Mechanisms of Competition betxeen Canopy-Forming and Turf-Forming Intertidal Algae

Abstract

Mechanisms of competition between two canopy algae and an understory alga were investigated by a field manipulative experiment using artificial thalli. The study was carried out in the upper intertidal zone at Nudibranch Point in Vancouver Island, British Columbia, Canada, where two fucoids, Fucus gardneri and Pelvetiopsis limitata, and a turf red alga, Mazzaella cornucopiae, were dominant in the algal community. The experiment was designed to test three hypotheses, shading, whiplash, and allelopathy, imposed by potential fucoid effects on M. cornucopiae. Only the shading effect was significant, indicating that adult fucoid thalli reduced M. cornucopiae biomass underneath the fucoids. Results indicated that reversal of competitive dominance existed between F. gardneri and M. cornucopiae depending on the life history stage of the competitors. By including the turf alga's effects on the fucoids, the well-balanced and non-hierarchical interaction networks among the major macroalgae support the high likelihood of species coexistence in the community.