Effects of algal canopy clearance on plant, fish and macroinvertebrate communities on eastern Tasmanian reefs

Changes in assemblages of plants, macroinvertebrates and fishes on three eastern Tasmanian reefs were monitored over 12 months in replicated control blocks and adjacent 10×12-m blocks cleared of fucoid, laminarian and dictyotalean algae. Removal of canopy-forming plants produced less change to biotic assemblages than reported in studies elsewhere, with the magnitude of change for fish and invertebrate taxa lower than variation between sites and comparable to variation between months.The introduced annual kelp Undaria pinnatifida exhibited the only pronounced response to canopy removal amongst algal taxa, with a fivefold increase in cleared blocks compared to control blocks. Marine reserves are suggested to assist reef communities resist invasion by U. pinnatifida, through an indirect mechanism involving increased predation pressure on sea urchins and reduced formation of urchin barrens that are amenable to U. pinnatifida propagation.Large invertebrates were more associated with turfing algae or the reef substratum than the macroalgal canopy. The herbivorous sea urchin Heliocidaris erythrogramma and abalone Haliotis ruber showed the strongest response to clearing amongst common macroinvertebrate species, with a halving of population numbers. Observed densities of the common monacanthid fish Acanthaluteres vittiger also declined by about 50%. The relatively high level of resistance shown by eastern Tasmanian reef biota to patch disturbance was attributed largely to high diversity and biomass of turfing macroalgae damping effects of canopy clearance.