Rangiroa: The Physical Environment
Rangiroa is the largest atoll island in French Polynesia and one of the largest atolls in the world. Though it has a land area of only 3.37 km2  it has a very large lagoon of over 1000 km2; the entire island of Tahiti could fit within the confines of the lagoon. With a foundational elevation of only two meters above sea level, Rangiroa appears fragile and impermanent surrounded by the ocean. As so eloquently stated by Charles Darwin in his 1839 publication The Voyage of the Beagle, “These low hollow coral islands bear no proportion to the vast ocean out of which they abruptly rise; and it seems wonderful that such weak invaders are not overwhelmed by the all-powerful and never-tiring waves of that great sea, miscalled the Pacific.” People have a tendency to perceive atoll islands as weak structures, and with the eminent threat of global sea level rise, the coral islets of Rangiroa are likely to become completely submerged within the next century. This imminent destruction of the terrestrial environments of the atoll certainly has a fundamental impact on the human habitants of the atoll. However, as indicated by the geological record and the existence of atolls today, the coral reef environment will persist.
Standing in the middle of one of the 241 islets that make up Rangiroa, one can look to one side and see the vast expanse of the Pacific Ocean, then turn to face the other direction and look upon the lagoon whose area is nearly three hundred times that of the total land mass. The lagoon is so large in fact that you cannot see across it to the islets on the far side. In a matter of minutes one can cross the breadth of the land on foot and the land is so flat and free of tall vegetation that the cruise ships anchored inside the lagoon tower above the islets like unwelcome giants. Any visitor can relate to Charles Darwin’s sense of wonder at the existence of a coral atoll like Rangiroa:
It is one of the smallest size, and has its narrow islets united together in a ring. The immensity of the ocean, the fury of the breakers, contrasted with the lowness of the land and the smoothness of the bright green water within the lagoon, can hardly be imagined without having been seen.
Darwin was also the first to provide an accurate, though incomplete, mechanism for atoll island formation.
As the barrier-reef slowly sinks down, the corals will go on vigorously growing upwards; but as the island sinks, the water will gain inch by inch on the shore – the separate mountains first forming separate islands within one great reef – and finally, the last and highest pinnacle disappearing. The instant this takes place, a perfect atoll is formed.
Darwin’s theory was limited because at the time of his discoveries the scientific community did not yet realize the significance of glacial cycling accompanied by sea level fluctuations. The structure of the Tuamotu Archipelago has been greatly influenced by sea level oscillation, particularly variation during the Holocene (12 ka to present). Rangiroa could not have been suitable for human occupation before 800 CE. During the last glacial period, global sea level was much lower than today due to the large volume of ice-bound water removed from the ocean basins. During the glacial maximum, eustatic sea level fell to 120 m below modern sea level, exposing carbonate platforms (the remains of the last-interglacial reefs). Postglacial eustatic sea level began rising about 19,000 years ago. Stable atoll islets did not begin to develop until after the mid-Holocene hydro-isostatic sea level highstand in the tropical Pacific Ocean. This sea level maximum (+1.0m at Rangiroa in 500 CE) was the result of the input of glacial meltwater back into the oceans. Carbonate platforms were resubmerged allowing for the growth of new Holocene coral reefs that would provide the foundation for modern-day atoll islands. As the glaciers receded, a huge quantity of mass was transferred from the continents to the ocean. Consequently, the continental plates became less dense, resulting in isostatic rebound of the continental crust. This rise of the continents relative to the oceanic crust resulted in a second period of sea level retreat. By 900 CE, sea level had fallen below the surface of the reef flats providing area for the deposition of clay and sand, the development of a terrestrial environment.
It is possible to directly observe evidence of past sea level fluctuation on Rangiroa today. The southern rim of the atoll features a forty-four mile long, narrow band of structures called feo. This local term that has been used to characterize ancient reef spits; Pirazzoli and Mantaggioni (1984) define feo as “very dissected remnants of elevated ancient reef rock.” The feo is composed of corals, crustose coralline algae, foraminifera, and other skeletal fragments. After studying the feo in 1903, Swiss geologist Louis Agassiz wrote,
The great wall of ancient elevated reef rock… was fully 12 feet high, and is the remnant of the ancient coralliferous limestone ridge which flanked the southern side of Rangiroa. …This old ledge is deeply pitted and honeycombed and eroded into all kinds of fantastic spires and pinnacles and walls cut through by crevasses extending from the low-water mark to the summit, which is more or less covered by the high sand beach accumulated behind it on the lagoon side. This beach completely conceals the extension of the old ledge under the island.
Today, the feo pinnacles range in height from six to ten feet and are characterized by lower erosional platforms, or notches, carved out by marine abrasion. These notches can serve as an indicator of past sea level since they are, and were, formed at the surface of the water. The height of the feo structure itself is also an indicator of past sea level; corals must grow underwater but near the surface, therefore the height of the feo provides evidence of a much higher past sea level.
The atoll of Rangiroa is composed of a ring of 241 small islets, or motu, separated by 150 hoa, which are channels of varying width and depth that connect the lagoon to the sea. The windward rim of the atoll is composed of relatively narrow (350-600 m wide) motu. These are Rangiroa’s largest motu and house the main residential and commercial locations, the towns of Avatoru and Tiputa. The southern, swellward rim is widest (800-1200 m); however, the motu here are very small and separated by multiple hoa. Finally, the western leeward rim is much like the northern rim with an average width of 500 m and slightly smaller motu with a higher degree of separation. The western rim is also most exposed to tropical storms and hurricanes.
The hoa play an extremely important role in facilitating exchange between the ocean and the lagoon. There are three types of ocean/lagoon exchange: flowing over the reef crest, alternating currents in the passes, and wind-powered currents, which draw water out of the lagoon, over the reefs. Tides follow a semi-daily, or semi-diurnal, cycle with two high tides and two low tides each twenty-four hour day. The average tidal amplitude is only 0.20 cm. The two main passes, Hotua’ara and Hiria, function similarly in terms of current flow. Both are characterized by alternating inflowing and outflowing currents traveling at three to four knots. In Hotua’ara, the outflowing current often carries a high concentration of detritic particles and a high organic load of mainly zoo- and phytoplankton. 
The hoa can be characterized by the level of exchange that they allow and the resultant differences in their respective biological communities. Permanently functioning hoa are the least frequent type in Rangiroa. They are the widest of the channels, ranging from 400 to 600 meters, and allow open circulation and exchange with the ocean. This category includes the two main passes through the northern part of Rangiroa’s peripheral reef, Hotua’ara (also known as Avatoru) and Hiria (also known as Tiputa). These two permanently functioning hoa allow for constant exchange between the ocean and lagoon waters; consequently, the lagoon waters are very similar to the surrounding ocean.
Hotua’ara pass is about 900 meters long and 450 meters wide; it is shallowest at the northern (ocean) end (15-20 meters) and grows deeper approaching the lagoon where the depth fluctuates between 20 and 30 meters. There is a large amount of flora and fauna found within the Hotua’ara pass. The Hiria pass is 800 to 900 meters long and slightly wider (550 meters). Unlike Hotua’ara, Hiria is only about 16 meters deep on the southern (lagoon) end and reaches a maximum depth of 38 to 39 meters at the outer reef slope. Hiria lacks the coral and faunal diversity of Hotua’ara pass, with only a few scarce coral colonies scattered along the bottom and the sides of the pass. The main fauna present in Hiria pass include fish, echinoderms, polychaete worms, and corals.
Hoa in the second category are classified as partly obstructed. These are channels that have been cleared of boulders, coral patches, and various detritic material by tsunamis, hurricanes, and tropical storms. Debris remains either inside the hoa or at one or two of its mouths. This category is further divided into two subcategories: hoa partly isolated from the ocean, and hoa largely open to oceanic influences. The first is usually isolated from the ocean side by coral boulders but open to the lagoon. Such passages are mostly present on the northwest rim of the atoll. They are colonized by corals, and less abundant numbers of echinoderms as well as some Actinians (sea anemones) and Annelids (segmented worms). Endofauna is more abundant in this type of hoa compared to the permanently functioning hoa. The second subcategory describes hoa that are open to the ocean but blocked from exchange with the lagoon by a sandy strip. These passages are most common on the southern side of the atoll. They are rich in coral, (with at least 12 species) echinoderms, and molluscs (about 30 species). Both types of partly obstructed hoa can only function, that is allow complete ocean-lagoon exchange, under very rough sea conditions.
The majority of the passes through the atoll ring fall under a third category of non-functional, entirely obstructed hoa. These channels are blocked, usually by coral rubble accumulations, on both the lagoon and ocean ends. This type of hoa is characterized by cyanophytes, encrusting sponges, and algal macrophytes. Molluscs are abundant, whereas corals are scarce and when present are mainly Porites.
Rangiroa’s lagoon is continually flushed by the tides allowing for a large exchange of water each day. Since there is so much exchange with the ocean, there is an estimated loss of 4.3 tons of biomass per tide (about ten tons per day for the two passes). This loss of zooplankton biomass enriches the surrounding ocean waters and contributes to high fish population in the passes. The lagoon can be divided into three separate sections: the northern fringe, the southern fringe, and the western fringe, in addition to the lagoon floor. The northern lagoon fringe is characterized by narrow beaches with silty, detritic sand and the presence of marine Phanerograms (seeding/flowering plants). Molluscs are present on the beach but they are low in abundance and unevenly dispersed. Halodeima atra, a species of sea cucumber locally known as rori, can be found in very large numbers in the muddy sand of the shallow northern fringe. Corals, mainly Acropora and Porites, are present where the slope increases. The sand of the southern fringe is white and less muddy; it covers the coral flat around the mouth of the hoa. The slope is gentle between 8 and 12 meters and drops off abruptly at 12 meters. Coral patches are more abundant in the shallow waters and decrease as the depth of the water increases. This is due to the light limitation as well as more sediment deposition in the deeper, lower energy environments which both increases turbidity and stifles coral growth because of the actual volume of deposition. The western fringe is similar to both the northern and southern fringes. It is characterized by a reduction of vegetation as the motu becomes narrower.
Coral pinnacles can be found throughout the lagoon. A pinnacle is a large patch of coral that rises up from the bottom and is just barely visible above the water, posing a potential hazard to watercraft. These pinnacles can have a sheltered and an exposed side, which influences the respective floral and faunal communities. The sheltered side is usually to the northwest and is characterized by a large platform (100-150m wide) composed of sand-covered coral stone. The biology here is limited to a few species of molluscs and crustaceans. The exposed side of the pinnacle is usually a narrow sandy platform. The biology here is much more abundant and diverse. The fish community includes surgeonfishes, jacks, butterfly fishes, parrotfishes, squirrelfishes, and wrasses. There are also many sessile bivalves and the outer edge of the slope is colonized with algae as well as algal beds, which are dispersed depending on the turbulence and the depth of the water. The lagoon floor around the pinnacles is normally at a depth of 20-25m and is very uniform. There are small hills and depressions of sand that are colonized with diatoms and cyanophytes. Coral patches are scattered across the floor but there is no dominant flora.
The flora and fauna inside the lagoon is generally very diversified. There are 90 families (about 800 species) of fish and 33 families of molluscs, representing 22 taxa of bivalves and 73 taxa of gastropods. The fish community of the coral reef contains migrant species as well as species that live on the reef after their post-larval stage of life. Inside the lagoon, fish aggregate around the corals and the inner slope of the lagoon. The main groups of fish found in this area of the lagoon are snappers, groupers, parrotfishes, squirrelfishes, and wrasses. There is a different dynamic at the inner reef flat, where it connects with the hoa. This area is a nursery for many fishes, but mainly the damselfish, the surgeonfish, and the parrotfish. Within the pass, there is a community of reef fish composed of damselfish, butterflyfish, wrasses, surgeonfishes, and lemon and grey sharks and also many migrant species of fish. As the outer slope begins, fish diversity decreases with depth. On the uppermost part of the slope, the main residents are surgeonfishes, parrotfishes, triggerfishes, wrasses and groupers. Maximum diversity occurs around depths of 10-20m where squirrelfishes, butterflyfishes, damselfishes, snappers, and surgeonfishes are all present. The deeper coral reefs are less diverse and more sparse, with a clearly different faunal community. There is a decrease in the herbivorous species and in increase in the size and number of important carnivorous species, such as groupers, wrasses, and snappers.
Rangiroa’s terrestrial flora and fauna is not nearly as diverse as that found in the marine environment. Vegetation is limited to coconut palms (Cocos nucifera) and other succulent plants that can handle the arid environment. Much of the soil is very depleted in nutrients and does not hold much moisture. Any moisture that is kept in the soil is not there for extended periods of time because of sparse annual precipitation (for eight months of year evaporation exceeds the amount of precipitation on the atoll). There is also a very low level of organic content in the soil, from only 2-3% to the highest being 15% around marshes. The dryness and infertility of the soil are the major limiting factors to terrestrial plant growth. There are more than 50 families of plants that have been identified on Rangiroa, and as of 1969, a list of 119 plant species had been identified, only 38 (32%) of which were indigenous to Rangiroa. A few of these plants were likely introduced from Tahiti via the transport of soil from the high island to the atoll.
The fauna of the limited terrestrial environment of the atoll is mainly composed of birds, a few reptiles, and some invertebrates. There are a total of 27 nesting species of birds, including the Tuamotu warbler, green pigeons, pacific reef herons, Tuamotu lorikeet, Sooty crakes, and the migrant New Zealand cuckoo. The coral atolls such as Rangiroa also have some of the last populations of the coconut crab (Birgus latro), which has recently become a threatened species. The marine green turtle (Chelonia mydas) and a few small lizards, such as the azure-tailed skink and the house gecko make up the reptiles found on the atolls. There are only a few representatives of mammals on the island, which include the Polynesian rat and the Norway rat, along with a few once-domestic cats that have become wild.
Invasive species have become a very big problem for the endemic species found on the islands of French Polynesia. Many of the terrestrial species that are found throughout the region are endemic, so there have been many extinctions due to the introduction of species within certain niches. There are almost twice as many introduced vascular plant species (1,700) as there are indigenous plants (893), and of these 600 of the introduced species have become naturalized and 70 considered invasive. Many species that are introduced are not problematic, but the ones that are decimate the native species that share the same resources.
Rangiroa experiences minor seasonal variation in climate. The trade winds blow to the east over the Tuamotus year round. The winds are most regular from May to October. From June to October, the wind blows predominantly east-south-east. From November to May, the winds usually blow east-north-east and are often light, alternating with heavy unstable squalls from north to south-west. Finally, May through September is characterized by strong winds from the south-east. Rangiroa and the other islands in the archipelago are susceptible to hurricanes especially during January and February (summer in the South Pacific). The atoll receives very little rainfall due to its exceptionally flat topography. Most rain occurs between November and January while rainfall becomes very scarce in April and May as well as September. Furthermore, rainfall tends to be very inconsistent, fluctuating from one year to the next; the average monthly precipitation is often skewed by large amounts of rain during single storm events. The average annual temperature range on Rangiroa is very low (27.3ºC to 27.8ºC). July is usually the coldest month of the year (25.6-26.3ºC) while February and March are the hottest (28.7-29.4ºC).
Low-lying atoll islands such as Rangiroa are very vulnerable to recent human-mediated climatic disruption. Rising global temperature has contributed to accelerated melting of the Greenland and West Atlantic ice sheets consequently increasing the rate of sea level rise. Inundation, “the diffusion of a high water level into or across an island” (Woodruffe, 2007), is one of the most publicized issues associated with sea level rise around island nations. A flooding event such as this would most likely submerge the coral reef foundation of Rangiroa; without a foundation above sea level, currents and storms would quickly erode the loose sands that form the highest points of land on the atoll today. This destruction of terrestrial environment is inevitable if climatic disturbance is not addressed in the immediate future. Global sea level rose at an average rate of 1.7 to 1.9 mm/yr during the bulk of the twentieth century. Data acquired from satellite altimetry has shown that the rate probably increased to about 2.5 mm/yr from 1990 to 2000 and has increased further to about 4 mm/yr from 2000 to 2009.  If this pattern continues, sea level is projected to surpass the level of mid-Holocene paleoreef remnants (the foundation of Rangiroa’s habitable islets) by the year 2070 at the earliest and by 2140 at the latest.
During the mid-Holocene hydro-isostatic highstand, atoll islands in the Pacific were inundated by the rising sea level. Paleoceanographic research has shown that sea level follows a cyclical pattern in accordance with glacial phases. Without a conscious effort on the part of humankind, Rangiroa will likely cease to be an island within the next century. This has huge ramifications for the human populace and the terrestrial biota; on the other hand the coral reef community will remain relatively unchanged. As Darwin wrote, “for as mountain after mountain, and island after island, slowly sank beneath the water, fresh bases would be successively afforded for the growth of corals.” Coral reefs are persistent and highly adaptable; the terrestrial environment is not. The identity of Rangiroa as an island is dependent on the existence of a terrestrial environment, but what defines Rangiroa and makes it unique is its extensive coral reef environment, which has been a permanent feature for thousands of years.
Heidi Hirsh, University of San Diego
Joan Hurley, University of South Carolina
 Le présent document doit être cité comme suit : Andréfouët S., Chauvin C., Spraggins S., Torres-Pulliza D, Kranenburg C. 2005, Atlas des récifs coralliens de Polynésie française, Centre IRD de Nouméa, février 2005, 38 pages + 86 planches Mise en page et fabrication : Jean Pierre Mermoud. Imprimé à l’atelier de Reprographie, Centre de Nouméa.
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 At least 12 feet higher based on Agassiz's documentation.
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