Quantifying the role of parrotfish in the production and cycling of carbonate in coral reef ecosystems

Abstract

Parrotfish are a diverse and ubiquitous group found on coral reefs worldwide. They are categorised into three main feeding modes; the browsers, scrapers and excavators, which together perform a number of important functional roles on coral reefs. Scraper and excavator parrotfish are common on most Indo-Pacific coral reefs where their roles in bioerosion, sediment production, grazing pressure and sediment reworking have been shown to influence benthic community composition, reef growth potential and sediment supply to reef habitats and reef associated sedimentary landforms. However, despite the widely known importance of parrotfish on coral reefs, our understanding of how their roles in carbonate cycling vary among species and among whole parrotfish communities in different reef habitats remains limited. This thesis produces original contributions to knowledge in the areas of species specific bioerosion estimates for the central Indian Ocean, bottom-up controls of habitat type on parrotfish assemblages and how variations in parrotfish assemblages translate to contributions to carbonate cycling processes among different reef habitats. The study was carried out across eight habitats on an atoll-edge reef platform in the central Maldives, where it was found that parrotfish community composition was driven by reef structural complexity and substrate type. Parrotfish occurred in six of the eight habitats, comprising ~44% of the platform area. Among these habitats, overall grazing pressure, bioerosion rates, sediment reworking and sediment production varied markedly. These processes were also found to have different spatial patterns over the reef platform, showing that they are not necessarily tightly coupled. In addition, reef habitats can vary in their importance for both sediment supply, and the relative importance of reworked sediment. Parrotfish produced a wide range of sediment size fractions, from <32 to 2000 µm and produced predominantly coral sands (>80%) between 125 and 1000 µm in diameter. This is comparable to the grain types found on local reef islands, and it is likely that the most significant supply of this material is from habitats on the atoll-edge side of the platform (which make up ~20% of the total platform area). Quantifying parrotfish functional roles and understanding the drivers behind these processes is important for informing future empirical and modelling studies, particularly as coral reefs undergo a time of dramatic environmental change.