Habitat engineering by decadal-scale bioturbation around shipwrecks on the Great Barrier Reef mid-shelf

Abstract

Conspicuous, systematic patterns of holes in soft sediment around wreck sites on the Great Barrier Reef mid-shelf are documented. Multibeam bathymetry data show concentric 'halos of holes' extending hundreds of meters around the wrecks. These halos at 5 wreck sites between 16 and 111 yr old consist of hundreds of tightly spaced depressions with diameters of up to 10 m. Seafloor imagery clearly indicates a biogenic origin of the holes. The halos of the 2 youngest wrecks (<25 yr) have fewer holes than older wrecks, suggesting a relationship between wreck age and excavation activity. Conversely, the size of a wreck does not appear to control the sediment turnover activity. Holes are comparatively long-lived, and some of them are actively maintained by bioturbation activity. The spatial distribution of holes is consistent with resource depletion by a central-place forager, albeit on yearly to decadal time scales. Tow-camera data show that some holes support diverse sessile faunal assemblages of sponges, soft corals, and hard corals in an otherwise flat seafloor devoid of sessile fauna. The (to date unknown) bioturbator(s) are thus allogenic habitat engineers. The systematic occurrence of halos around wrecks suggests that 'the ecosystem wreck' extends beyond the spatial confines of the hull of a wreck to the seafloor, by a distance 10-fold greater than the size of a wreck itself. This has implications for the evaluation of ecological effects of artificial reefs such as wrecks and offshore wind farms on decadal time scales.