Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

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.

Overview

Data processing included organization of raw seawater chemistry measurements, conversion of pH values to the total scale, calculation of carbonate system parameters, buoyant-weight-based calcification calculations, and post-hoc biochemical and skeletal data extraction. All steps described below represent the procedures used to generate the submitted dataset. No statistical analysis, modeling, or graphing is included here.

Seawater Chemistry Data Processing

pH Conversion

Daily pHNBS measurements from the YSI multiparameter meter were converted to total scale (pHT) using Tris buffer reference standards prepared and certified by the Dickson Laboratory. Temperature-dependent conversion equations followed Dickson et al. (2007).
All pH values were standardized across seasons using:

pHT = pHNBS + (Δbuffer offset)

where Δbuffer offset was determined daily from Tris-buffer calibration measurements.

Total Alkalinity (TA)

Raw potentiometric titration files from the Metrohm 877 Titrino Plus were exported using Metrohm tiamo software (version 2.5).
Processing included:

Drift correction and electrode slope verification.

CRM standardization using certified values for TA (Dickson Laboratory, batch number provided in metadata).

Extraction of equivalence point and TA value using open-cell titration algorithms embedded in tiamo.

TA values were reported in µmol kg-1.
If sample density was required, seawater density was calculated using temperature and salinity data following Fofonoff and Millard (1983).

- Loaded ecosys_dat.csv as resource "995155_v1_diel-sampling" in CSV format, treating empty strings and "NA" as missing values, with headers on row 1
- Converted date field from M/D/YYYY format to ISO 8601 YYYY-MM-DD date format
- Renamed five fields to valid identifiers: "tank area (m2)" to tank_area, "tank depth (m)" to tank_depth, "flow rate (L sec-1)" to flow_rate, "ta (umol kg -1)" to ta, and "o2 (mg/L)" to o2
- Set data types for all 16 fields: date as date (%Y-%m-%d), experiment and mesocosm and tp as integer, flow_rate, o2, ph_nbs, sal, ta, tank_area, tank_depth, and temp as number, and season, tod, treatment, and type as string
- Output written to 995155_v1_diel-sampling.csv

NSF Award Abstract:
Corals build calcium carbonate skeletons to maintain the three-dimensional structure of a coral reef, which provides habitat for many organisms and protects shorelines from bioerosion and storm damage. However, changes in ocean chemistry threaten the ability of corals to build and sustain these ecological important structures. To further the understanding of how climate change impacts coral reefs, this project investigates how changes in ocean carbonate chemistry directly influence coral calcification. The researchers are conducting a series of experiments on corals grown in seawater tanks to study corals responses to seawater chemistry in a changing ocean. Broader impacts of the project include student research opportunities, science-inquiry labs, and virtual learning. This project supports the training of several early career researchers, Ph.D. students, undergraduates, and high school students in the disciplines of chemistry, engineering, and marine ecology. Researchers partner with the Texas State Aquarium to communicate with the general public through a virtual research expedition series that will focus on coral reef health. This series includes interviews, behind the scene tours, and virtual dives on coral reefs in Hawaii.

This project examines the fundamental connections between seawater chemistry and coral physiology by investigating the modulation of seawater chemistry in the microenvironment surrounding corals. Specifically, this project 1) examines the response of corals to differing carbonate chemistry and 2) characterizes the proton gradient across the corals' boundary layers under differing ocean acidification conditions. Results of this work isolate whether carbonate ions or hydrogen ions have a stronger influence on calcification rates. This work utilizes a state-of-the-art experimental mesocosm facility that combines an automated systems to simultaneously and independently control both total alkalinity and carbon dioxide in the tanks to examine coral response under different carbon chemistry scenarios. Small-scale gradients in carbon chemistry surrounding the corals are being characterized using an innovative solid-state, reagentless sensor capable of making simultaneous measurements of two critical carbon system parameters. Coral biological response variables are quantified during short-term incubations and long-term mesocosm manipulations to understand physiological implications across multiple scales (i.e., individual and community scales) and across boundary layers.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.