Canopy removal experiments in Cystoseira-dominated rockpools from the Western coast of the Mediterranean (Ligurian Sea)

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The effects of canopy removal on algal abundance and diversity were evaluated in Cystoseira-dominated rockpools on the west coast of Italy (western Mediterranean). Four large pools ranging from 0.1 to 0.3 m above MLLW, were selected for experiments. Two canopy removals and a control were established in each pool in a randomized block design. One treatment was initiated in July 1989 (removal in summer) and the other in January 1990 (removal in winter). Algae were collected monthly by a destructive sampling method, until July 1990. Cystoseira removal induced some modifications in the algal assemblage. Articulated corallines, delicately branched and thick blades and branched algae were positively affected by canopy removal. The effect of summer and winter removal experiments was similar. In July 1990, control plots were still dominated by Cystoseira plants, while treatment areas supported a dense assemblage of turf-forming algae. At the specific level, however, strong negative interactions only occurred between Cystoseira and a few other algae, such as Corallina elongata, Padina pavonica and Rytiphloea tinctoria. These species accounted for many of the differences between treatment and control plots. As a consequence, diversity, expressed either as the Shannon-Weaver index ( H′) or as species richness ( S), was not significantly affected by canopy removal. Many of the algae that grew as epiphytes on Cystoseira or that lived in the understorey were able to persist in treatment areas in the turf matrix or as epiphytes on the new dominants. These results support the idea that communities are organized in small “components”, where strong interactions occur only between a few sets of species. Factors that could explain species coexistence in Cystoseira-dominated rockpools are also discussed.