Climate change impacts on corals in the UK overseas territories of BIOT and the Pitcairn Islands

BIOT

The British Indian Ocean Territory (BIOT) consists of five atolls of low-lying islands, including the largest atoll in the world, Great Chagos Bank, and a number of submerged atolls and banks. Diego Garcia is the only inhabited island. The BIOT Marine Protected Area (MPA) was designatedin 2010. It covers the entire maritime zone and coastal waters, an approximate area of 640,000 km2. The marine environment is rich and diverse, attracting sea birds, sharks, cetaceans and sea turtles and with extensive seagrass and coral reef habitats. It includes the endangered Chagos brain coral (Ctenella chagius), an endemic massive coral unique to BIOT. BIOT reefs have suffered extensive bleaching and mortality, and they remain vulnerable to current and future climate change and other pressures, including:

Bleaching
The heavy mortality has been caused by recurrent marine heatwaves since the 1970s. Reefs have not yet recovered from the most severe bleaching in 2016 and 2017, with increasingly severe events expected. Deeper fore-reefs may act as refuges, but those colonies are likely to be more sensitive to temperature change. Limiting other pressures will not guarantee resilience to future bleaching.

Ocean acidification
There has been a low impact of ocean acidification on coral reefs so far, but when combined with future bleaching therisk of decalcification and erosion will increase. Under high emissions scenarios, BIOT is projected to become less suitable for corals by the end of the century.

Changes in sea level and extreme weather events

In BIOT, sea level is rising twice as fast as the global average. Extreme sea levels appear related to El Niño or La Niña events. When combined with bleaching and acidification sea level rise will reduce the effectiveness of reefs to perform as breakwaters. BIOT has experienced considerable shoreline erosion, which suggests loss of breakwater effects from protective fringing reefs.

Other human pressures
Rats have caused a crash of seabird populations, disrupting guano nutrient flows to the detriment of some reef organisms including sponges and corals. Around Diego Garcia, small scale fishing is allowed to residents but illegal, unreported and unregulated (IUU) fishing is also known to occur, and there are concerns of pollution and anchor damage within the lagoon.

The Pitcairn Islands (Pitcairn, Oeno, Henderson, and Ducie) comprise a volcanic outcrop of four small islands in the Southern Pacific Ocean. Pitcairn is the only inhabited island.

The Pitcairn Islands MPA was established in 2016 and covers 841,910 km2. The marine environment is less varied in terms of species richness than BIOT, due to its isolation, and includes fewer endemics. Coral reefs are abundant, particularly around Ducie Island, are found at deeper waters than usual due to exceptional water clarity, and are dominated by massive resistant species as well as the more sensitive cauliflower and soft fire corals. While the location of the Pitcairn Islands has provided a relative shelter from the effects of warming, these sensitive corals are vulnerable to expected future climate impacts and other pressures:

Bleaching
No evidence of past bleaching has been found, and no change in average sea surface temperature (SST) has been detected. Low levels of thermal stress are expected in the future. The South Pacific Circulation Gyre may slow down future warming, and the deeper reefs may be relatively sheltered from future climate change. However, these corals may be more sensitive if stress occurs, and their isolation may challenge their recovery. Monitoring is needed to ensure any future bleaching is recorded.

Ocean acidification
Saturation state of calcium carbonate-based minerals is reaching the point where coral skeletons weaken and growth slows down. Low pH is changing the composition of colonies, favouring algal cover and encouraging bioerosion. Even under low emissions scenarios, saturation of calcium carbonate based minerals and pH levels are unlikely to support net coral calcification in this island group by the end of century.

Changes in sea level and extreme weather events
Sea level has risen faster here than the global average, although island margins appear to have adjusted, however, as sea level rise accelerates it is unclear whether these islands will persist. Corals have suffered damage from waves around Henderson and cold water intrusions at Ducie. Heavy rainfall on Pitcairn has caused sedimentation and turbidity although strong prevailing currents seem to mitigate this risk.

Other human pressures
There are concerns around overfishing, and Henderson Island has some of the highest levels of plastic pollution on the planet. Cruise ships and sediment runoff are causing damage to the seabed and the corals. Crown of thorns starfish may be present around Pitcairn. There are also rats on Henderson Island, which reduce numbers of seabirds with potential negative effects on surrounding reefs through the loss of guano nutrients, as it has been observed in BIOT.

Lincoln S., Cowburn B., Howes E., Birchenough S., Pinnegar J., Dye S., Buckley P., Engelhard G. E. & Townhill B., 2021. Climate change impacts on corals in the UK Overseas Territories of BIOT and the Pitcairn Islands. BlueBelt Report Card, UK Blue Belt Programme. Full report.

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