Detached kelps from distant sources are a food subsidy for sea urchins

Trophic subsidies link habitats and can determine community structure in the subsidised habitats. Knowledge of the spatial extents of trophic interactions is important for understanding food webs, and for making spatial management practices more efficient. We demonstrate trophic linkages between detached (drift) fragments of the kelp Ecklonia radiata and the purple sea urchin Heliocidaris erythrogramma among discrete rocky reefs separated by kilometres. Sea urchins were abundant at one inshore reef, where the biomass of drift was usually high. There, sea urchins trapped detached kelp at high rates, although local kelp abundance was low. Most detached kelp present on the reef was retained by sea urchins. Detached seagrass, which was abundant on the reef, was not retained by sea urchins in large quantities. Experiments with tethered pieces of kelp showed that sea urchins only consumed detached fragments, and did not consume attached kelps. Comparisons of the morphology of detached fragments of kelp collected from the inshore reef to attached kelps from reefs further offshore showed that a large proportion (30-95%, varying among dates) of the fragments originated at distant reefs (≥2 km away). At the inshore reef, the sea urchin H. erythrogramma is subsidised by detached kelps, and detached kelp fragments have been transported across landscapes. Cross-habitat resource subsidies therefore link discrete reef habitats separated by kilometres of non-reef habitat.