Doi.org/10.1038/s41467-018-05741-0

Target Article

Frade, P.R., Bongaerts, P., Englebert, N., Rogers, A., Gonzalez-Rivero, M., Hoegh-Guldberg, O. (2018). Deep reefs of the Great Barrier Reef offer limited thermal refuge during mass coral bleaching. Nature Communications 9, article number: 3447. Available at: https://www.nature.com/articles/s41467-018-05741-0 doi: doi.org/10.1038/s41467-018-05741-0

Significant Claim

Frade et al., (2018) reports on a survey of deep-water corals that were affected by the mass coral bleaching event of 2016, which Hughes et al., (2017) claimed to have killed up to around 30% of shallow-water coral on the Great Barrier Reef (GBR). In this previous study, Hughes et al., (2017) used results only of coral with water depth at low tide of less than 2 m. Frade et al., (2018) on the other hand considered a far more representative group of corals to depths of between 5 and 40 m, consequently, providing a more meaningful result than that by Hughes. Furthermore, Frande et al., (2018) found the mortality to be around 10% for all corals in the far Northern GBR where the bleaching was worst, and where often 50% of the surface corals died.

Possible caveat with this article

The results of Frade et al. (2018) were far more comprehensive than Hughes et al., (2017), but the implications of this work regarding the estimation of the bleached fraction of the GBR is missing. This work indicates that the 2016 bleaching event was much less serious than previously reported by Hughes et al. (2017), as explained below.

The level of bleaching for the deep-water of the Northern GBR was around only 10%, therefore, it is reasonable to assume that the deep-water corals for the Central and Southern GBR, which are approximately between 16.5S and 24.5S, would have faced close to zero mortality, since water temperature in these latitudes is commonly lower than that in the Northern GBR. Thus, taking the areas of these three regions into account, the total mortality for the entire GBR for the corals between 5 m and 40 m is around 3%. Surprisingly, figures are not available for the relative proportion of corals at different water depths, but if we assume that shallow-water corals represent 20% of all GBR coral (with 30% mortality) and the deeper corals representing 80% of all GBR coral (with 3% mortality), then it is logical to say that the total mortality for the entire GBR was about 8%. This is a better estimation of coral mortality during the bleaching event in 2016 than the 20% assumed by Frade et al., (2018).

Considering the ability of corals to recover a drop in cover of 8% may not be regarded as cataclysmic. For example, the Southern GBR increased coral cover by 250% in just 6 years between 2011 and 2017 following massive coral loss after cyclones Yasi and Hamish (https://www.aims.gov.au/reef-monitoring/gbr-condition-summary-2017-2018).

Comments

In WL-Letters you have the freedom to respond to any caveat that may be inaccurate.
-- Andutta (talk) 18:12, 6 July 2022 (UTC)