The effects of substrate texture, grazing, and disturbance on macroalgal establishment in streams

Abstract

Cladophora glomerata (a filamentous chlorophyte) and Nostoc parmeloides (a colonial cyanobacterium) are strongly associated with rough substrates in Rattlesnake Creek, Santa Barbara County, California. The dominant grazer, the caddisfly Agapetus celatus, was excluded from tiles. In the absence of Agapetus, Cladophora was found nearly equally on rough and smooth surfaces. Grazers strongly reduced Cladophora establishment on all surfaces; texture did not generally influence grazing effects in autumn, but refuges (pits) greatly increased algal survival in spring when recruitment was greater. Two other grazing insects (Baetis spp. and Micrasema) increased on exclusion tiles, apparently due to competitive release, but did not eliminate effects of Agapetus. Manual disturbance at 3- and 7-wk intervals kept Cladophora densities much lower than on control and once-scoured tiles. Densities were higher on rough tiles, but the interaction between disturbance and texture was not significant for macroalgae; basal filaments, and new sporelings, were protected within depressions. Nostoc attained densities in autumn of 50-100 colonies/100 cm2 on rough tiles and was virtually absent from smooth tiles. It was little affected by grazing, but was greatly reduced by disturbance. Rough texture provided minor protection, but mature Nostoc formed basal crusts that resisted disturbance, thereby maintaining space in the absence of severe scour. -from Authors