Assessment and communication of the spatial variability in bleaching severity throughout the Great Barrier Reef following back-to-back bleaching events in 2016 & 2017

Neal Cantin

Led by: Dr Neal Cantin, AIMS


Project Summary

This desktop analysis of existing bleaching observations and data will deliver three key outcomes:

  1. a quantitative assessment of the spatial variability of severe coral bleaching and mortality throughout the Great Barrier Reef (GBR) on regional and within individual reef scales,
  2. a quantitative comparison of bleaching observations from a range of observers AIMS, GBRMPA, JCU Centre of Excellence and the GBRMPA COTS control team (implemented by AMPTO) which used a variety of methods and spatial assessments,
  3. clear science communication and engagement with key stakeholder groups (tourist operators, reef managers and media outlets) in an effort to improve the communication and understanding of the variability and the spatial impacts of coral bleaching throughout the GBR.


Project Publications
Final Report





Project Description


The GBR experienced the worst coral bleaching event in recorded history in early 2016, with an estimated 25–35 % of corals killed in the northern and Far Norther GBR, which repeated in 2017 to extend the zone of severe bleaching further south impacting reefs between Townsville and the Torres Strait Islands (Hughes et al. 2017). Such heat waves will inevitably become more severe and frequent in the future, even if global warming is contained to <2°C, which means the future threat of coral bleaching due to ocean warming will remain a major concern in the coming decade.

Despite the unprecedented spatial impact and scale of both high temperatures and severe coral bleaching observations in 2016 and 2017, conflicting messages throughout the news media, social media outlets and from scientific reports has had direct negative economic impacts on reef stakeholders (eg. Tourist operators) and has reinforced the negative perception of the future of the GBR. This perception could have significant impacts on management actions such as shifting land use practices to mitigate water quality if individuals think these actions are useless and could influence society’s future investment into the protection of the World Heritage listed asset (Eagle et al. 2018). Due to the large spatial scale of the GBR (2000km length, >3000 individual reefs), the variation of bleaching impacts have been observed on local scales within and between reefs across regional scales throughout the marine park. This project seeks to assess all available bleaching observations to provide a proactive, balanced assessment of the severity of the 2016 and 2017 major bleaching events and a clear interpretation of the spatial variability in coral mortality. This is critical to provide clarity in the perception of bleaching impacts and the threat of ocean warming for industries reliant upon the GBR.


The PI Neal Cantin is lead investigator on the NESP TWQ funded project 3.3.1 and has led the AIMS Coral Bleaching Response surveys with the National Coral Bleaching taskforce in 2016 and 2017. As part of these projects, coral bleaching severity and mortality datasets from AIMS (2016 & 2017) and GBRMPA (2016) have been compiled and are being analysed for NESP 3.3.1. David Westcott has access to and is compiling the GBRMPA CoTS Control team bleaching observations and will contribute this data to the proposed project. Further data requests will be made to compile additional observations that are not included in the current datasets.

The project will assess and document the survey methods used by each of the research organizations to determine comparability. Survey methods include both permanent transect based line intercept approaches, quadrat based quantitative scoring, qualitative visual estimates from a range of observers and aerial scores across larger spatial scales.

Initial comparisons of bleaching severity will be made at reef locations that have observations from various observers using different survey methods. These comparisons will evaluate any differences in the conclusions drawn from each set of observations, taking the date of the survey into account in relation to heat exposure (later in the summer will provide the most reliable estimate of outcome) and location. This will also allow us to evaluate the value of combining different datasets for assessments of bleaching severity and the suitability of the various currently used survey methods. Surveys will be assessed within reef if the data is available at multiple sites and across a depth range to include as many deeper reef slope locations in the severity assessment if possible. AIMS in situ temperature loggers at (2 and 7m depths) will be used in conjunction with satellite-based SST products to characterize the heat exposure throughout the entire GBR Marine Park.


NESP 2017 Research Priority Alignment

This project will directly contribute to the NESP Tropical Water Quality Hub research priorities in Theme 1 and Theme 3 by contributing directly to the understanding and documentation of the current status of the Great Barrier Reef, a priority tropical ecosystem for Australia and the World. This project will examine and clarify the spatial impacts of the 2016-17 coral bleaching on the Great Barrier Reef, to improve the scientific and public understanding of the impacts of this widespread, highly publicised disturbance event. Ongoing data processing efforts from NESP 3.3.1 (Cantin) and NESP 4.2 (Steinberg) will increase the efficiency of the analysis and interpretation of this project. Ongoing work assessing the oceanographic drivers of heat stress in NESP 4.2 will contribute to the assessment and interpretation of the spatial variability in bleaching response in this proposed project.


Project Funding

This project is jointly funded through AIMS and the Australian Government’s National Environmental Science Program.