Drift algal subsidies to sea urchins in low-productivity habitats

Highly productive kelp beds off the coast of Nova Scotia, Canada, export a large quantity of detrital material to adjacent low-productivity habitats. We used a combination of dietary tracers (fatty acids, stable isotopes, and gut contents) and gonad index to evaluate the importance and spatial extent of this energy subsidy to green sea urchins Strongylocentrotus droebachiensis offshore from kelp beds along 240 m transects perpendicular to the shore at 4 sites. Gut contents and δ ¹³C values indicated the presence of kelp in the diets of sea urchins collected up to 240 m offshore from kelp beds. We observed a corresponding decrease in gonad index with distance from the kelp at all sites but one, where patches of live kelp offshore from the main kelp bed provided an additional food source. Sea urchins that fed on a large pool of detrital kelp at another site had ̃15% larger gonads than sea urchins at other locations. δ ¹⁵N values were more enriched for sea urchins at 160 and 240 m from the kelp bed, suggesting that these sea urchins consume more animal matter, which was also evident in their gut contents. Our findings suggest that drift kelp represents an important energy source for sea urchins in subtidal habitats on the scale of tens to hundreds of meters offshore from kelp beds and that this resource is increasingly patchy in space and time with distance from the kelp bed.