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).

Benthic community composition:
Benthic community composition was recorded in August 2020 using the method described by Brown et al. 2018. During each survey, three replicate 15-meter (m) transects were laid north, east, and west from a permanent reference point. Quadrats (0.25 square meters (m²)) were alternated right and left every 0.5 m along the transect line, totaling 30 quadrats per transect. Percent cover of each quadrat was recorded in situ from 22 specific categories. The four main categories were hard coral, other invertebrates (including soft corals and sponges), algae, and substrate. Hard corals were differentiated into family and growth form: Acroporidae- tabular/corymbose/digitate (ARC-TCD), Acroporidae- branching (ACR-BRA), Acroporidae- plating/encrusting (ACR-PE), Pocilloporidae (POCI); Poritidae-massive (POR-M); Poritidae-encrusting/plating varieties (POR-PE); Poritidae-branching (POR-BRA); Faviidae-Lobophyllidae (FAV-LOB); and other hard corals (including non-scleractinian corals). Bleaching status was recorded to family and growth form for all live corals and due to low bleaching prevalence across sites (Fig. S2 of Brown et al. 2023), combined as 'bleached hard coral' for analyses. Macroalgae were separated as: fleshy macroalgae, calcifying algae of the genus Halimeda, articulate/crustose coralline algae (ACA/CCA), and turf algae/cyanobacteria. Substrate was divided into sand/sediment, coral rubble, and recently dead hard coral (i.e., coral skeletons with epithetic algal community with turf height <3 millimeters (mm)). Recently dead hard coral was also recorded to family and growth form where possible (Fig. S2 of Brown et al. 2023).

Benthic community composition in 2020 was compared to data collected in July 2015 and August 2016 along the same transects (Brown et al. 2018). All surveys were conducted within the same season (austral winter) because macroalgae display significant shifts in composition and abundance across seasons (Brown et al. 2018). The relative changes in hard coral (including bleached coral), algae, and hard substrate (including coral rubble and recently dead coral) were determined by subtracting the mean between years and dividing by the initial cover.

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