The impact of anthropogenic and natural stresses on the coral reefs of Rodrigues, western Indian Ocean
This study investigated the interaction between natural and anthropogenic impacts on the reefs of Rodrigues. Rodrigues remains undeveloped, however as a result of deforestation in the 1800s, the reefs are subjected to episodic fluvial inputs following high rainfall, resulting in high sedimentation/turbidity within the lagoon. The island is also affected by natural impacts such as coral bleaching and cyclones. Despite these factors the reefs in Rodrigues are currently in good health. This study assessed whether fluvial inputs are having a sub-lethal effect on coral colonies on the fore reef slopes, by studying coral growth rates, larval settlement and recovery from injury at 3 sites with varying sediment regimes. The severity of a coral bleaching episode, which affected the island in 2002, was also assessed. The results show that at the 2 inshore sites (Totor and Trou Malabar) sediment deposition was well above `tolerable' levels for coral reefs (up to 96 mg crn2 d-1), suggesting that this threshold is not applicable for these reefs. It is suggested that high turbidity and sediment deposition are related to high rainfall, resulting in land run-off, combined with high wind causing sediment resuspension. This periodic high sedimentation and turbidity appeared to be having a sub-lethal effect on coral colonies, resulting in a decrease in growth rates of Acropora austera and Porites rus, low coral recruitment and a low ability of Montipora spp. to recover from injury. The bleaching event was not widespread and coral mortality was restricted to sites in the north and west of the island. Where bleaching did occur, it was severe, resulting in mortality of up to 75 % of coral colonies at some sites, particularly branching and tabular Acropora spp. One year later, dead coral colonies had become heavily eroded and overgrown with turf and macro-algae, although some recovery had occurred.