Effects of an experimental reduction in grazing by green sea urchins on a benthic macroalgal community in eastern Newfoundland

Abstract

An urchin removal experiment was performed over 5 yr at a heavily grazed, sublittoral site in Conception Bay, Newfoundland. Three transects were used: one unrnanipulated, one from which urchins were removed manually for 3 yr, and one from which urchins were removed using quicklime. Each site was sampled in the upper (2 to 3 m) and middle (6 to 9 m) urchin-dominated zone. Macroalgal biomass increased following urchin removal by both methods. At 2 to 3 m a closed canopy of Alaria escdenta developed,with few other algal species, except during the summer when the canopy was reduced by blade erosion. The community that developed at 6 to 9 m consisted mainly of Desmarestia acdeata, with lesser amounts of other canopy-forming algae (A. esculenta, Agarum cribrosum, and Laminaria digitata). When urchins were allowed access to the urchin removal sites, they grazed away a l l algae except A. cribrosum and Ptilota serrata. Some A. cribrosum survived for at least 2 yr after urchin reinvasion. Algal biomass in the unmanipulated area was low for the first 3 yr, but both biomass and species richness increased during the summers of 1982 and 1983. These algae were removed by urchin grazing and natural senescence during the autumn and winter. The species composition of this temporary summer community differed, however, from the community on the removal transects, and was dominated by annual algae such as Acrothrix novae-angliae, Chordaria flagelliformis, Dictyosiphon foeniculaceus, Ectocarpus siliculosus, and Eudesme virescens, whereas perennials such as A. escdenta and D. aculeata dominated in the urchin removal area.