Context-dependant survival of the invasive seaweed Codium fragile ssp. tomentosoides in kelp bed and urchin barren habitats off Nova Scotia

This study examines the fate of the invasive alga Codium fragile ssp. tomentosoides at destructive grazing fronts of the green sea urchin Strongylocentrotus droebachiensis along the margins of algal beds and in barren grounds formed in the wake of these fronts. We monitored the first reported encounter between an urchin front and an algal bed containing C. fragile, and conducted a series of manipulative experiments at a grazing front and in a barrens habitat. Urchin density had a significant effect on survival of C. fragile. At low densities, urchin fronts were more likely to bypass the invasive alga, though urchins following behind the front eventually consumed most individuals. Urchins' preferred food, laminarian kelps, affected the survival time of C. fragile by slowing the forward propagation of the front, but did not divert urchins from consuming C. fragile. The presence of dense stands of the unpalatable macroalgae Desmarestia viridis and periods of high water temperature and wave action appeared to facilitate the survival of C. fragile by affecting urchin foraging behaviour. Our results suggest that, although urchins have the potential to exert strong control over populations of C. fragile, the outcome of interactions between the 2 species is likely to depend on their biotic and abiotic context.