Study location:
This study was conducted across eight sites at Heron Island, southern Great Barrier Reef (23°27’ S 151°55’ E), previously characterized in detail (Brown et al. 2018, 2020). Briefly, sites included each geomorphological habitat of Heron Reef: reef slope, reef crest, reef flat, shallow lagoon, and deep lagoon (Phinn et al. 2012) (Fig. 1 of Brown et al. 2023). The geomorphological habitats of Heron Reef are distinguished by diverse benthic communities, with hard coral cover higher within the reef slope and macroalgae cover greater within the lagoonal habitats (Brown et al. 2018; Roelfsema et al. 2021). Reef-wide coral cover in 2015 and 2016 was amongst the highest observations in the past 60 years (Connell et al. 1997; Brown et al. 2018; Roelfsema et al. 2021) (e.g., ~75% within the south-west reef slope and ~20% within lagoon), making these years optimal as a recent baseline record.

Within the reef slope habitat, four sites were established: two within the north-east section of the reef (Fourth Point, 4.2 meters (m) and 8 m) and two within the south-west (Harry’s Bommie, 6.1 m and 8.2 m) (Fig. 1 of Brown et al. 2023). The northeast of Heron Reef is the exposed side, subject to extreme wave forces during cyclones, whereas the south-west is sheltered from waves generated by both the south-east trade winds and extreme wave action of cyclones (Connell et al. 1997). One site was established in each other geomorphological habitat, with each site sharing its name: Reef Crest (RC; 0.9 m), Reef Flat (RF; 0.7 m), Shallow Lagoon (SL; 1.3 m) and Deep Lagoon (DL; 2.6 m) (Fig. 1 of Brown et al. 2023). Inside the lagoon, semidiurnal tidal fluctuations result in higher variability in temperature and pH than reef slope sites (Brown et al. 2018; Cyronak et al. 2020) (Fig. 2, Fig. S1 of Brown et al. 2023). Photosynthetically active radiation (PAR; µmol quanta m⁻²s⁻¹) is lower within reef slope habitats (HB5: 75.9, HB8: 72.8, FP5: 179.4, FP8: 58.9) than within the lagoon habitats (RC: 199.2, RF: 371.7, SL: 201.8, DL: 198.8), due to differences in depth (Brown et al. 2018; Cyronak et al. 2020). Mean depth and PAR were determined by use of Conductivity Temperature Depth (CTD) units that continuously recorded between July 2015 and November 2016 (SBE 16plus V2 SEACAT fitted with an auxiliary PAR sensor, Satlantic/ECO-PAR sensor) (see Brown et al. 2018 for more detailed methodology).

Evaluation of thermal stress:
Seawater temperatures were recorded at hourly intervals using cross-calibrated HOBO Pendant loggers (UA-001-64; ± 0.552 degrees Celsius (°C) accuracy) between September 2019 and August 2020. Logger accuracy was assessed at the end of the deployment period using a water bath (Thermo Scientific Precision TSGP20). Due to logger failures, only a partial temperature record was obtained at HB8 (from December 2019), RC (from February 2020), and DL (from February 2020) and there is no temperature record at FP5 (Table 1, Fig. S1 of Brown et al. 2023). No loggers were deployed at FP8 during 2019-2020.

Seawater temperatures recorded in 2019-2020 were compared to temperatures recorded at the same locations in 2015-2016 (see Brown et al. 2018 for detailed methodology). The mean, maximum, minimum, and mean daily amplitude were calculated at each site for the two periods September 2015 to August 2016 and September 2019 to August 2020 (Table 1 of Brown et al. 2023). Daily (24-hour) mean temperatures (T) were extracted from the logger data. The climatological maximum monthly mean (MMM) of Heron Reef is 27.3°C (Weeks et al. 2008). Temperature anomalies, or ‘hotspots’ (HS), were calculated from the logger data using U.S. National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) methodology (Eakin et al. 2010). If T, representing the daily mean temperature, exceeded the region's coral bleaching threshold (MMM + 1°C; 28.3°C), then the MMM was subtracted from T. Importantly, we did not use nighttime-only temperatures, as is done with NOAA CRW, instead choosing to use 24-hour mean temperatures due to the diel variability across sites. Thermal anomalies were then summed across a rolling 12-week (90-day) period to determine the extent of thermal stress in degree heating weeks (DHW; °C per week).

For more detailed information, please see:
Brown, K.T., Eyal, G., Dove, S.G. et al. (2023) Fine-scale heterogeneity reveals disproportionate thermal stress and coral mortality in thermally variable reef habitats during a marine heatwave. Coral Reefs 42, 131-142. https://doi.org/10.1007/s00338-022-02328-6