Carbon Cycle Model of a Hawaiian Barrier Reef under Rising Ocean Acidification and Temperature Conditions of the Anthropocene

Abstract:

A CO₂-carbonic acid system biogeochemical box model (CRESCAM, Coral Reef and Sediment Carbonate Model) of the barrier reef flat in Kaneohe Bay, Hawai‘i was developed to determine how increasing temperature and dissolved inorganic carbon (DIC) content of open ocean source waters, resulting from rising anthropogenic CO₂ emissions and ocean acidification, affect the CaCO₃budget of coral reef ecosystems. CRESCAM consists of 17 reservoirs and 59 fluxes, including a surface water column domain, a two-layer permeable sediment domain, and a coral framework domain. Physical, chemical, and biological processes such as advection, carbonate precipitation/dissolution, and net ecosystem production and calcification were modeled. The initial model parameters were constrained by experimental and field data from previous coral reef studies, mostly in Kaneohe Bay over the past 50 years. The field studies include data collected by our research group for both the water column and sediment-porewater system.

The model system, initially in a quasi-steady state condition estimated for the early 21^st century, was perturbed using future projections to the year 2100 of the Anthropocene of atmospheric CO_{2 ­}concentrations, temperature, and source water DIC. These perturbations were derived from the most recent (2013) IPCC’s Representative Concentration Pathway (RCP) scenarios, which predict CO₂ atmospheric concentrations and temperature anomalies out to 2100. A series of model case studies were also performed whereby one or more parameters (e.g., coral calcification response to declining surface water pH) were altered to investigate potential future outcomes. Our model simulations predict that although the Kaneohe Bay barrier reef will likely see a significant decline in NEC over the coming century, it is unlikely to reach a state of net erosion – a result contrary to several global coral reef model projections. In addition, we show that depending on the future response of NEP and NEC to OA and rising temperatures, the surface waters could switch from being a present-day source of CO₂ to the atmosphere to a future sink. This ecosystem specific model can be applied to any reef system where data are available to constrain the initial model state and is a powerful tool for examining future changes in coral reef carbon budgets.