Growth of Western Australian Corals in the Anthropocene

Heat or Acid? The question of how tropical coral reefs will respond to increasing atmospheric greenhouse gas concentrations and concomitant climate change is widely debated. Model predictions and laboratory experiments suggest that decreasing carbonate saturation and decreasing pH may reduce calcification in carbonate-depositing organisms, including corals, yet field data are sparse, and recent declines in coral growth rates have been variously attributed to thermal stress or ocean acidification. Cooper et al. (p. 593) demonstrate that there has been no large-scale decline in calcification rates of massive Porites on coral reefs along the Indian Ocean coast of Western Australia. Instead, coral growth has increased significantly in the past 110 years, particularly at high latitudes. Thus, coral calcification appears to increase as ocean waters warm, but—at excessive temperatures—coral bleaching and reduced ocean carbonate saturation may lead to growth declines as observed on the Great Barrier Reef. Cores taken from massive corals indicate that temperature rather than ocean acidification has governed reef growth. Anthropogenic increases of atmospheric carbon dioxide lead to warmer sea surface temperatures and altered ocean chemistry. Experimental evidence suggests that coral calcification decreases as aragonite saturation drops but increases as temperatures rise toward thresholds optimal for coral growth. In situ studies have documented alarming recent declines in calcification rates on several tropical coral reef ecosystems. We show there is no widespread pattern of consistent decline in calcification rates of massive Porites during the 20th century on reefs spanning an 11° latitudinal range in the southeast Indian Ocean off Western Australia. Increasing calcification rates on the high-latitude reefs contrast with the downward trajectory reported for corals on Australia’s Great Barrier Reef and provide additional evidence that recent changes in coral calcification are responses to temperature rather than ocean acidification.

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