The paradox of neutral carbonate budgets on coral-dominated reefs.

Coral reefs deliver vital services via a complex three-dimensional framework sustained by the balance between calcium carbonate production and erosion, or the net carbonate budget state. In many tropical western Atlantic reefs, ecological decline has reduced carbonate production, yielding near-neutral or negative budgets. Yet some reefs retain high coral cover and, theoretically, should also have high net positive budgets, yet often show modest carbonate accumulation. We used the remote reef of Cayo Arenas in the Campeche Bank, Gulf of Mexico, to test whether in reefs under suboptimal (variable) environmental conditions, high coral production is offset by robust bioeroder communities, producing neutral budgets. At 14 sites, we quantified carbonate producers and bioeroders to estimate gross production, bioerosion and net budget states. Despite relatively high live coral cover, mean net carbonate budgets did not differ significantly from a neutral (zero) state, despite a positive net budget (0.7 ± 0.5 SE kg CaCO3 m-2 yr-1) and substantial site-to-site variability (range: -2.3 to 4.3 kg CaCO3 m-2 yr-1). Crucially, this neutrality arose not from depressed biological activity (as in degraded reefs) but from an active equilibrium: elevated carbonate production coupled with substantial bioerosion. These reefs, therefore, represent a contemporary, functional reef state in net stasis. Distinguishing active-neutral reefs, where both production and bioerosion are elevated, from impoverishment-neutral reefs, where both are suppressed, is critical for predicting reef trajectories under environmental change and for targeting management, although near-stasis emerging from high carbonate turnover can appear functionally intact yet operate with limited buffering capacity against net carbonate loss.