POST-SPATE DEVELOPMENT OF EPILITHIC ALGAL COMMUNITIES IN DIFFERENT CURRENT ENVIRONMENTS

Development of epilithic algal communities was monitored in fast- (29 cm.s-1) and slow-current (12 cm.s-1) stream channels for 33 d following a simulated spate. Disturbance reduced diatom densities in both current regimes to equal levels, but recovery was more rapid in slow current. Maximum algal abundances, which seemed to reach carrying capacity in both current environments, were lower in fast current. The percentage of live cells in diatom assemblages decreased during community development. The small, monoraphid diatom Achnanthes minutissima dominated communities in both current regimes from 0-12 d, principally because of its persistence on substrata through disturbance. Post-spate diatom accumulation was more rapid in slow current, presumably due to higher cell export in fast current. Differences in community structure between current regimes during early (0-6 d) recovery were attributed to differences in attachment strength and growth form of persistent species. In both current regimes, initial dominance by A. minutissima shifted to dominance by Synedra spp. and dense, floating mats of zygnematalean green algae. Zygnematalean mats enhanced diatom deposition and probably reduced water-column nutrient concentrations, altered light availability, and increased habitat heterogeneity. Zygnematalean proliferation affected diatom succession in slow but not in fast currents, suggesting that autogenic changes were more pronounced in slow-current habitats. -Authors