Recovery of coral growth rates and reef carbonate budget after severe bleaching events at Lizard Island
Corals are the building blocks of remarkably diverse ecosystems, housing thousands of reef fish and associated organisms, but are extremely sensitive to anthropogenic stressors, such as ocean warming. The corals at Lizard Island experienced heat stress in unprecedented back-to-back bleaching events during 2016 and 2017 (Hughes et al. 2018) causing widespread mortality. Unfortunately another heat stress event impacted the GBR in 2020 as well. Coral reefs are resilient and can recover if given enough time without stress, but this is dependent on the growth and reproduction of remaining corals, and settlement of larvae from other reefs. The aim of this study was to use non-invasive sampling techniques to monitor coral health, growth, population turn over, and reef carbonate budgets at Lizard Island from 2018-19 and 2019-20).
Field work was conducted in April 2018, March 2019 and March 2020. This allows for two years of comparable data on recovery and population turn over (2018-19, 2019-20). At each site (Palfrey, Watsons, Lizard head) permanent stakes were driven into the reef to allow for relocation of the site. At each site, the ReefBudget methodology (Perry et al. 2015) was employed to determine carbonate production over time at each site. At Watsons and Palfrey, five 10m x 1m Belt transects were set up with each quadrant (1 x 1 m) being photographed for juvenile population turnover, surface area and coral health.
A total of 840 corals were photographed and quantified in 18 Families and 38 Genera. In 2018 and 2019 over 90% of corals were in the health categories of 4+ that are heavily pigmented. During the 2020 heat stress event, 20% of corals had health scores of less than 3 signifying fluorescing and bleaching (see Fig. 1). At this time, 50% of Acroporidae encounter had a bleaching score of 3 or less compared to 15% of the more thermotolerant Poritidae in 2020.
The mortality of individual colonies was greater than recruitment at our sites. 14% (91/669) of all individuals tracked from 2018 to 2019 succumbed to mortality after a year however there was 8% recruitment (0.54 individuals/m2). By 2020, 9% of those new recruits died and a further 13% of all individuals died, but there was 6% recruitment (0.33 individuals/m2). Therefore, total mortality was 24% with only 13% recruitment of coral colonies over the two years.
Carbonate production was greatest at Lizard head, a P. cylindrica dominated site (3.1-4.5 kg CaCO3/mr/yr) compared to Palfrey (0.19-0.30 kg CaCO3/mr/yr) and Watsons (1.1-1.8 kg CaCO3/mr/yr) where there was more sand and rubble patches. Lizard head carbonate production stayed relatively similar through the years however Watsons and Palfrey rates decreased from 2018 to 2019 and 2020 (Fig. 2).
The majority of corals at Lizard Island were healthy, however, mortality of individual colonies is greater than successful recruitment. If this imbalance continues then the reefs around Lizard Island may turn to a state of net erosion over carbonate production, sooner than anticipated. Especially if unprecedented rates of heat stress continue to impact the reefs and there is a limited window for coral recovery.
I would like to thank the sponsors of the Isobel Bennett Marine Biology Fellowship, which was awarded through the Lizard Island Reef Research Foundation, for their continued support in this project. Only with continued monitoring of corals and the impacts due to bleaching will we be able ascertain the long-term recovery projections of this beautiful place.
References
Hughes TP, Kerry JT, Baird AH, Connolly SR, Dietzel A, Eakin CM, Heron SF, Hoey AS, Hoogenboom MO, Liu G, McWilliam MJ, Pears RJ, Pratchett MS, Skirving WJ, Stella JS, Torda G (2018) Global warming transforms coral reef assemblages. Nature 556:492-496
Perry, C. T., Steneck, R. S., Murphy, G. N., Kench, P. S., Edinger, E. N., Smithers, S. G., & Mumby, P. J. (2015). Regional-scale dominance of non-framework building corals on Caribbean reefs affects carbonate production and future reef growth. Global Change Biology, 21(3), 1153-1164. doi:doi:10.1111/gcb.12792
Dr. Kristen Anderson-King
Ed: Kristen was the 2018 recipient of the Isobel Bennett Marine Biology Fellowship, funded by the Hermon Slade Raiatea Foundation. This postdoctoral fellowship is named in recognition of the late Dr Isobel Bennett, one of Australia’s eminent marine biologists. Kristen is a research fellow at JCU’s ARC Centre of Excellence for Coral Reef Studies and has previously undertaken research at Lizard.