New approaches to quantifying bioerosion by endolithic sponge populations: applications to the coral reefs of Grand Cayman

Abstract

Bioerosion is a critical process on coral reefs, influencing reef structural integrity and complexity and generating significant amounts of sediment. Excavating sponges are important bioeroders, especially in the Caribbean where sponges dominate macroborer communities. However, the contribution of bioeroding sponge communities to total bioerosion on coral reefs is not well understood; census surveys are rarely employed by monitoring agencies, and there is little data on the erosion rates of different species. Here, we investigated bioerosion by two Caribbean sponge species with different growth forms (Siphonodictyon brevitubulatum—α-form and Cliona tenuis—β-form). We also described new approaches to estimating bioerosion by sponge communities. By categorising the growth form of different species, we applied newly developed bioerosion rates, along with a previously published rate for C. delitrix, to census surveys and use these to estimate bioerosion by sponge communities on Grand Cayman reefs. Results indicate distinct habitat preferences for the two most abundant sponge species, C. tenuis and C. caribbaea. Mean sponge bioerosion across eight sites was 0.1 kg CaCO3 m−2 yr−1. Visible cover by α-growth-form excavating sponges caused a disproportionately high level of bioerosion in comparison with cover by β-growth-form species. Therefore, it is important to consider growth forms and excavation strategies when assessing bioerosion by sponge communities. Our present level of understanding of bioerosion by sponge species is limited, and more research is clearly required. However, the approaches described here can generate instant, meaningful results on sponge abundance and bioerosion and would complement many current benthic monitoring regimes. Furthermore, they create a framework for the provision of data, which is relevant to both coral reef management and to developing our understanding of how bioeroding sponge populations influence reef structure and carbonate budgets.