During two ship-based research expeditions in August and October 2020 on the R/V Coral Reef II, we measured the thermal tolerance of 229 Acropora cervicornis colonies from six coral nurseries along Florida's Coral Reef (figure 1a; electronic supplementary material, table S1 of Cunning et al. 2021). The nurseries, from north to south, are operated by Nova Southeastern University (NSU), the University of Miami (UM), the Coral Restoration Foundation (CRF), Reef Renewal (RR), the Florida Fish and Wildlife Conservation Commission (FWC) and Mote Marine Laboratory (MML). Thermal tolerance of nursery corals was measured by Coral Bleaching Automated Stress System ("CBASS"), which are portable, field-deployable experimental tanks used to apply rapid, acute heat stress challenges. Fragments of each coral colony were independently exposed to each of eight temperature stress profiles of increasing magnitude (with maximum temperatures between 30 and 38 degrees Celsius (°C)) for 7 hours, after which maximum photochemical efficiency (Fv/Fm) was measured as an indicator of each fragment’s stress response. These data were used to construct a dose-response curve for each colony, from which the effective dose of heat stress required to reduce Fv/Fm by 50% (ED50 value) was calculated as a metric of each colony’s thermal tolerance. A full description of the study methodology can be found in Cunning et al. 2021 (DOI: 10.1098/rspb.2021.1613).

Instruments:
CBASS were constructed following the general design of Voolstra et al. (2020), using beverage coolers (Coleman 24 Can Party Stacker) partitioned into two independent halves (tanks) with an acrylic divider. Each 8-liter (L) tank was equipped with one titanium aquarium heater (Finnex TH-300 W) and two thermoelectric chillers (Nova Tec IceProbe). Seawater was circulated within each tank using a submersible powerhead (SUNSUN JVP 530 GPH), and fresh incoming seawater was supplied to each tank at a rate of approximately 1 milliliter per second (mL s-1) (turnover = approx. 2.2 hours). Light was provided by LED aquarium lights (Phlizon 165 W) mounted above each cooler and manually adjusted to provide 550 micrmoles photons per square meter per second (μmol photons m-2 s-1) at the center of each tank, measured at the depth of the coral fragments with an underwater photosynthetically active radiation sensor (Apogee Instruments). Temperature profiles were executed by custom controllers (ELEGOO Mega 2560) with temperature sensors (Vktech DS18b20) activating the heaters and/or chillers as needed to achieve prescribed set points. For the tanks heated to 34°C or below on the October field expedition, Inkbird (ITC-308) temperature controllers were used instead. Temperature profiles consisted of a 30°C baseline temperature, a 3-hour ramp up to a prescribed maximum temperature (ranging from 30°C to 38°C), a 3-hour hold at the maximum temperature and a 1-hour ramp down to 30°C. Profiles were timed such that the end of the maximum temperature hold period coincided with local sunset, and the temperature ramped back down in darkness. After returning to 30°C, Fv/Fm was measured for each coral fragment using a DIVING-PAM-II (Walz, Effeltrich, Germany) chlorophyll fluorometer. Fluorometer settings included: measuring light intensity = 1, measuring light frequency (ML-F) = 4, ML-F high = off, damping = 2, F0mode = off, saturatingpulseintensity = 8 and satur-atingpulsewidth = 0.8 s. The gain setting was adjusted as needed to produce an F0 measurement above 100. Two Fv/Fm measurements were taken for each coral fragment (if possible depending on size), from non-overlapping areas of tissue that were facing upward, perpendicular to incident light. See Cunning et al. (2021) for additional information and figures.