THE EFFECTS OF SUBSTRATE TEXTURE, GRAZING, AND DISTURBANCE ON MACROALGAL ESTABLISHMENT IN STREAMS

Two common macroalgae, Cladophora glomerata (a filamentous chlorophyte) and Nostoc parmeloides (a colonial cyanobacterium), are strongly associated with rough substrates in Rattlesnake Creek, a second-order stream in coastal Santa Barbara County, California. To test the hypothesis that rough texture reduces algal mortality from invertebrate grazers and from winter storms, we conducted a series of experiments on two substrate types (tiles) that varied only in surface heterogeneity. The dominant grazer, the caddisfly Agapetus celatus, was excluded from tiles using petroleum jelly barriers, and algal establishment was compared with that on control tiles; grazing experments were repeated during spring and autumn. In a separate experiment, storm scouring was simulated by manually scrubbing naturally colonized rough and smooth tiles at different intervals (7 wk, 3 wk, and once, with an undisturbed control) from December to June, during which algal establishment and survival were monitored. In the absence of Agapetus, Cladophora was found nearly equally on rough and smooth surfaces. Grazers strongly reduced Cladophora establishment on all surfaces (often by > 80%); 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; however, microscopic examination showed that basal filaments, and new sporelings, were protected within depressions. Nostoc attained densities in autumn of almost-equal-to 50-100 colonies/100 cm2 on rough tiles and was virtually absent from smooth tiles. Unlike Cladophora, Nostoc 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. Substrate heterogeneity provides refuges from herbivory and disturbance for establishing macroalgae. While it increasingly appears that herbivory plays a profound role in determining algal dynamics in streams, herbivores and substrate texture may also interact to modify the rate and direction of benthic succession. By inhibiting the establishment of intermediate taxa (e.g., Cladophora), grazers may restrict succession on smooth substrates to an early, diatom-dominated assemblage, or accelerate dominance by later taxa (e.g., Nostoc) on rugose substrates.