Resilience of the sessile benthic community to rockweed harvest

Ascophyllum nodosum (rockweed) is a dominant, canopy-forming seaweed found on intertidal rocky shores in the North Atlantic. Commercial uses for agriculture and food additives have supported industrial-scale harvesting by multiple countries in both the North American and European portions of its range. Rockweed removal changes the three-dimensional structure of the intertidal ecosystem, altering the characteristics of light, temperature, and wave energy for the benthic community. The potential for community impacts could be considerable. We conducted a three-year Before-After-Control-Impact (BACI) study to quantify how partial canopy removal affects the sessile benthic community along the coast of Maine (USA), where harvest regulations are more conservative than those found in all other countries. Using 22, marked, 16 m² plots, we measured rockweed canopy height, biomass, and sessile taxa composition before and for two years after industrial harvest conducted by commercial partners. Presence and abundance models showed limited taxon-specific responses. While most benthic taxa were similar between control and impact sites, a few algae and invertebrates showed temporary reductions in line with changes in rockweed biomass, canopy height, or treatment category. Crustose red algae, on the other hand, exhibited somewhat higher abundance in harvested plots. Overall, the sessile community demonstrated stability after a single-event rockweed harvest. These results suggest that the current conservative commercial harvesting practices in Maine have limited short-term impacts on rockweed-associated benthic assemblages. Without the addition of other stressors, these practices would not be expected to lead to long-term changes in the benthic community.