Patterns of coral reef resilience at Aldabra Atoll, Seychelles

Abstract

Climate change-induced coral bleaching events are increasing in frequency and severity, threatening the persistence of coral reefs worldwide. With diminishing time frames for recovery between such events, improving our understanding of coral reef resilience is critical. Remote, well protected coral reefs such as those at Aldabra Atoll (Seychelles) serve as unique natural laboratories to study reef resilience in absence of direct anthropogenic stressors like eutrophication and overfishing. Using spatiotemporal monitoring data of benthic and fish communities in combination with in-situ water temperature measurements, this thesis assesses how Aldabra’s coral reef system responded to and recovered from the 2014–2017 global coral bleaching event. Immediately following the bleaching, hard coral cover decreased by 51–62% on Aldabra’s seaward reefs at shallow (5 m) and deep (15 m) water depths, but inside its semi-enclosed lagoon (< 3 m water depth), coral mortality was lower (34%), likely due to three-fold higher daily water temperature variability there. Primarily driven by herbivores, Aldabra’s fish abundance increased by >120% following the bleaching event. Although there were substantial fluctuations in fish abundance and biomass throughout the study period, pre- and post-bleaching values were similar. During the four years post-bleaching (2016–2019), hard coral cover on the deeper seaward reefs did not change; turf algae and the calcifying macroalgae Halimeda remained the dominant taxa there. In shallow water, however, hard coral cover in 2019 reached 54–68% (seaward reefs) and 93% (lagoon) of pre-bleaching coral cover. Independent of water depth, coral juvenile abundance more than doubled over the period 2016–2019 at all locations, whilst coral larvae settlement was one order of magnitude higher inside the lagoon compared to the seaward reef. The results of this thesis highlight the substantial natural variation in bleaching susceptibility and recovery driven by environmental conditions. High daily water temperature variability and water flow in Aldabra’s lagoon is likely to confer lower bleaching-induced mortality. As the early recovery of hard coral cover was predominantly driven by the growth of remnant colonies, slower recovery rates on the seaward reefs are likely due to slower coral growth at depth and higher wave exposure. The rapid increase in coral juvenile abundance indicate that reef recovery will speed up substantially also on Aldabra’s seaward reefs. These recovery trajectories are likely to be connected to the high abundance of herbivores at Aldabra, which limit turf algae cover, thus promoting coral growth and the survival of coral juveniles. While coral reefs at remote, strictly protected locations may not be less susceptible to bleaching-induced hard coral mortality, post-bleaching recovery can be rapid in absence of local anthropogenic stressors. Nevertheless, while local management of coral reef systems is crucial to support their recovery from mass coral bleaching events, immediate action to tackle the root cause of the climate crisis is indispensable to preserve even the world’s most strictly protected and remote coral reefs.