Physical disturbance by kelp abrades erect algae from the understorey

Positive and negative interactions among organisms are key determinants of pattern inthe distribution and abundance of many species. Beneath subtidal canopies of kelp Ecklonia radiata (Laminariales), we observed sparse covers of erect algae (articulated coralline algae and filamentousturf-forming algae) that formed extensive covers where canopies were absent. Moreover, articulated corallines occurred in greater abundance beneath canopies of E. radiata mixed with canopy-forming species of Fucales than beneath monospecific canopies of E. radiata. We experimentally tested the hypotheses that (1) canopies negatively affect the abundance of articulated corallines and filamentous turfs, (2) physical abrasion of the substratum by canopies contributes to such negative effects, and (3) the effect of abrasion on articulated corallines is greater beneath monospecific canopies than mixed-species canopies, but the effect on filamentous turfs does not differ between types of canopy. Experiments revealed large negative effects of canopies on the abundance of articulated corallines and filamentous turfs, to which abrasion made a substantial contribution (~54 to 67% for articulated coralline and ~58% for filamentous turf). Moreover, the intensity of abrasion was greater beneath monospecific than mixed-species canopies, which was consistent with differences in the effect of canopies and abrasion on articulated corallines (monospecific > mixed-species) but not filamentous turfs (monospecific = mixed-species). Although abrasion is one of many possible influences of algal canopies, our results show that it can substantially contribute to the heterogeneity of understorey habitat on subtidal rocky coasts. Comparison with prior research suggests that the effects of E. radiata canopies on understorey algae may be largely explained by the combined effects of shade, sedimentation and abrasion.