Calcium carbonate production in the southernmost subtropical Atlantic coral reef.

Emerging evidence have been supporting the idea that the better known South Atlantic coral reefs (located between 18°S and 24°S) are now essentially senescent structures that have experienced little or no additional vertical reef growth over the past millennia. This has often coincided with a shift to a dominance of non-coral calcifying organisms becoming the main CaCO3 producers in these high latitude and marginal marine settings. Here, we used Calcification Accretion Units (CAUs) and census-based methods to measure non-coral rates of CaCO3 production on the geologically senescent reef and adjacent rhodolith beds within the southernmost subtropical Atlantic reef (i.e., Queimada Grande Reef, QGR). The reef habitat is currently producing CaCO3 at rates of ∼126 g m-2 yr-1. In contrast, fragments of dead corals skeletons deposited adjacent to the reef over the last ∼2000 years are now colonized by crustose coralline red algae. These form a rhodolith bed that produces CaCO3 at rates of 858 g m-2 yr-1. Our results indicate that, whilst not sufficient to promote active net framework accumulation, CaCO3 production by coralline algae and bryozoans on the QGR appears to be sufficient to at least limit net large-scale erosion of the underlying reef structure, allowing the reef structure to persist in a state close to budgetary stasis. Finally, our results are also of relevance for providing insights regarding the balance of CaCO3 production/dissolution/erosion processes in coral reefs, especially in these less understood marginal reefs.