Dataset: Coral physiological and biological data collected during diel sampling in outdoor mesocosms in Hawaii from January 2022 to March 2024

Overview

Seasonal mesocosm incubations of Montipora capitata and Pocillopora acuta were conducted at the Hawai‘i Institute of Marine Biology (HIMB) from June 2022 through March 2024. Experiments were run during two seasonal windows each year: winter (January 10–March 17) and summer (June 10–August 17). Due to facility limitations, all treatments could not run simultaneously. Therefore, each seasonal experiment consisted of two sequential 30-day exposures, each using newly collected coral colonies. At the beginning (Day 0) and end (Day 30) of each experiment, a 26-hour diel sampling was conducted to characterize variability in total alkalinity (TA) and carbonate system parameters under each treatment. All methods were standardized across replicates, with only TA and pH manipulations differing among treatments.

Coral Collection and Pre-Experiment Holding

Coral colonies of M. capitata and P. 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 HIMB mesocosm facility and held for one week in flow-through tanks identical to the experimental mesocosms. One day before each 30-day experiment, colonies were stained with alizarin red for eight hours to mark the starting skeletal growth band (Jokiel and Morrissey 1993). At the start of each experiment, colonies were buoyant weighed following Jokiel (1978) and randomly assigned to mesocosms. Each mesocosm contained 20 colonies of each species arranged in a standardized alternating layout to avoid species-level clustering.

Mesocosm Facility Design

Experiments were conducted in a flow-through mesocosm facility described by Jokiel et al. (2014). The system consisted of twelve 450 L fiberglass tanks arranged in four rows of three mesocosms, each row supplied by a dedicated header tank. Natural seawater was pumped from Kāne‘ohe Bay at ~3 m depth and delivered unfiltered to preserve natural diel and seasonal environmental variability. Each header tank fed seawater into a 100 L mixing reservoir where TA and pH manipulations were applied before seawater entered the mesocosms. Residence time in each mesocosm was greater than one hour. Each tank contained two submersible circulation pumps and a continuously running airstone to maintain internal water movement and oxygenation.

Treatment Conditions and Carbonate Chemistry Manipulation

Four carbonate chemistry treatments were created across each pair of 30-day experiments through controlled manipulation of total alkalinity (TA), dissolved inorganic carbon (DIC), and pH:

Ambient control

Low pH (target ΔpH approximately −0.3 from ambient)

High or low TA (±100 µmol kg−1 from ambient)

Combined low pH and altered TA

TA manipulations were accomplished in header mixing tanks using peristaltic pumps delivering either 1.0 M HCl (for low TA; ~3 mL min−1) or 1.0 M Na2CO3 (for high TA; ~2 mL min−1). pH reductions were achieved by bubbling pure CO2 or a CO2-air mixture directly into mesocosms using a Maxi-Jet 1600 pump-driven venturi injector. Because each row was supplied by a single header tank, only one direction of TA manipulation (increase or decrease) could be performed within a given 30-day period. The second 30-day experiment used the opposite TA manipulation to ensure all treatments were completed each season.

Seawater Sampling and Measurements

Daily Measurements

Temperature, salinity, dissolved oxygen, and pHNBS were measured daily at mid-day in each mesocosm and header tank. During diel sampling, these parameters were measured every hour on the hour for 26 consecutive hours. Measurements were collected using a YSI ProDSS or YSI 556 MPS multiparameter meter.

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

Total Alkalinity Sampling and Storage

Seawater samples for TA analysis were collected hourly during each 26-hour diel cycle. Bottles were rinsed three times with sample water and filled in acid-cleaned 100 mL borosilicate bottles with no headspace to prevent gas exchange. Samples were stored in the dark at ambient temperature and analyzed within 12 hours of collection. Sampling followed best practices described in Dickson et al. (2007).

TA Determination and Derived Carbonate Chemistry

Total alkalinity was measured by open-cell potentiometric titration on a Metrohm 877 Titrino Plus with a Metrohm 9101 Herisau glass pH electrode. All titrations were standardized using certified reference materials (CRMs) from the Dickson laboratory. Electrode slope, offset, and drift were checked daily to ensure analytical consistency. Carbonate system parameters, including DIC, pCO2, bicarbonate, carbonate ion concentration, and omega aragonite, were calculated using the R package seacarb.