Real-time observations of benthic ocean chemistry on Hawai‘i coral reefs

Steven Colbert and Jim Beets, University of Hawai‘i at Hilo

Underwater picture of instrument on the shallow sea floor near a coral reefA benthic observatory continuously measures pH and other water-quality variables.

Coral reefs form the literal foundation of some of the most complex and productive ecosystems in the world. They are not only ecologically and culturally valuable, but also provide a range of important services including food, recreation, employment, and coastal protection. However, the benefits that people receive from coral reefs are not well reciprocated. Reefs in Hawai‘i and around the world are experiencing a number of threats related to climate change, including ocean acidification. As carbon dioxide from the atmosphere and land-based runoff dissolves in seawater, chemical reactions occur that lower the water's pH and decrease the availability of calcium carbonate in a form usable by marine organisms for skeletal or shell growth. For corals, this depletion of compatible carbonate material can affect the physiology and growth rates of the entire reef. It is therefore important to collect baseline measures of ocean acidification from the seafloor where the reefs are, in order to determine how conditions are changing and to be able to separate natural variability from long-term trends.

Underwater picture of instrument on the shallow sea floor near a coral reefCollecting alkalinity samples.

We deployed benthic observatories to measure temperature, salinity, oxygen, carbon dioxide, and pH at three sites around Hawai‘i Island. At all sites, aragonite saturation, a measure of how easily organisms can access carbonate building blocks, was lower than typical open ocean conditions, and lower on average than at most coral reef sites worldwide. Hawaiian reefs are likely already experiencing effects from ocean acidification. But the causes are not just global: composition of local surface runoff and groundwater can add substantially to the problem. Differences in pH and total dissolved carbon dioxide between our observed sites reflect different watershed characteristics and illustrate how land usage can impact inorganic carbon levels in the coastal marine environment.

Our results are alarming, as they show acidification levels that had not been expected to occur for 40 more years. This helps to underscore both the value and the urgency of long-term coastal water quality monitoring, analogous to the climate monitoring stations on land. Statewide monitoring of inorganic carbon in reef areas can allow us to track the effects of acidification and would provide a basis for management plans to better support stressed reef areas.

Quick Summary

     Coral reefs are some of the most productive and valuable ecosystems on the planet, but they are experiencing a number of threats, including ocean acidification.
     Our benthic monitoring shows that 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, in order to assess the effects of acidification on reefs and support development of reef management plans.

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This project was supported by the Pacific Islands ClimateAdaptation Science Center (Cooperative Agreement # G13AC00314 from the US Geological Survey). Contact Steven Colbert ( for more information on this project. To learn more about climate science at PI-CASC, contact David Helweg at or visit: