concept

Carbonate budget

The accounting sheet of a reef, measured as the mass of calcium carbonate (CaCO₃) produced minus the mass removed, per square metre per year. On the credit side, corals and coralline algae secrete limestone; on the debit side, parrotfish grazing, boring sponges, urchin scraping and chemical dissolution take it away. If the total is positive the reef rises and tracks sea level; if negative, the foundation breaks down faster than it regrows. Scientists measure each term in the field (Perry et al.'s ReefBudget census method) to tell whether a reef is building or dying — a reef that still "looks like it is there" can already be running at a loss.