Parrotfish erosion underpins reef growth, sand talus development and island building in the Maldives

Abstract

Parrotfish play a key functional role on coral reefs as external bioerosion agents and produce large quantities of carbonate sediment as a by-product of grazing on reef surfaces. Parrotfish are therefore an important potential source of sediment for island construction and maintenance within atoll reef environments, particularly under future scenarios of sea level rise and island morphological change. Here, we present the first field-based estimates of excavating parrotfish erosion (Chlorurus sordidus and Chlorurus strongylocephalus) within the Indian Ocean and quantify the contribution of parrotfish to the carbonate and sediment budgets of an atoll interior reef platform in the Maldives. We note that parrotfish erosion rates are high (6.3kgm−2 y−1), generating large amounts of new coral-based sediment (2.6kgm−2 y−1) that has a comparable grain size distribution to island deposits. Mean erosion rates by individual C. strongylocephalus (405kg individual y−1) were higher than C. sordidus (55kg individual y−1), but their contribution to erosion per unit area of reef was less due to a lower relative biomass (C. strongylocephalus: 1.3kgm−2 y−1; C. sordidus: 5.0kgm−2 y−1). Parrotfish also facilitate sediment export from reefs (0.7kgm−2 y−1), which contributes extensively to the development of the sand talus on the fore-reef slope and to the evolution of the wider atoll basin. Our results provide strong evidence that parrotfish erosion (and sediment generation) underpins island morphology on Maldivian reefs and highlight the importance of larger parrotfish as producers of island-grade sediment. Ecological processes must therefore be considered within future coastal management strategies for enhancing island stability.