Microspatial variation in marine biofilm abundance on intertidal rock surfaces

The effect of substrate surface roughness on small-scale patchiness and the ability of molluscan grazers to feed on intertidal biofilms was examined in a factorial experiment. Granite slabs were treated to create 4 different levels of surface roughness, and biofilm and macroalgae were allowed to recruit. Biofilm cover varied greatly with slab roughness, and was spatially patchy at a scale of millimetres. Diatoms dominated the biofilm, but were less abundant on surfaces with the smallest pits. Cover of diatoms and cyanobacteria was affected by surface roughness, with increased abundance around surface features. Different species of grazer varied in their success at removing certain diatoms and cyanobacteria from slabs of varying roughness, due to either the morphology of the different food types or grazer radula structure. Cover of macroalgal species on the slabs of different roughness also varied, and one species, Hypnea sp., did not recruit on smooth slabs. Rock roughness, therefore, affects both the biofilm and algal species that recruit and their abundance. Grazers were able to remove algae from slabs of all roughness with no apparent species-specific differences in their ability. However, grazer species appear to be more or less efficient at feeding according to the level of roughness, and this combination of variation in rock roughness and grazer efficiency may explain the observed small-scale patchiness on rocky shores in Hong Kong