Within a global context, mankind has irrevocably destroyed 20% of the coral reefs over the past fifty years and 50% of the reefs are at risk within the next 20 years. French Polynesia is distinguished by its particularly resistant healthy reefs. Here, the effects of climate change are minimal and local pollution is under control.
With the exception of the Marquesas Islands, all of the islands in French Polynesia have coastal marine ecosystems that are almost exclusively comprised of coral. For biogeographical reasons, the abundance of reef species is certainly lower than in tropical regions situated more to the west of the Pacific. However, with close to 200 species of coral, 1200 species of fish, 1000 species of crustaceans and 2500 species of mollusks—to name the most common known groups—these ecosystems remain extremely rich compared to temperate zones. One square kilometer of coral reef contains a number of animal and plant species comparable to the entire coastal marine area of France or the state of California.
The most obvious role of a coral reef is to create a protective barrier around coastlines, allowing the development of human activities along the protected coastal plain. On a larger scale, the land mass of an atoll is entirely constructed of reef organisms. Without coral reefs, the atolls and their villages would simply not exist (there are 83 atolls in French Polynesia, of which 76 are located in the Tuamotu Islands). Besides, reefs create resources useful to mankind, like traditional fishing, aquaculture (i.e. pearl shells that produce Tahiti’s famous black pearl) and collecting ornamental fish popular among aquarists. Then of course, coastal landscapes and underwater reef paradises with warm clear water are ideal for visitors, which makes tourism the number one industry in the country. We now know how to evaluate the benefits of a natural system and call it “ecosystem services.” For instance, the total value of reef-generated products on the island of Moorea is estimated at $85 million per year (9 billion French Pacific Francs/74 million Euros). Beyond the financially quantifiable aspects, the reefs provide other valuable features deemed as non-use, such as the cultural, social, traditional and spiritual aspects practiced by the people who live there and who have depended upon the reef for many generations.
Outer Slopes: A Vital Part of the Reef
This powerful reef dynamic is especially obvious in areas directly facing the ocean called the outer reef slopes. Compared to the lagoon, this part constructs an ecosystem which is the most aesthetic, the most abundant in species and the most important as far as the functioning of the reef. Between 0 to 30 meters in depth, coral attains record levels of coverage and growth through annually producing up to 10kg/22 lbs. of limestone per square meter from elements dissolved in sea water. It is this limestone production and calcareous encrusted algae that construct the entire coral structure. On islands older than atolls, calcareous buildup over several tens of millions of years can reach almost 1000m/3280 ft. thick!
French Polynesia has been spared
Due to its remote location in the Pacific Ocean that offers somewhat of an immunity to physical and chemical changes, due to the very widespread distribution of islands over a surface area of more than 5 million km2/2 million sq. mi. and because of relatively varied climates, French Polynesia’s coral reefs appear to be somewhat protected from threats linked to climate change. Even though temperature limits have been exceeded several times in the past 25 years, the occurrence of coral bleaching has been much less intense compared to other regions in the world where mortality rates for coral have sometimes reached 100% of a reef’s colony. The effects of acidification of sea water in the natural environment are not very noticeable at the moment. Local disruptions are limited to some of the most populated zones and haven’t reached the natural environments located in most of the islands. This isn’t due to a rigid and methodical regulation of the ecosystems, but rather because of limited or nonexistent human populations (more than a quarter of the islands are uninhabited). Lastly, barrier reefs located near the majority of the islands isolate and preserve the outer reef of the islands. For all of these reasons, out of all of the 15000 km2 /6000 sq. mi of coral reefs and lagoons belonging to the 118 islands in French Polynesia, 90% are still considered to be in good ecological health.
Cyclones, as well as coral-eating crown-of-thorns starfish (Acanthaster planci or taramea in Tahitian) are the two main factors that naturally control the state of coral reefs in our islands. Cyclones, which occur most frequently on the western edges of French Polynesia, randomly affect anything in their path, including reef coastlines exposed to the swells of the storm. The coral-eating crown-of-thorns starfish (taramea) can proliferate at quite an impressive rate (with two major phenomena observed since the late 1970s). They cause extensive damage that can attain the scale of an entire archipelago. The 76 atolls of the Tuamotu islands have somehow been spared from this type of invasion for reasons that are still unknown. The last episode of a taramea attack observed between 2004-2010 killed off the majority of the outer coral slopes within the Society Islands. However, the reefs in French Polynesia are resilient to natural stressors and display a quick recovery. About a decade is necessary for the coral to make a full recovery from these types of disturbances.
Endangered and threatened
For several decades, this natural dynamic has been threatened due to consequences of human activities. Climate change is to blame for part of it. Coral is very sensitive to elevations in sea water temperature and often lives in an environment that is very close to its maximum tolerance levels. This maximum, which for the moment is set at 29.2 C/84.56 °F, has been more frequently exceeded during austral summers throughout the periods of El Niño. The coral then takes on florescent colors for a few weeks before bleaching. Depending on the intensity and duration of the phenomenon, some of the colonies end up dying. Another endangering effect is linked to climate change; i.e. the acidification provoked by an increase in carbon dioxide (CO2) in the atmosphere and in sea water since this gas is constantly absorbed by the ocean. When the CO2 content rises, there is a change in chemical balance which increases the acidity of the water. The synthesis of the limestone by calcifying marine organisms (including coral) becomes disrupted. Human-induced distress at the local level is not as subtle but more concrete. Today, human activities are the main reason for any observed damage to the reef in the past 50 years. The over-exploitation of resources, waste water discharge, hyper-sedimentation caused by the careless destruction of terrestrial plant cover and the physical modification of the coastline (rock retaining walls, embankments for a specific purpose, etc.) are the main stressors that coral reefs face.
Some of the most studied and monitored reefs in the world
With several research centers (Criobe: Insular Research Center and Environmental Observatory, Ifremer, IRD formerly Orstom, Malardé Institute, University of California, Berkeley, University of French Polynesia), French Polynesia benefits from a powerful research facility that over the natural environment of the islands. Within this framework, scientists from four archipelagoes have been closely monitoring the state of health of reefs in French Polynesia for more than 40 years. The island of Moorea is the most studied tropical island system in the world based on the number of scientific publications over this subject. With two research stations Gump/UC Berkeley (USA) and Criobe (France), it is also the island whose reefs are the most closely monitored with its network of studied reefs that started in 1983 and that are still active today.
On a larger scale, Criobe’s Polynesia Mana monitoring program has been evaluating strategically selected reefs for the condition of their health since 1991. Every two years and on almost 15 islands spread over the entire geographical area of French Polynesia, bio-indicator parameters are collected over coral and fish colonies for each targeted reef area. This monitoring, along with scientific reef research, makes it possible to understand how ecosystems function and evolve over time. In time, and in this era of sustainable development and integrated management of ecosystems, this knowledge should be increasingly used as a means for policy makers and administrators to make the right decisions concerning these precious and fragile coastal marine ecosystems.
Criobe – Centre de Recherches Insulaires et Observatoire de l’Environnement (Insular Research Center and Environmental Observatory) – Moorea