Does intertidal canopy removal of Ascophyllum nodosum alter the community structure beneath?

- Effects of harvesting A. nodosum on sediments and invertebrates were examined.
- Harvesting did have significant effects on mobile megafauna invertebrate densities.
- Harvesting did not have significant effects on densities of infauna.
- Harvesting did not have consistent significant effects on sediment characteristics.
- These results are valuable for management decision making.

Changes to associated communities when habitat is altered have been well documented in terrestrial and marine systems. In marine ecosystems, subtidal canopy removal of overlying vegetation is known to impact both physical and biological processes below. Recently, in Maine (USA), there has been much concern about the impact of harvesting (i.e., canopy removal) of Ascophyllum nodosum (rockweed) on the intertidal community. To examine the impacts of rockweed harvesting on the sediment structure and invertebrates associated with the sediment, control and experimentally harvested plots were established in May 2011 at Sears Island (Maine, USA). Monthly data collection through the summer and early fall included sediment cores, sieved infauna samples, and mobile megafauna quadrats. Sediment samples were analyzed for organic content as well as percent coarse sand, fine sand, silt, and clay. The experiment was repeated in 2012 in the same location with a modified design. In 2013 a commercially harvested site was sampled in the same manner before and after harvesting. Although differences in abundances of mobile megafauna were evident in the experimentally harvested sites, they were not evident in the commercially harvested site. Sediments and smaller invertebrates were mostly unaffected by canopy removal in both the experimentally and commercially harvested sites. While these data are contrary to those for subtidal canopy removal experiments, the rocky intertidal is a more physically dynamic habitat that may be tolerant of greater disturbance.