354 - Réunion Island coral reefs in poor health
The once splendid colours are dulled, algae growing everywhere and the biodiversity is impoverished Who would believe that this forlorn picture depicts Réunion Island’s coral reef, known for its beauty and rich living communities? Since the 1980s, these corals have lost much of their splendour, victims of high visitor pressure, waste water outflow and runoff, coastline development and deforestation which provokes ground erosion. Ten years’ monitoring of the coral reef has led to a report for the Réunion Marine Nature Reserve authority, co-published by an IRD research scientist( 1). It gives a stark warning. Water quality in the lagoon is deteriorating and the corals are dying. The authors recommend restricted-access measures for the reef, with the aid for example of waymarked pathways, special wildlife refuge areas and intensified surveillance of fishing and leisure activities. Such an approach is vital if human activities and ecosystem conservation, all essential for the island’s economic development, are to be reconciled.
An IRD researcher has co-written( 1) a report, based on ten years’ monitoring of the coral barrier, for the Réunion Marine Nature Reserve (RMNR). The report signals a serious deterioration of the island’s reef ecosystem.
Biodiversity declining …
In 1998 and 2008, the entire reef fish community was studied in detail. Moreover, every year the RMNR wildlife wardens conducted sampling studies of particular target fish species (including groupers, butterflyfish, damselfish and mullet) at 14 measuring stations spread out over the whole coral reef flat, situated between the sandy zone and the coral barrier, and from the outer slope of the reef. The scientists could therefore follow trends in distribution of the different species.
In one decade, the number of herbivores, the fish which graze on algae such as surgeon fish or Stegastes (a genus of damselfish), has increased by 15 to 20 %. They now dominate the ecosystem by far: they represent on average 68 % of individuals on the reef flat and 40 % on the outer slope. Furthermore there are now very few piscivores in the fish communities (less than 3 %), which are the prime target for fishing.
The overall effect of this upheaval in the fish communities and the change in their trophic structure( 2) is a decline in the coral reef’s biological richness.
… because the corals are in poor health
This loss of fish community biodiversity is a sign of deterioration of the coral habitat. The increase in density of herbivores means there has been a fall in the extent of reef cover to the advantage of algae.
The poor state of the coral would be a result of strong anthropogenic pressure in Réunion. In many drainage basins, deforestation is aggravating soil erosion which causes more earth and fertilizer to be washed down to the lagoon floor. Population growth( 3) means that the capacities of sewage purification works have become overloaded and can no longer properly treat waste water. These waters often end up in the sea; still laden with pollutants. Urbanization, agriculture and coastal zone development contaminate the groundwater which enriches the reef waters in nitrates and phosphates and stimulates the proliferation of algae to the detriment of the coral through eutrophication processes. All these developments also increase run-off of polluted waters, particularly by hydrocarbons flowing down from the roads, and degrade the quality of water in the lagoon. A further problem is excess visitor pressure, especially from tourism, which contributes to the coral reef’s deterioration. Frequentation by holidaymakers and bathers along Réunion’s 25 km of beach, can reach about 10 000 some week ends. The coral colonies suffer substantial damage (from flippers, people walking all over the corals and so on).
A highly vulnerable reef
Réunion’s reef may well be 10 000 years old, but in reality that is very young in relation to geological time. The island itself is only 3 million years old. The greater the age of a volcanic island the further the coral barrier is from the coast, as the reef grows out towards the open sea, the case of Mauritius located not far away. Then over the millennia the island gradually sinks under its own weight. Another factor is the erosion around the island’s flanks which results in gentle slopes. Conversely, on a young island like Réunion, the reef is closer to the coast and is hit by the full effect of the effluents and run-off from the steep slopes of the island’s drainage basins. The Réunion reef’s high vulnerability is therefore linked to its young age.
Desperate situations, desperate measures
To slow down this degradation, the IRD researchers recommend coordination of the coral reef management with that of the drainage basins by concerted harmonious development in order to limit input of terrestrial pollution from the island. Control of access and uses of the reef is also essential, with waymarked paths, areas of prohibited access or special wildlife refuges areas, intensified surveillance of fishing and leisure activities and so on. It was with this in mind that Réunion Marine Nature Reserve was created in 2007. It plays an essential role in the management of the area associated with the coral reef and its protection. The reserve has already brought benefits, especially in the form of improved information for people who use it, along with a decrease in poaching and a heightened compliance with the regulations.
The corals are particularly sensitive to environmental variations and are therefore at the front line in the present situation of global change (climate, population and so on). The coral reef is effectively a natural barrier against coastal erosion by the ocean and constitutes a reservoir of biodiversity as well as an important source of revenue for local communities thanks to the fisheries and tourism it generates. To reconcile human activity and conservation of this fragile ecosystem, both essential for the island’s economic development, those involved in management of the littoral zone are introducing stronger access-control and use regulations for Réunion’s coral reef.
- This study was conducted in partnership with an officer of the environmental consultancy PARETO. The report was compiled for the Réunion Marine Nature Reserve.
- The trophic structure is the whole set feeding relations (predator-prey relationships for example) between the various species in an ecosystem.
- The Réunion population increased by 11 % between 1999 and 2006, according to INSEE (Institut National de la Statistique et des Etudes Economiques ). It is expected to reach one million by 2030 (see FAS n° 294 (April 2008)– Population growth in Reunion : a strong point for the island’s economic expansion?).
Copy editor Gaëlle Courcoux, DIC, IRD.
Translation - Nicholas Flay