Effects of canopy-mediated abrasion and water flow on the early colonisation of turf-forming algae

Algal canopies form predictable associations with the benthic understorey, and canopy-mediated processes may maintain these associations. Three canopy-mediated processes that are inherently linked are water flow through a canopy, abrasion of the substrate by the canopy, and light penetration. These processes were experimentally reduced to test the hypotheses that turf-forming algae would be: (1) positively affected by reduced abrasion by kelp canopies; (2) positively affected by reduced water flow; and (3) negatively affected by shading (reduced light). Biomass of turf-forming algae was greater when abrasion was reduced, but less when light was reduced. In contrast to predictions, reduced water flow had a negative effect on the percentage cover and biomass of turf- forming algae, rejecting the second hypothesis. It seems, however, that this negative effect was caused by an increase in shading associated with reduced canopy movement, not a reduction of water flow per se. None of the factors accounted for all of the change seen in understorey algae, indicating that it is important to study the interactive effects of physical processes.