Beyond the reef: cross-habitat linkages shape soft-sediment biodiversity in a coral reef lagoon

Soft-sediment habitats cover approximately 80% of the seafloor and comprise 95% of the benthic habitat in the Great Barrier Reef, Australia. They support diverse and abundant macroinfaunal communities that play key roles in ecosystem functioning. However, macroinfauna are among the least-studied components of coral reef ecosystems, and we lack knowledge of their biodiversity, variation, and the scale-dependent processes that shape them. Here, we investigated spatial variation in macroinfaunal community structure and composition across multiple scales within the coral-reef lagoon of One Tree Reef on the Great Barrier Reef. Across the lagoon, total abundance, polychaete richness, and the abundances of nematodes, oligochaetes, and a Syllid sp. declined as the proportion of fine sediment increased. At patch-reef scales, mollusc abundance and richness, Sabellid sp. abundance, and Gastropod sp. abundance increased with distance from reefs, whereas a Lumbrinereid sp. and Mediomastus sp. decreased. These findings advance our understanding of the drivers shaping soft-sediment assemblages and provide baseline data for assessing how reef decline and increased fine-sediment inputs may affect adjacent soft-sediment habitats.