gradients in ambient seawater quality, local scale variation in physical environmental conditions, such as habitat structure, are important for the assemblage structure as well.