• Agarum clathratum;
  • Agarum cribrosum;
  • Alaria esculenta;
  • community stability;
  • disturbances;
  • grazing;
  • kelp;
  • mixed linear models;
  • positive interactions;
  • Ptilota serrata;
  • recruitment;
  • resilience;
  • spatial and temporal scales

The brown alga Agarum clathratum (Dumortier) is the only large, perennial, fleshy macrophyte commonly found on urchin-dominated barrens in the northwestern North Atlantic. We examined the spatial and temporal stability of A. clathratum stands and their impact on algal recruitment in the Mingan Islands, northern Gulf of St. Lawrence. The stands were highly stable in space and time, with only small intersite variations. The percent cover of A. clathratum in 144-m2 areas increased by 6.5%–11.4% over a 2-year period, and most changes in abundance occurred at the edge of the stands. The surface area of small (<13 m2) single stands of A. clathratum increased by approximately 1.8%·month−1, although marked increases (>95%) occurred during winter, largely because adjacent stands merged into larger single stands. Mature stands of A. clathratum appear to enhance algal recruitment, as juvenile A. clathratum and the understory red alga Ptilota serrata (Kützing) were orders of magnitude more abundant inside than outside the stands. The experimental removal of the A. clathratum canopy (1-m2 portions) had no long-term effect on the abundance of A. clathratum, which within 14 months had recolonized most of the cleared areas. In contrast to juvenile A. clathratum, the abundance of P. serrata rapidly decreased after canopy removal. Our results demonstrate that A. clathratum stands are a stable component of urchin barrens in spite of the heavy grazing that typically occurs there. Maintenance and expansion of A. clathratum stands and associated flora appear to depend on positive interactions with self-defended adult A. clathratum.