Resilience of Central Pacific reefs subject to frequent heat stress and human disturbance

Abstract

Frequent occurrences of coral bleaching and associated coral mortality over recent decades have raised concerns about the survival of coral reefs in a warming planet. The El Niño-influenced coral reefs in the central Gilbert Islands of the Republic of Kiribati, which experience years with prolonged heat stress more frequently than 99% of the world’s reefs, may serve as a natural model for coral community response to frequent heat stress. Here we use nine years of survey data (2004–2012) and a suite of remote sensing variables from sites along gradients of climate variability and human disturbance in the region to evaluate the drivers of coral community response to, and recovery from, multiple heat stress events. The results indicate that the extent of bleaching was limited during the 2009–2010 El Niño event, in contrast to a similar 2004–2005 event, and was correlated with incoming light and historical temperature variability, rather than heat stress. Spatial and temporal patterns in benthic cover suggest growing resistance to bleaching-level heat stress among coral communities subject to high inter-annual temperature variability and local disturbance, due to the spread of “weedy” and temperature-tolerant species (e.g., Porites rus ) and the cloudy conditions in the region during El Niño events.

Large geographic variability in the resistance of corals to thermal stress

“…Differential rates and clearly identified causes of mortality and recovery among taxa create challenges for large‐scale evaluations of climate impacts (Darling et al, 2019; McClanahan et al, 2001). Estimating mortality requires inter‐annual monitoring to evaluate changes that could be poorly tied to heat stress alone (Darling et al, 2013; Donner & Carilli, 2019). While we acknowledge this weakness, bleaching is currently the most commonly used and quantifiable way to measure sensitivity to heat stress (Donner et al, 2017; Sully et al, 2019).…”

“…The CTA model is based on the concept of cumulative degree‐heating weeks or months, or the amount of excess temperatures above a summer baseline, and is the most commonly used metric to assess the probability of coral bleaching and the future state of coral reefs (Donner & Carilli, 2019; Eakin et al, 2010; Van Hooidonk et al, 2013). To estimate CTAs, we extracted daily SST time series for each site from the 5‐km NOAA Coral Reef Watch version 3.1 products between 1985 and 2015, available from the NOAA website (https://coralreefwatch.noaa.gov/product/5km/).…”

Extreme temperature events will drive coral decline in the Coral Triangle

“…In fact, a modeling study projecting future coral bleaching (Logan, Dunne, Eakin, & Donner, ) found that calculations based on the previous 60 years (a rolling window that approximates genetic adaptation) decreased predicted bleaching frequencies by up to 80% when compared to calculations based on a static window. Furthermore, there is evidence that temperature variability is beneficial to coral TST: past exposure to a greater range of temperatures may lead to increased thermotolerance in some coral species (Bay & Palumbi, ; Carilli, Donner, & Hartmann, ; Mayfield, Chan, Putnam, Chen, & Fan, ; Putnam, Barott, Ainsworth, & Gates, ; Rivest, Comeau, & Cornwall, ) and coral communities as a whole (Ainsworth et al, ; Donner, ; Donner & Carilli, ; Guest et al, ; Hughes et al, ; McClanahan & Maina, ; Sully, Burkepile, Donovan, Hodgson, & Woesik, ; Thompson & van Woesik, ).…”

“…This implies that there is higher vulnerability to regional-scale disturbance at locations that rely heavily on external recruitment. coral communities as a whole (Ainsworth et al, 2016;Donner, 2011;Donner & Carilli, 2019;Guest et al, 2012;Hughes et al, 2019;McClanahan & Maina, 2003;Sully, Burkepile, Donovan, Hodgson, & Woesik, 2019;Thompson & van Woesik, 2009).…”

Higher thermal resistance of corals in the global marine biodiversity center

“…We considered two exposure models, one based on a single variable of excess temperature exposure (cumulative degree heating weeks, the CTA model) and one based on a multivariate environmental index of climate exposure (CE) that integrates temperature, winds and water flow into a weighted metric of exposure. The CTA model is based on the concept of cumulative degree heating weeks or months, or the amount of excess temperatures above a summer baseline, and is the most commonly used metric to assess the probability of coral bleaching and the future state of coral reefs (Eakin et al 2010;van Hooidonk et al 2013;Donner & Carilli 2019). For each site, we extracted daily SST time series from the 5-km NOAA Coral Reef Watch version 3.1 products between 1985 and 2015, available from the NOAA website (https://coralreefwatch.noaa.gov/satellite/bleaching5km/index.php).…”