Coral bleaching is a stress response associated with hot, calm, and clear conditions when SST passes an ecological stress threshold, accompanied by limited wind-driven mixing and cooling of the ocean, and high light exposure. Coral cover and bleaching data are scarce for the Pitcairn Islands, but local evidence suggests that there have been minor impacts due to heat stress. Cold-water intrusions around the Pitcairn Islands were recorded in 1972 and 2016 and can cause an effect similar to bleaching, however the process behind this thermal stress response is less well understood.
Ocean acidification negatively affects calcification, accelerating the dissolution of coral skeletons thus weakening skeletons and triggering stress-responses. This stress-response affects the rate of tissue repair, feeding, reproduction, growth, and early life-stage survival. It is likely that the response of corals to more acidic waters in the Pacific are not yet outside natural variability and may be more influenced by local conditions and mechanisms such as circulation patterns and pollution.
In the tropical Pacific, the proportion of intense versus weaker cyclones has increased substantially in the last 40 years. Deep ocean swells from extra-tropical cyclones could be affecting Pitcairn Islands’ reefs but no assessment on these impacts has been conducted for the region.
Medium evidence, high agreement
Confidence in the data is medium due to the paucity of data and limited monitoring. However global and regional observations provide useful insight on the response of reefs and their associated communities.
Coral reefs in the Pitcairn Islands grow in deeper, cooler waters south of the western Pacific warm pool so relatively low heat stress is expected over the rest of the century. Lower heat stress means that corals are less at risk from thermal bleaching than their Pacific counterparts. However, by 2100, the combined effects of ocean warming and acidification could still decrease coral cover by over 50% in the Pitcairn Islands. There may be additional risks from cold water stress causing declines in reef condition.
Decreasing ocean pH would result in marginal calcification conditions throughout most of the Pacific region, threatening corals and other calcifying organisms. However, there is likely to be substantial variability across reefs due to local conditions and species composition. Ocean acidification levels projected by 2100 may also influence critical behaviours of reef fish.
Coastal inundation could provide extra ‘space’ because Pitcairn Islands’ reefs are relatively deep and corals grow vertically. However, the current rate of sea level rise is accelerating and there is a risk that corals will not be able to ‘keep pace’.
Medium evidence, medium agreement
Assessments of future reef condition are not possible due to the paucity of data. Projections are also hampered by a lack of evidence on geographical range shifts of calcifying species.