Real-time observations of benthic ocean chemistry on two coral reefs in west Hawaiʻi

A boxy instrument sits near a coral outcrop on the rocky sea bottom.
A benthic observatory deployed at Puakō, HI, sits on a shallow, near-shore sea floor, continuously measuring pH and other water-quality variables. (Photo: Tyler Phelps)

Ocean acidification represents a shift in the fundamental chemical balance of the oceans. Acidification is driven by carbon dioxide from human use of fossil fuels and agricultural activities dissolving in surface seawater, causing chemical reactions that reduce the pH of seawater. Exposure to lower pH can affect physiology as well as calcification rates in many benthic organisms, including coral. A baseline measure of ocean acidification from the seafloor, where our reefs are growing, is needed to effectively evaluate inter-annual differences or climate related changes. Carbon dioxide dynamics were assessed at three sites: Hōnaunau-Keei, Pelekane Bay-Puakō, and Wai‘ōpae. A benthic observatory was deployed to measure temperature, salinity, oxygen, carbon dioxide and pH to study fundamental processes of photosynthesis, respiration, and calcification occurring on reefs. At all sites, aragonite saturation, a measure of how easily organisms can produce calcium carbonate, was lower than typical open ocean conditions, and lower on average than at most coral reef sites worldwide. At Wai‘ōpae, greater daily variability of pH reflects more vigorous carbon cycling. Changes in management to reduce the acidity of groundwater inputs or increase herbivory on the reef could help stabilize these conditions, benefitting the corals. In fact, the quality of groundwater flowing into to the coastal ocean statewide could be managed better. The inputs typically have less total inorganic carbon than seawater, creating conditions that make it difficult for organisms in the coastal waters to precipitate calcium carbonate. Coral reefs on Hawai‘i Island are currently exposed to ocean acidification conditions that previous predictions suggested would not occur for 40 more years. Statewide measurements of total inorganic carbon at the reef and in groundwater are needed as a part of regular water quality monitoring to determine how conditions change over seasonal time scales and to assess the impact of acidification on reefs.

A scuba diver floats above a coral outcrop holding a water sampling tube.
Student Rebecca Ziegler scuba dives to collecting alkalinity water samples from within the coral framework off the coast of Hawaiʻi Island. (Photo: Jenna Budke)




Steven Colbert
Assistant Professor of Marine Science, UH Hilo


Jim Beets
Professor of Marine Science, UH Hilo