The knotted wrack Ascophyllum nodosum has both an ecological and economical importance on sheltered rocky shores of the North Atlantic, being the commercial harvest of this macroalga an important activity of the West of Ireland. The biotic assemblage associated with A. nodosum commonly includes Vertebrata lanosa, an obligate red epiphytic alga growing on their thalli. Several invertebrates use this epiphyte as a habitat. Therefore, assessing environmental impacts is crucial to ensure habitat resilience and the sustainability of the seaweed extraction industry.
This study was conducted on an intertidal site in south Galway Bay (Ireland) and focused on the potential effects of harvesting A. nodosum using two non-mechanical methods (hand-harvesting with knives and using cutter rakes from boats) on V. lanosa biomass and the associated biodiversity. The study differentiated potential differences in the responses to impact from A. nodosum communities on hard substrates (e.g., large boulders) and those on mixed, gravelly muddy substrates. The results showed a decrease in V. lanosa biomass, an overall loss in invertebrate abundance and a change in species composition after the harvesting. However, it was not possible to conclude if these changes were solely the result of seaweed extraction. It was found that seasonal and spatial variability in V. lanosa and the associated assemblages may obscure the detection of canopy harvesting effects.
