Dataset: Coral physiological and biological data collected during acute exposures in static incubation chambers in Hawaii from June 2023 to January 2025

Coral Collection and Pre-Experiment Holding

Coral colonies of Montipora capitata and Pocillopora acuta were collected by hand while snorkeling from shallow (1 m) fringing reef habitat surrounding HIMB in Kāne‘ohe Bay, Hawai‘i. Colonies were transported immediately to the mesocosm facility and held for one week in flow-through tanks that were identical in design to the experimental mesocosms.

Incubation Design

Incubations were conducted in a temperature-controlled water bath, under full-spectrum lighting (Wattshine 400w LEDs) set to ~800 PAR µmol m^-2 s^-1. Incubation chambers were closed-cell, with stirbars spinning via a belt-controlled magnet dial that was submerged entirely.  Corals were placed in a plug holder in the middle of each chamber, and chambers were numbered and haphazardly placed on the submerged racks. Corals were acclimated for 1 hr to treatment conditions before incubation, then closed in each chamber for either 1, 2, or 4 hr intervals based on preliminary assessments of calcification rates.  Seawater after this period was collected in borosilicate bottles for total alkalinity analysis, and separately in beakers for physical and chemical analysis with a YSI. 

Treatment Conditions and Carbonate Chemistry Manipulation

Four carbonate chemistry conditions were generated across consecutive 30-day exposures by altering total alkalinity (TA), dissolved inorganic carbon (DIC), and pH. Target conditions included:

1. Ambient control

2. Low pH (ΔpH ~ -0.3 from ambient)

3. High or low TA (±100 µmol kg-1 from ambient)

4. Combined low pH and TA alteration

5. High pH  (ΔpH ~ 0.3 from ambient)

6. Combined high pH and TA alteration

TA adjustments were made using a peristaltic pump delivering either 1.0 M HCl (for low TA) at ~3 mL min-1 or 1.0 M Na2CO3 (for high TA) at ~2 mL min-1 directly into each row’s mixing tank.
pH reductions were achieved by bubbling pure CO2 or a CO2-air mixture directly into designated mesocosms using a Maxi-Jet 1600 pump-driven venturi injector.

Seawater Sampling and Measurements

Daily Parameters

Temperature, salinity, dissolved oxygen, and pHNBS were measured daily at mid-day in all mesocosms and header tanks using a YSI multimeter (YSI ProDSS or YSI 556 MPS).

pH electrodes were calibrated daily using NIST-traceable pH 4, 7, and 10 buffers and corrected to the total scale using Tris buffer from A. Dickson (Scripps Institution of Oceanography). Dissolved oxygen calibrations followed manufacturer water-saturated air calibration protocols.

Total Alkalinity Sampling and Storage

Discrete seawater samples (100 mL) for total alkalinity (TA) were collected twice weekly.

Sampling procedures followed best practices described in Dickson et al. (2007):

Rinsed sample bottles three times with sample water.

Collected seawater in acid-cleaned 100 mL borosilicate bottles.

No headspace was left to prevent gas exchange.

Samples were analyzed within 12 hours of collection and stored at ambient temperature in the dark until analysis.

TA Determination

TA was determined using open-cell potentiometric titration on a Metrohm 877 Titrino Plus equipped with a Metrohm 9101 Herisau glass pH electrode.

All titrations were standardized using certified reference materials (CRM, batch number provided by Dickson Laboratory). Electrode slope, offset, and drift were checked daily.

Carbonate system variables (DIC, pCO2, HCO3-, CO32-, and Omega aragonite) were calculated using the R package seacarb.

Calcification Measurements

Calcification rates were quantified using buoyant weighing following Jokiel et al. (1978). Colonies were weighed on day 0 and day 30. Calcification rate (G) was calculated as:

G (g CaCO3 d-1) = 1.54 × (Wf – Wi) / d

where 1.54 g cm-3 is the density of aragonite, Wf and Wi are final and initial buoyant weights, and d is the exposure duration in days.

Three colonies per species per mesocosm were randomly selected for further biometric analysis. Three branch tips per colony were sampled immediately before and after the exposure period. Collected fragments were frozen at -20 C until post-processing.

Mortality was low and did not differ across treatments; dead colonies were excluded from analysis. All corals were returned to the reef at the conclusion of the experiments.