VARIATIONS IN BIOFILMS COLONIZING ARTIFICIAL SURFACES - SEASONAL EFFECTS AND EFFECTS OF GRAZERS

The colonization of microscopic organisms, commonly called a biofilm, was examined on fibreglass panels situated intertidally at Quibray Bay of Botany Bay in New South Wales, Australia. Panels were examined by incident light microscopy, measuring percentage cover, and by a computer image analysis technique, measuring optical density. Optical density was positively correlated with and was therefore a reliable estimate of total percentage cover of the biofilm. Optical density has not been used before in this application and, although some drawbacks are discussed, it is a much more efficient sampling method than microscopic examination of panels. Variations in the percentage cover and composition of the biofilm colonizing surfaces were assessed with regard to: (i) different seasons and periods of exposure and (ii) the presence of grazing gastropods. The greatest cover of the biofilm occurred on panels in summer and spring. Diatoms and bacteria dominated the assemblage in summer, the alga Enteromorpha sp. in spring and the protozoan Vorticella sp. dominated panels for one month in winter. Also, the presence of grazers significantly decreased total microscopic cover (as measured by optical density). These results demonstrated complex variations in the percentage cover and composition of microscopic assemblages which should be taken into account when considering the effects of a biofilm on macroscopic patterns and processes.