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Of the life processes on a coral reef, calcification produces the most conspicuous end product--the reef framework and sediments. Most of the information relevant to coral-reef calcification comes from studies of the rates of CaCO3 retention by the reef or studies of individual organism calcification rates. Neither of these types of studies really assesses the rate at which the reef community produces CaCO3.
Alkalinity depletion as water flows across a reef, together with volume transport of that water, can be used to compute the rate of reef calcification. This procedure has been employed across a predominantly coral community and across a predominantly coralline algal community on windward inter-island reef flats of Eniwetok Atoll, Marshall Islands.
The mean alkalinity of water approaching the reef is about 2.30 meq/l, and the alkalinity as the water crosses the reef is typically depleted by less than 0.01 meq/l. The product of ^Dgr alkalinity times volume transport, divided by the reef length, averages approximately 0.0025 (meq/sq m)/sec, with no significant difference in depletion rate between the 2 calcifying communities examined. This alkalinity depletion rate is equivalent to a CaCO3 production rate of 4 × 103 (g CaCO3/m2)/year.
If the porosity of the sediment produced by calcification is 50%, then the CaCO3 production rate is sufficient for an upward reef growth rate of about 3 mm/year. Because the present rate of eustatic sea level rise is considerably less than 3 mm/year, the reef is either catching up with sea level, or most of the CaCO3 produced is being removed. Sediment accumulations downstream from actively calcifying reef areas favor the latter hypothesis.
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