Decadal Trends in Community Structure of the Coral Reefs of the Chagos Archipelago. John Turner1, Charles Sheppard2, Ronan Roche1, Anne Sheppard2, Courtney Couch3

1Bangor University, 2University of Warwick, 3University of Hawai’I Manoa

The remote reefs of the Chagos Archipelago serve as an important reference site, providing a benchmark for reversing damaged ecosystems elsewhere, and allow climate change impact to be assessed in the absence of local anthropogenic impacts. Coral communities have been subjected to extreme warming events in 1997, 2015 and 2016, and occasional COT outbreaks (eg. in 2013). Since 2006, visual and video assessments at permanent monitoring sites across the atolls have established an archive of community structure state. Northern atolls have exhibited higher coral cover (~40-50%) than the Southern atolls (~30%) with increasing algal and soft coral cover with depth, and lagoons exhibiting greater hard coral cover than seaward reefs at all depths. Seaward coral reefs were dominated by Acropora tables, most having established since the 1997 ENSO event, but by 2013, senescence was apparent in the largest of these and by 2015 many had collapsed. Acropora White Syndrome (AWS) was identified in 2014 and increased in prevalence by 2015. Reassessment of archived video from 2006 confirmed that AWS was a new threat to the reefs of Chagos despite their relative isolation. Assessments of coral recruits (~40% being tabular Acropora species) indicate that the capacity for recovery has been good, although many are lost when dead coral substrate is removed from shallow reefs by storms. The long term resilience of the Chagos Archipelago reefs was severely tested by two consecutive warming events in 2015 and 2016, causing bleaching induced mortality in most species except Porites to at least 15 m depth, with a significant reduction in cover and structure as the reefs entered an erosional state.