Spatial variability in bleaching severity on the Great Barrier Reef during 2016 & 2017 (NESP TWQ 5.7, NESP TWQ 4.2 and NESP TWQ 3.3.1, AIMS)

This dataset is a compilation of available ocean temperature data, aerial and in-water bleaching observations during the 2016 and 2017 bleaching events on the Great Barrier Reef in order to estimate the total reef area impacted by coral bleaching and thermal heat stress. A total of 982 reefs (56.8% reef area of the GBR) were surveyed in 2016 and 781 reefs (50.9% reef area of the GBR) surveyed in 2017.
This analysis provides an improved understanding of the variability and the increase in spatial impacts from coral bleaching throughout the GBR in a warming ocean. This data compilation of available bleaching survey data was used to provide:
i) a quantitative assessment of the spatial variability of accumulated heat stress, severe coral bleaching and mortality throughout the GBR on regional and within individual reef scales, in comparison with previous widespread bleaching events in 1998 and 2002;
ii) a quantitative comparison of bleaching observations from a range of observers which used a variety of methods and spatial assessments.

The recent heat waves in 2016 and 2017 were unprecedented for the GBR. Temperatures remained above historical summer maximums for more than three months, and maximum anomalies in some locations were as high as 2.9°C (2016) and 3.2°C (2017) above the recent historical average summer maximum (1985-2012).

During the 2016 - 2017 bleaching events, both coral bleaching and coral mortality occurred at a lower level of accumulated heat stress than previously proposed by the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) Degree Heating Week (DHW) product. The combined data recorded widespread coral bleaching occurring at a NOAA DHW value of 2-4 degree-weeks, severe bleaching, and some coral mortality at 5-8 DHW’s, and extensive mortality at reefs exposed to more than 8 DHW’s.

Prior to this event, the general guideline based upon previous observations was that coral bleaching would occur at a DHW >4 and that coral mortality would begin at a DHW >8. In 2016 and 2017 on the GBR at heat stress exposures of 2.5°C-weeks we observed coral bleaching, after 5°C-weeks severe bleaching and some mortality and severe mortality following 8°C-weeks.

Methods:
All available aerial and in-water bleaching observations were assessed to quantify the relationship between coral bleaching and accumulated heat stress from ocean SST products and bleaching severity and coral mortality.

Sea Surface Temperature
Data was sourced from SSTAARS Climatology, NOAA-CRW Degree Heating Week (DHW).
Broad spatial coverage of satellite Sea Surface Temperature (SST) monitoring products (NOAA Coral Reef Watch) were combined with the established relationship of observed bleaching severity with the accumulated heat stress product NOAA Degree Heating Weeks (DHWs), to scale up estimates of bleaching to the whole GBR.

The NOAA Degree Heating Week product is a thermal stress index which combines the intensity of the temperature anomaly (Hotspot) at least 1°C above the historical summer Maximum Monthly Mean (MMM, Version 3.1) temperature (upper thermal threshold) with the duration of time exceeding this threshold over a 12 week period.

Coral Bleaching Surveys Aerial
Data was sourced from:

Aerial – Berkelmans 1998 - Aerial survey of the GBR during the 1998 bleaching event [Region: Whole of GBR, Dates: 1998-03-09, Number of sites: 495 reefs]. AIMS

Aerial – Berkelmans 2002- Aerial survey of the GBR during the 2002 bleaching event [Region: Whole of GBR, Dates: 2002-03-20, Number of sites: 1295 reefs]. AIMS

Aerial - Hughes 2016 – Aerial survey of the GBR during the 2016 bleaching event [Region: Whole of GBR, Dates: 2016-03-22/2016-04-28, Number of sites: 924 reefs]. JCU and TSRA

Aerial - Hughes 2017 – Aerial survey of the GBR during the 2017 bleaching event [Region: Whole of GBR, Dates: 2017-03-15 / 2017-04-05, Number of sites: 725 reefs]. JCU

Aerial – AIMS 2017 - Aerial survey of the GBR during the 2017 bleaching event [Region: Townsville and Cairns, Dates: 2017-03-09, Number of sites: 70 reefs]. AIMS and GBRMPA

Data handling:
The data from Berkelmans 1998 and Berkelmans 2002 was presented with five-point rating (1-5) but converted to align with the Hughes dataset format (Category 0-4).
Berkelmans Hughes
5 (<1% bleached) = 0
4 (1–10% bleached) = 1
3 (10–30% bleached ) = 2
2 (30–60% bleached) = 3
1 (>60% bleached) = 4

Methods:
Aerial Bleaching Surveys: conducted from a combination of light fixed wing aircraft and helicopter, flying at an elevation of approximately 150m. This method accurately captures the proportion of healthy and bleached coral colonies in the shallow reef flat (0-3m) and in the upper reef slope (0-6m). Reefs were scored independently (isolated communications) by 2-4 observers looking out the left and right windows of the aircraft. Bleaching scores were recorded on spatial GBRMPA zoning reef maps, with mapping handheld GPS devices recording the flight path and confirming the reef location during the flight (Berkelmans & Oliver 1999; Berkelmans et al. 2004; Hughes et al. 2017).
Surveys in 1998 and 2002, did not have GPS recorders. Reefs were categorised by visual assessment based upon the proportion of living coral visible to the observer that appeared bleached, severely bleached white, or fluorescent, into five community bleaching severity categories:
0 (no bleaching) <1%, 1 (moderate) 1-10%, 2 (high) 10-30%, 3 (very high) 30-60%, 4 (extreme bleaching) > 60%;

These categories were used to align with the protocols developed in 1998 by Ray Berkelmans and GBRMPA (Berkelmans & Oliver 1999; Berkelmans et al. 2004). Photographs of each categorised reef in 2016 were recorded, stored, geo-located with GPS locations and made available to the public through the JCU Centre of Excellence for Reef Studies website (https://www.coralcoe.org.au/coral-bleaching-map). Images from the GBRMPA flight in 2017 between Townsville and Cairns will be available through the eAtlas.

Coral Bleaching Surveys In-water:

Sectors:
• Far North (9-14°S; including the Torres Strait) • Northern (14-18°S; Innisfail to Cooktown) • Central (18-21°S; Mackay to Innisfail) • Southern sector (21-24°S; Mackay to Gladstone).

AIMS 2016 - In water survey of reefs in the Townsville-Cairns area in 2016 [Region: Central and Northern sectors, Method: in water – 1m belt Transect, Dates: 2016-03-02 / 2016-04-06, Number of sites: 27 reefs] Reported metrics: Bleaching categories 1-4, % Mortality, % Bleaching

AIMS 2017 - In water survey of reefs in the Townsville-Cairns area in 2017 [Region: Central and Northern sectors, Method: in water – 1m belt Transect, Dates: 2017-03-14 / 2017-04-01, Number of sites: 19 reefs] Reported metrics: Bleaching categories 1-4, % Mortality, % Bleaching

GBRMPA 2016 - In water survey of reefs in the GBR area in 2016 [Region: all sectors, Method: in water – RHIS, Dates: 2016-03-02 / 2016-06-11, Number of sites: 102 reefs] Reported metrics: Bleaching categories 1-4, % Mortality, % Bleaching

GBRMPA EotR 2015-2017- Eye on the Reef CoTS Control team observers in water survey of reefs in the GBR area in 2016/2017 [Region: all sectors, Method: in water – RHIS (5m radius), Dates: 2016-02-01 / 2017-04-30, Number of sites: 1200 reefs] Reported metrics: Bleaching percent by coral type, Mortality percent by coral type, % Bleaching, % Mortality

Frade 2016 - Local survey up to 40m depth to test depth as refuge for bleaching. [Region: Far North and Northern sectors, Method: in water – 1m belt Transect, Dates: 2016-05-14/23, Number of sites: 6 reefs / 9 sites] Reported metrics: Bleaching categories 1-4. University of Queensland Global Change Institute.

Baird 2016 - Local survey up to 27m depth to test depth as refuge for bleaching.[Region Far North, Method: in water – 1m belt Transect, Dates: 2016-04-11/14, Number of sites: 7 reefs / 11 sites] Reported metrics: Bleaching categories 1-6 to genus level coral ID

Methods:

In-water quantitative transect based surveys conducted by AIMS: In-water surveys were conducted between Townsville and Port Douglas by the AIMS at 54 sites within 19 reefs in 2016 and 40 sites within 12 reefs in 2017, from 13 March – 6 April 2016 and from 14 March – 1 April 2017. Surveys were conducted following the onset of maximum temperature exposure to capture the bleaching response at its peak, with an additional 0 – 2.9 DHW accumulating following the time of the survey. Follow up surveys were conducted in June 2016, September 2016 and September 2017 to assess survivorship. For each reef, surveys were conducted at three habitats: (1) shallow, sheltered reef flat at 2m; (2) exposed shallow reef slope at 3m; and (3) deeper exposed reef slope at 7-9m along permanent AIMS Long-term monitoring sites or equivalent on the northern exposed flank of each reef. Scleractinian hard corals >5cm in diameter were identified to genus level and bleaching severity categorised as: (1) healthy; (3) minor-moderate bleaching (paling/ 1-50%; (4) major bleaching (51-90% bleached); severe bleaching (100% white) and (6) recently dead (full colony or partial sections of the colony).

In water surveys conducted by the University of Queensland Global Change Institute (Frade et al. 2018) were conducted between 14 - 23 May 2016 at nine sites within six outer shelf reefs in the Northern Sector of the GBR, at a depth of 5, 10, 25 and 40m, in order to determine the effect of bleaching over a large depth range. Surveys were conducted along 1m belt transects covering 75m length at each depth. Scleractinian hard corals >10cm in diameter were identified to genus level and bleaching severity categorised as: (1) healthy; (2) minor bleaching (paling/ 1-50%; (3) severely bleached (51-100% bleached) and (4) recently dead.

In-water qualitative Reef Health Index Surveys (RHIS): In water surveys conducted by the GBRMPA field management team in 2016 were conducted at a total of 62 reefs across seven inshore-offshore transects following guidelines outlined in the Coral Bleaching Risk and Impact Assessment Plan (GBRMPA 2011). The survey plan aimed to conduct 15 or more Reef Health Index Surveys (RHIS) (that is, replicate samples) across three different locations on each reef, corresponding to three different aspects (north-east, north-west and south-west). At each of the western locations (i.e. north-west and south-west), three RHIS surveys were conducted at the same depth (approximately one to four metres), making a total of six RHIS. At the north-eastern location, three surveys were conducted at three different depths (approximately one to three metres, six metres and nine metres), making a total of nine RHIS. Surveys followed the standard protocol for Reef Health Impact Surveys (RHIS) (Beeden et al. 2014). Key information recorded includes:
i) qualitative estimates of the percentage of the benthos (sea floor) made up by macroalgae, live coral, recently dead coral, live coral rock, coral rubble and sand;
and
ii) observations of coral impacts, and this is done over a series of five-metre radius point surveys (with one RHIS form completed for each circular plot of 78.5 square metres).

The percentage of coral cover (if any) that had recently died from each impact-type (that is, bleaching, disease, predation, damage) was estimated as described in Beeden et al. (2014) for each RHIS survey by examining all coral colonies within the point surveys for any impacts. These data were used to categorise the percentage of bleached corals within the total living coral within each RHIS area. For comparison to all aerial survey data and the in-water transect based quantitative estimates, the average RHIS per cent bleached scores for each reef were converted to align with the aerial bleaching categories described above (Berkelmans et al. 2004).

In-water surveys conducted by the COTS Targeted Control Program were also collected for this analysis from the Eye on the Reef Program from 2016 and 2017 (RRRC and AMPTO 2016). Three RHIS surveys were conducted at each reef and followed the standard protocol for RHIS (Beeden et al. 2014), recording the percentage of bleached corals among the living coral cover in addition to records of coral cover, coral type and potential COTS presence, absence and damage. Bleached corals were categorised according to the GBRMPA Coral Bleaching Response Plan (GBRMPA 2011) among four RHIS Colony bleaching levels:
(1) upper surfaces
(2) Pale/Fluoro
(3) Totally white
(4) Recently dead.

Data handling:
All in-water bleaching observations were converted to align with the National Bleaching Taskforce aerial bleaching categories 0-4. For community level bleaching response transect and RHIS based observations were converted into a percentage (%) bleached of all corals counted from the survey method, which was then converted into the 5 category scale used by the aerial survey scores. The in-water and aerial bleaching observations to establish the relationship between total heat stress accumulation (DHW) and the overall coral community level bleaching response (% of living corals bleached) were combined. The in-water transect and RHIS were combined to provide estimates of the proportion of individual coral colonies that were severely bleached or recently dead.

Further information can be found in the report: Cantin, N. E., Klein-Salas, E., Frade, P. (2021) Spatial variability in coral bleaching severity and mortality during the 2016 and 2017 Great Barrier Reef coral bleaching events. Report to the National Environmental Science Program. Reef and Rainforest Research Centre Limited, Cairns (64pp.).

Format:
GIS Spatial layers, Tif and KML files.

Data Dictionary:

GBR Community Bleaching Surveys - over the 2016-2017 time frame
0 (<1% bleached)
1 (1–10% bleached
2 (10–30% bleached )
3 (30–60% bleached)
4 (>60% bleached)

GBR: NOAA Coral Reef Watch – GBR Maximum Monthly Mean (MMM; raster) - Historical average maximum of the monthly mean temperatures for 1985-2012 climatology (v3.1) from annual sea surface Temperature anomalies. Represents the upper thermal limit for each location.
25 °C
26 °C
27 °C
28 °C
29 °C
30 °C

GBR: NOAA CRW – DHW max [2002, 1998, 2016, 2017]
Maximum accumulation of Degree Heating Weeks for each bleaching year. The NOAA DHW product is a summation of temperature anomalies at least 1°C above the MMM value over a 12 week period, which generates a measure of thermal stress as a function of both the strength of the anomaly above the MMM and the duration of the heating event. The DHW max value used here is the maximum DHW value accumulated for each summer heat wave.
0 °C – weeks
2 °C – weeks
5 °C – weeks
8 °C – weeks
15 °C - weeks

References:
Beeden R, Turner M, Dryden J, Merida F, Goudkamp K, Malone C, Marshall Paul A, Birtles A, Maynard J (2014) Rapid survey protocol that provides dynamic information on reef condition to managers of the Great Barrier Reef. Environmental Monitoring and Assessment 186:8527-8540

Berkelmans R, Oliver JK (1999) Large-scale bleaching of corals on the Great Barrier Reef. Coral Reefs 18:55-60

Berkelmans R, De'ath G, Kininmonth S, Skirving W (2004) A comparison of the 1998 and 2002 coral bleaching events on the Great Barrier Reef: spatial correlation, patterns and predictions. Coral Reefs 23:74-83

Cantin, N. E., Klein-Salas, E., Frade, P. (2021) Spatial variability in coral bleaching severity and mortality during the 2016 and 2017 Great Barrier Reef coral bleaching events. Report to the National Environmental Science Program. Reef and Rainforest Research Centre Limited, Cairns (64pp.).

Frade PR, 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 9:3447

GBRMPA (2011) Coral Bleaching Response Plan. Great Barrier Reef Marine Park Authority, Townsville

Hughes TP, Kerry JT, Álvarez-Noriega M, Álvarez-Romero JG, Anderson KD, Baird AH, Babcock RC, Beger M, Bellwood DR, Berkelmans R, Bridge TC, Butler IR, Byrne M, Cantin NE, Comeau S, Connolly SR, Cumming GS, Dalton SJ, Diaz-Pulido G, Eakin CM, Figueira WF, Gilmour JP, Harrison HB, Heron SF, Hoey AS, Hobbs J-PA, Hoogenboom MO, Kennedy EV, Kuo C-y, Lough JM, Lowe RJ, Liu G, McCulloch MT, Malcolm HA, McWilliam MJ, Pandolfi JM, Pears RJ, Pratchett MS, Schoepf V, Simpson T, Skirving WJ, Sommer B, Torda G, Wachenfeld DR, Willis BL, Wilson SK (2017) Global warming and recurrent mass bleaching of corals. Nature 543:373-377

Data Location:

This dataset is filed in the eAtlas enduring data repository at: data\nesp5\5.7_Bleaching-Assessment