Coral-algae metabolism and diurnal changes in the CO(2)-carbonate system of bulk sea water

Precise measurements were conducted in continuous flow seawater mesocosms located in full sunlight that compared metabolic response of coral, coral-macroalgae and macroalgae systems over a diurnal cycle. Irradiance controlled net photosynthesis (P(net)), which in turn drove net calcification (G(net)), and altered pH. P(net) exerted the dominant control on [CO(3)(2−)] and aragonite saturation state (Ω(arag)) over the diel cycle. Dark calcification rate decreased after sunset, reaching zero near midnight followed by an increasing rate that peaked at 03:00 h. Changes in Ω(arag) and pH lagged behind G(net) throughout the daily cycle by two or more hours. The flux rate P(net) was the primary driver of calcification. Daytime coral metabolism rapidly removes dissolved inorganic carbon (DIC) from the bulk seawater and photosynthesis provides the energy that drives G(net) while increasing the bulk water pH. These relationships result in a correlation between G(net) and Ω(arag), with Ω(arag) as the dependent variable. High rates of H(+) efflux continued for several hours following mid-day peak G(net) suggesting that corals have difficulty in shedding waste protons as described by the Proton Flux Hypothesis. DIC flux (uptake) followed P(net) and G(net) and dropped off rapidly following peak P(net) and peak G(net) indicating that corals can cope more effectively with the problem of limited DIC supply compared to the problem of eliminating H(+). Over a 24 h period the plot of total alkalinity (A(T)) versus DIC as well as the plot of G(net) versus Ω(arag) revealed a circular hysteresis pattern over the diel cycle in the coral and coral-algae mesocosms, but not the macroalgae mesocosm. Presence of macroalgae did not change G(net) of the corals, but altered the relationship between Ω(arag) and G(net). Predictive models of how future global changes will effect coral growth that are based on oceanic Ω(arag) must include the influence of future localized P(net) on G(net) and changes in rate of reef carbonate dissolution. The correlation between Ω(arag) and G(net) over the diel cycle is simply the response of the CO(2)-carbonate system to increased pH as photosynthesis shifts the equilibria and increases the [CO(3)(2−)] relative to the other DIC components of [HCO(3)(−)] and [CO(2)]. Therefore Ω(arag) closely tracked pH as an effect of changes in P(net), which also drove changes in G(net). Measurements of DIC flux and H(+) flux are far more useful than concentrations in describing coral metabolism dynamics. Coral reefs are systems that exist in constant disequilibrium with the water column.