Observations and Modeling of the CO2-Carbonic Acid System on Hawaiian Coral Reefs: Implications of Future Ocean Acidification and Climate Change

Abstract

The CO2-carbonic acid system of nearshore and coral reef ecosystems is highly variable, and often poorly constrained. In addition to the natural processes altering the carbon system of coral reefs, increased atmospheric and seawater carbon dioxide (CO2) concentrations, from the anthropogenic burning of fossil fuels and land use changes, have the potential to alter the fragile biogeochemical balance of these ecosystems. Autonomous seawater CO2 monitoring buoys are becoming an increasingly utilized method for studying the CO2 chemistry of coastal waters, and these systems provide accurate, high-resolution CO2 data that were previously unobtainable a decade ago. This research presents the results of the longest running, continuous CO2 time-series for a coral reef environment in the world. A network of three monitoring buoys was established in 2008 around Oahu, Hawai‘i, providing high-resolution data. Net annualized air-sea CO2 gas exchange was calculated at each study location and was comparable to estimates at other reef locations around the world. An in-situ study of the permeable carbonate sediment-porewater system at two locations showed that porewater carbon biogeochemistry in permeable reef sediments is strongly controlled by microbial respiration of organic matter. The short residence time of the porewater, due to increased advection, is another major control on the biogeochemical parameters such as total alkalinity and pH. Finally, the data collected by the observing buoys, along with the in-situ porewater data and previous data collected in Kaneohe Bay, were used to create a carbon biogeochemical box model, the Coral Reef and Sediment Carbonate Model (CRESCAM), for the Kaneohe Bay barrier reef flat. The model was forced using the Representative Concentration Pathway CO2 emissions scenarios from the 2013 Intergovernmental Panel on Climate Change 5th Assessment Report. Several case studies were conducted to determine important parameters and to identify possible future conditions on the barrier reef under increasing ocean acidification, rising temperature, and land use changes. Model runs predicted that the barrier reef flat could experience a 20% decrease in coral calcification, by 2100. Although carbonate dissolution is expected to increase in the sediments, dissolution will not provide a sufficient buffer to mitigate any decreases in surface water pH.