Calcification by crustose coralline algae on the northern Great Barrier Reef, Australia

Calcification by four species of crustose coralline algae was estimated on the windward reef at Lizard Island, northern Great Barrier Reef, Australia, by combining measurements of O 2 , pH, and total alkalinity with equations describing the seawater carbonate equilibrium. Calcification ( C ) was regressed against irradiance ( I ) and modeled using a general exponential function. C‐I models yielded estimates of gross calcification that ranged from 9.6 mmol CaCO 3 m −2 h −1 at 0 m to 2.0 mmol CaCO 3 m −2 h −1 at 18 m. A significant proportion of all samples exhibited CaCO3 dissolution in the dark. Integration of C‐I models with half sine‐curve approximations of whole‐day irradiance yielded estimated net deposition rates of 0.82 to 9.1 g CaCO 3 m −2 d −1 . Net 24‐h calcification was linearly correlated with noontime irradiance. Daily CaCO 3 deposition as a function of reef surface relief (3.1 for the crest and 5.0 for the slope) indicated potential contributions to reef accretion of 4.1 to 28.1 g m −2 d −1 , assuming 100% coralline cover. These estimates predict annual deposition rates of 1.5 to 10.3 kg CaCO 3 m −2 yr −1 , provided that measurements made between late summer and mid‐winter are representative of calcification throughout the year. Since observed accretion falls far short of the quantities predicted by these measurements, erosive agencies must remove much of the CaCO 3 deposited annually by crustose coralline algae on windward reef margins.