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 regres sed against irradiance (T) and modeled using a general exponential function . C-I models yielded estimates of gross calcification that ranged from 9.6 mmol CaCO3 m(-2) h(-1) at 0 m to 2.0 mmol CaCO3 m(-2) h(-1) at 18 m. A sign ificant 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 CaCO3 m( -2) d(-1). Net 24-h calcification was linearly correlated with noontime irr adiance. Daily CaCO3 deposition as a function of reef surface relief (3.1 f or the crest and 5.0 for the slope) indicated potential contributions to re ef accretion of 4.1 to 28.1 g m(-2) d(-1), assuming 100% coralline cover. T hese estimates predict annual deposition rates of 1.5 to 10.3 kg CaCO3 m(-2 ) yr(-1), provided that measurements made between late summer and mid-winte r are representative of calcification throughout the year. Since observed a ccretion falls far short of the quantities predicted by these measurements, erosive agencies must remove much of the CaCO3 deposited annually by crust ose coralline algae on windward reef margins.