Effects of gap size and the season of clearing on algal patch recovery were investigated in a high rocky intertidal shore of Bntish Columbia, Canada, to understand community dynamics in relation to physical disturbance. The comn~unity studled contains 3 domlnant perennial macroalgae, Mazzaella cornucopiae (= Iridaea corn ucopiae, Khodoph y ta), Fucus distichus and Pelvetiopsis lin~itata (Phaeophyta), and some ephemeral algae as well as barnacles and limpets. The experimental design comprised 3 gap sizes of clearing (5 X 5 cm, 10 X 10 cm and 20 X 20 cm) and 4 seasonal clearings (August 1991, October 1991, February 1992. April 1992. The recolonization rate of all algal species was fastest in the medium gaps, followed by the small and large gaps. Thus, medium gaps were filled by a relatively well-mixed combination of all algal species including M. cornucopiae (rare in large gaps) and E djstichus (low in small gaps). Responses of each alga to the different slzes of clearing were speciesspecific and related to the alga's reproductive characteristics. The results Indicate that negative factors (e.g. whiplash, desiccation) were probably responsible for slow colonization in the 2 extreme sizes of gap. Different algal species responded differently to the gaps cleared in different seasons. New substrata created a short time (e.g. 2 mo) before the peak of fucoid recruitment in unmanipulated plots received a greater number of fucoid propagules, and this initial recruitment secured fucoid persistence in the later successional period. On the other hand, for M. cornucopiae, which propagates mostly vegetatively, the timing of clearing generally did not influence dominance of this alga in the patches. Barnacle densities during the experimental period were less in small gaps than in medium and large gaps, but limpet abundance was not significantly influenced by clearing size. The kinds of algal species which move in when a gap of a particular size is created in a particular season were predicted for this community using the interaction terms of size and season effects of disturbance.

Pterygophora californica Ruprecht, Eisenja arborea Areschoug and Laminana setchellii Silva are 3 common kelps (Laminariales, Phaeophyta) found in the shallow subtidal of open coastal British Columbia, Canada. Earlier laboratory studies showed that P. californica stipes had a cr~tical flaw length (CFL, the length of a flaw which, if exceeded, results in structural failure) of 0.2 mm. In this paper w e report on studies to determine whether this CFL is valid in situ, whether all 3 species of kelp respond similarly to stipe damage and whether tissue that regrows in the site of a wound increases the mechanical strength of the stipe. We tested the effect of razor cuts less than the CFL and also razor cuts and cuts imposed by a triangular file that exceeded the CFL. Results were not as predicted. Neither razor cuts nor file cuts greater than the CFL significantly decreased the survivorship for P californica over control thalli. L. setchellii had a significantly lower survivorship when cut by a triangular file but not when cut by a razor Tissue regrowth does not significantly increase mean strength of the stipe. We hypothesize that the low survivorship of L. setchellii a s a result of stipe damage is one factor responsible for preventing successful establishment of adults of this species in sites where sea urchins a r e common. In contrast, the comparatively h~g h e r survivorship of P. calilornica adult plants subjected to similar damage may b e one factor enabling this species to survive in such sites.