Note: Figure and table references refer to Brown et al., 2024.

Study location

This study was conducted across six sites spanning less than 5 km at Heron Island, southern Great Barrier Reef (23°27’ S 151°55’ E; Figure 1), which included at least one representative from each geomorphological habitat of Heron Reef (site; depth ± standard deviation): reef slope [Fourth Point (FP; 4.2±0.72 m); Harry’s Bommie (HB; 6.1±0.82 m)], reef crest (RC; 0.9±0.59 m), reef flat (RF; 0.7±0.59 m), shallow lagoon (SL; 1.3±0.74 m) and deep lagoon (DL; 2.6±0.59 m) (Phinn et al. 2012) (Figure 1). PAR levels (µmol m-2 sec-1), averaged across the year-long period from August 2015– September 2016, were lower within reef slope habitats (HB: 75.9, FP: 179.4) than within the lagoon habitats (RC: 199.2, RF: 371.7, SL: 201.8, DL: 198.8) (Brown et al. 2023a). Historically, hard coral cover has been greatest within the reef slope (~60%), which is dominated by corals of the family Acroporidae, whereas within the lagoon, sites have peaked around 20% coral cover and are principally composed of Pocillopora and massive Porites (Connell et al. 1997; Roelfsema et al. 2021; Brown et al. 2023a). Experiments were performed in the austral spring to avoid any potentially confounding thermal stress that is becoming increasingly common during the summer (Marzonie et al. 2022). While absolute thermal tolerance can differ across seasons (Berkelmans and Willis 1999), relative thermal tolerance between individuals across seasons remains consistent (Cunning et al. 2021; Evensen et al. 2022).

Sample collection

Three morphologically distinct coral species with distinct life-history strategies were examined:

Acropora cf. aspera (competitive; branching open), Pocillopora cf. damicornis (weedy; branching closed), and Porites cf. lobata (stress-tolerant; massive) (Darling et al. 2012) (Figure 3). Coral fragments were collected from 27 September to 6 October 2022 (Table S1). Ten colonies of each genus were sampled from each site except where noted (n = 5–10 colonies per species per site; Figure 3). Acropora cf. aspera was not collected at the Shallow Lagoon and Deep Lagoon as it was absent or rare. Following collection, corals were transported to Heron Island Research Station (HIRS) and placed in outdoor, flow-through seawater troughs under ambient temperatures (22.86 ± 0.02°C) until experimentation. Each colony was divided into five fragments (i.e., genetic clones) of ~5 cm using bone cutters (Acropora and Pocillopora) or a brick saw (Porites). Acropora and Pocillopora were then suspended within the experimental tanks using fishing line. Porites fragments were placed on plastic grating at the bottom of the experimental tanks. Coral thermal tolerance experiments were initiated within 3–48 hours of collection (Table S1).

Acute heat stress experiment

A standardized temperature profile was used to measure heat tolerance in corals (e.g., (Voolstra et al. 2020; Cunning et al. 2021; Marzonie et al. 2022; Evensen et al. 2023)) with minor modification in temperature profiles. The climatological maximum monthly mean (MMM) of Heron Reef is 27.3°C (Weeks et al. 2008). A pilot experiment with all three genera indicated no difference in Fv/Fm between MMM, MMM+3°C and MMM+6°C, so the latter two treatments were increased to more accurately assess the decline in performance, such that the five treatments used here included ambient, MMM, MMM+4°C, MMM+6.5°C, and MMM+9°C. Generally, experiments began at ~12:00 with a 3-hr ramp to respective treatment temperatures (23°C, 27.3°C, 31.3°C, 33.8°C, 36.3°C), a 3-hr hold, and a 1-hr ramp down to MMM (Evensen et al. 2023) (Figure S1). Lights were turned off at the onset of the 1-hr ramp down to correspond with sunset. Due to experimental constraints (space, equipment, and time), only two treatments (n = 1 tank) were performed per day and each site was done in isolation. Accordingly, a complete assay took two days per site, with treatments tested each day selected randomly (Table S1, Figure S1). A fragment from each coral colony was randomly placed into each treatment, so that all colonies were present in each treatment. Temperatures were controlled using an Apex controller (Neptune Systems). Apex temperature probes were calibrated against a high-precision temperature probe (HANNA HI-98190; accuracy: ±0.4°C at 25°C; resolution: ± 0.10°C) at the onset of the experiment. Temperatures were also recorded using cross-calibrated temperature loggers (HOBO UA-001-64, accuracy: ± 0.29°C at 25°C). Photosynthetically active radiation (PAR) was static and controlled using aquarium lights (NICREW HyperReef LED, Shenzhen NiCai Technology Co.), averaging 250 µmol m-2 sec-1.

Organism identifiers (Genus, Lifesciences Identifier [LSID]):
Pocillopora, urn:lsid:marinespecies.org:taxname:206938
Porites, urn:lsid:marinespecies.org:taxname:206485
Acropora, urn:lsid:marinespecies.org:taxname:205469

926905_v1_heron-island-heatstress-temp.csv