Plenary Session: Predicting the future of coral reefs in a high-CO2 world

Katharina Fabricius

Australian Institute of Marine Science, Townsville, Qld 4810, Australia

Background
Tropical and subtropical coral reefs are the most biodiverse marine ecosystems on earth. Their future integrity, and that of the hundreds of thousands of species associated with coral reefs, depends on (1) the physiological capacity of these species to deal with ocean acidification (OA), and (2) the flow-on effects of the multitude of ecological changes likely to be caused by OA.

Methods
To date, most knowledge about ecological responses of reefs to OA has been derived from field studies around carbon dioxide seeps and from mesocosm experiments. I will (1) review some of the known and predicted principles about direct and indirect OA effects on coral reefs, (2) present new results from these studies, and (3) outline the most significant gaps in our knowledge.

Findings
Field and mesocosm studies have shown that ecological shifts from corals to seaweed, and increasing bioerosion, are only some of the many widespread consequences of OA. Data suggest that functional losses will progressively worsen with increasing levels of atmospheric CO2. Additionally, acclimatisation by corals to high CO2 is rarely observed today, and it has not occurred in the geological past. However, the OA effects on coral reefs are so complex that predicting the key functional bottlenecks in a high-CO2 world will remain a key challenge in the future.

Conclusions
More than 500 million people depend on healthy coral reefs for their livelihoods, as do many hundreds of thousands of marine species. OA will be irreversible on time scales of thousands of years, and is causing major upheaval for coral reefs. These facts alone make it a moral, ecological and economic imperative to restrict future increases in atmospheric CO2 beyond present day levels.