Decades of Coral Reef Degradation and the Climate Catastrophe

Abstract

Photo 1. Seringapatam reef, within the Scott system of reefs, rises from hundreds of meters depth near the edge of northwest Australia’s continental shelf. Having escaped the chronic pressures degrading other reefs around the world, these oceanic reefs are increasingly affected by recurrent heat stress and coral bleaching. Photo credit: Nick Thake. Photo 2. During the first global coral bleaching event in 1998, coral cover across the Scott reefs was reduced by ~80%. Given their isolation and the lack of larval connectivity to other reefs in the region, recovery was predicted take decades. Photo credit: Luke Smith. Photo 3. Twelve years after (2010) the 1998 mass bleaching, the Scott reefs had largely returned to their previous cover and diversity. Recovery was faster than expected due to high growth and survival of coral recruits, thanks to healthy fish stocks, good water quality, and the lack of heat stress and coral bleaching for more than a decade. However, not all coral groups had fully recovered and ocean temperatures had continued to increase. Photo credit: James Gilmour. Photo 4. Following moderate bleaching in 2010, 2011, and 2013, the Scott reefs were again affected by extreme heat stress and mass bleaching in 2016. Most of the branching corals that provided structure critical to fish and other reef-associated organisms were again lost, with smaller reductions in the more massive and encrusting corals. Photo credit: James Gilmour. Photo 5. Five years (2021) after the 2016 mass bleaching, coral recruits were evident at many sites. However, the corals that had not fully recovered from the 1998 mass bleaching were absent from most sites and many of the new recruits were “weedy” species that are more susceptible to ongoing temperature increases. Indeed, patchy bleaching was observed in 2021 and recurrent heat stress had occurred during most recent years (2017, 2019, and 2020). As the frequency of mass bleaching increases, and the window for recovery decreases, even our healthiest coral reefs will transition to a chronically degraded state. Photo credit: James Gilmour. These photographs illustrate the article “A tale of two reef systems: local conditions, disturbances, coral life histories, and the climate catastrophe” by James P. Gilmour, Kylie L. Cook, Nicole M. Ryan, Marjetta L. Puotinen, Rebecca H. Green and Andrew J. Heyward published in Ecological Applications. https://doi.org/10.1002/eap.2509.