Factors determining the upper limits of intertidal canopy-forming algae

Abstract

The causes of zonation of canopy-forming algae have been investigated by canopy removal experiments, combined with observations at the upper limits of various species on certain British shores during the exceptionally hot weather of summer 1983. Fucus vesiculosus, F. serratusand Laminaria digitata all extended their range upshore when the species zoned above were experimen- tally removed, indicating that competition can be directly responsible for setting upper limits of lowand mid-shore canopy algae. Removal of species zoned immediately below permitted down-shore extension of Pelvetia canaliculata, F. spiralis, F. vesiculosus, F. serratus and Himanthalia elongata, confirming the setting of lower limits of these species by competition. Amongst canopy-forming species, P. canaliculata, F. spiralis, L. saccharina, L. digitata and the 'button' stage of H elongata showed signs of damage during summer 1983. There were no signs of damage to F. vesiculosus, F. serratus and Ascophyllum nodosum. Some species of low-shore, turf-forming understorey and encmsting red algae seemed more vulnerable to hot weather than the canopy-forming species. Corallina officinalis and 'lithotharnnia' suffered extensive damage during summer 1983.