Dissolved inorganic carbon and total alkalinity of a Hawaiian fringing reef: chemical techniques for monitoring the effects of ocean acidification on coral reefs

There is an interest in developing approaches to “ecosystem-based” management for coral reefs. One aspect of ecosystem performance is to monitor carbon metabolism of whole communities. In an effort to explore robust techniques to monitor the metabolism of fringing reefs, especially considering the possible effects of ocean acidification, a yearlong study of the carbonate chemistry of a nearshore fringing reef in Hawaii was conducted. Diurnal changes in seawater carbonate chemistry were measured once a week in an algal-dominated and a coral-dominated reef flat on the Waimanalo fringing reef, Hawaii, from April of 2010 until May of 2011. Calculated rates of gross primary production (GPP) and net community calcification (G) were similar to previous estimates of community metabolism for other coral reefs (GPP 971 mmol C m−2 d−1; G 186 mmol CaCO3 m−2 d−1) and indicated that this reef was balanced in terms of organic metabolism, exhibited net calcification, and was a net source of CO2 to the atmosphere. Average slopes of total alkalinity versus dissolved inorganic carbon (TA–DIC slope) for the coral-dominated reef flat exhibited a greater calcification-to-net photosynthesis ratio than for the algal-dominated reef flat (coral slope vs. algal slope). Over the course of the time series, TA–DIC slopes remained significantly different between sites and were not correlated with diurnal averages in reef-water residence time or solar irradiance. These characteristic slopes for each reef flat reflect the relationship between carbon and carbonate community metabolism and can be used as a tool to monitor ecosystem function in response to ocean acidification.