Interactive Effects of Sediments and Urchins on Tropical Macroalgal Forests

Abstract Macroalgal forests are a prominent component of tropical seascapes providing food and shelter for many species whilst subsidising secondary productivity in coastal ecosystems. Monitoring and managing macroalgae requires an understanding of key processes that alter these assemblages, especially at the local scale. Here we consider how sediment depth/cover and abundance of the macroalgal grazing urchin Tripneustes gratilla (Linnaeus, 1758), influence variation in community composition and physical structure of macroalgal patches within the World Heritage listed Ningaloo Reef, Western Australia. Our study found high urchin densities in sites with lower sediment loads and that sediments, urchin abundance and seascape position combined, explained 32% of the spatial variation in macroalgal forest community composition. We also found a greater occurrence of Sargassopsis, Caulerpa, Sirophysalis and Hormophysa in the lagoon where sediment cover/depth was high. Canopy height and cover of canopy forming macroalgae was also greater where sediment loads were high and urchin abundance low. Macroalgal genera with a strong and robust thallus were found in sediment depths up to 6 cm as opposed to those with softer, fragile thalli that were found in sediments < 2 cm. Our results demonstrate that high densities of urchins and mobile sediments can have profound effects on the composition and structure of tropical macroalgal forests. Changes to macroalgal forest community composition and physical structure will have flow-on effects for ecological processes such as productivity and habitat use which can alter the ecosystem goods and services macroalgal forests provide.