Corals Do Better in Polluted Waters But Die in Pristine Sites

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Porites rus coral in more polluted areas of Kiribati was found to be growing like dandelions

A study on the impact of climate change on coral reefs is raising 'sticky' questions about conservation.

It found coral in more polluted waters near to a centre of human population handled extreme heat events better than pristine reefs in a more remote, untouched area.

The research project in the Pacific nation of Kiribati, which straddles the equator and is prone to extreme heat events, looked at two atolls 59 kilometres apart.

'Because of El Niño-Southern Oscillation, which causes ocean temperatures to fluctuate along the equator from year-to-year, these coral reefs experience heat stress more often than reefs in other parts of the world,' says the study’s lead author Sara Cannon, a PhD student at Canada's University of British Columbia’s Institute for the Oceans and Fisheries.

'The reefs in Kiribati could foreshadow how reefs in other places may respond to warmer oceans in the future.'

They found while extreme heat from climate change caused devastating bleaching among the less disturbed coral reefs near the atoll of Abaiang, while the coral reefs near the more populated and polluted waters of Tarawa, the capital of Kiribati, fared far better.

Tarawa's reefs were dominated by a weedy species of coral, Porites rus, which flourishes in waters with high nutrient concentrations from pollution. They found Porites rus was 'growing like dandelions' and could endure both pollution and high water temperatures.

On the other hand, the rich, mixed corals in Abaiang bleached during periods of high ocean temperatures, and the last surviving varieties of corals on the reef were then devoured by a crown of thorns infestations.

The results of the six-year study raises 'sticky questions' about how to move forward to protect coral reefs, notes Cannon.

'Marine protected areas (MPAs) are the most common tool that scientists recommend to protect reefs from human-caused stressors, including climate change,' Cannon says. 'But if MPAs create the conditions where more sensitive corals can thrive—by reducing local disturbances such as fishing or pollution, like in Abaiang—then it could backfire by making those reefs more vulnerable to heat stress.

'An important implication of this study is challenging common dogma in conservation biology, that places with less local threats to wildlife will be better able to resist climate-driven threats,' says Cannon.

'The reality is more complicated than that. Also, even if local stressors make reefs less vulnerable to climate change, we don't want to imply that local threats are good for reefs, because we don't fully understand the potential trade-offs – particularly for people in Kiribati.'

While the variety of coral growing in Tarawa may be able to survive extremes, Cannon notes, questions remain about its usefulness as a fish habitat and its ability to prevent erosion. This is especially important for protecting low-lying atoll countries like Kiribati, vulnerable to rising sea levels. Also, it is unlikely that Tarawa's reefs can ever be restored to their original, robust condition while the reefs on Abaiang could become healthy once again.

The study Coral reefs in the Gilbert Islands of Kiribati: resistance, resilience, and recovery after more than a decade of multiple stressors is published in PLOS One.




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