National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2016

Latitudinal trends and drivers in the CO2-carbonic acid system of Papahānaumokuākea Marine National Monument

Kealoha, A.K., S.E. Kahng, F.T. Mackenzie, S.R. Alin, R.K. Kosaki, R.E. Brainerd, and C.D. Winn

Aquat. Geochem., 21(6), 535–553, doi: 10.1007/s10498-015-9273-z (2015)

Emissions of anthropogenic carbon dioxide (CO2) to the atmosphere and the consequent effects of climate change and ocean acidification on coral reef ecosystems have motivated significant interest in describing and understanding the CO2–carbonic acid system of diverse coral reef environments. Although numerous studies have been successful in monitoring reef metabolism both in the field and in the laboratory, physical and biological forcings produce distinct conditions among environments. Due to the geographic isolation and associated difficulties with measuring marine carbon chemistry in waters of the Papahānaumokuākea Marine National Monument (PMNM), relatively few studies have described the CO2–carbonic acid system and carbonate saturation state gradients of these waters. Yet, PMNM remains one of the largest conservation areas in the world with extensive and diverse coral reef ecosystems that could offer valuable insight into our current and future understanding about regional and global impacts of ocean acidification. In order to provide a broad overview of latitudinal trends and features of the marine carbon system in PMNM waters, continuous measurements for surface seawater fugacity of CO2 (fCO2) and pH were collected during August 2011 and July 2012 cruises of the NOAA Ship Hi’ialakai. These measurements indicate that pH and fCO2 are three times more variable in nearshore monument waters relative to open ocean transect measurements. This variability can be observed up to 50 km away from the nearest reef and is likely the result of the direct and significant impact of coral reef metabolism on marine carbon chemistry around the islands and atolls. The relationship between total alkalinity and dissolved inorganic carbon is consistent with net calcification which creates an alkalinity sink throughout PMNM waters. Additionally, our measurements show clear latitudinal trends in fCO2, pH, and aragonite saturation state that are influenced by environmental forcings, including temperature, wind speed, and residence time of the water. Collectively, our results suggest that coral reefs located at the northernmost atolls of PMNM may be the most susceptible to the adverse impacts of climate change and ocean acidification.

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