DISTURBANCE OF STREAM PERIPHYTON BY PERTURBATIONS IN SHEAR-STRESS - TIME TO STRUCTURAL FAILURE AND DIFFERENCES IN COMMUNITY RESISTANCE

The resistance of stream periphyton to structural disturbance by increases in shear stress (simulating a spate) was investigated in a laboratory flow tank. We monitored loss of biomass from filamentous community (dominated by Melosira varians) under four different levels of shear stress. In each case, any loss that was going to occur did so within 10 min for this community. In a second experiment, we tested the resistance of four different communities (two dominated by nonfilamentous diatoms and two dominated by filamentous green algae/diatoms) to increases in shear stress. Nine different levels of shear stress were used, ranging from 1- to 70-fold higher than the conditions to which the communities were acclimated. All communities were 14 days old, but some differences in initial biomass occurred that influenced the degree of resistance independently of species composition. Overall, the nonfilamentous diatom communities were the most resistant, and the filamentous communities were the least resistant. The Kinetics of the sloughing process varied among community types, with a community dominated by Melosira varians/Gomphonema parvulum losing 50% of its biomass with only a 3-fold increase in shear stress. In contrast, a community dominated by the nonfilamentous diatoms Fragilaria vaucheriae/Cymbella minuta lost <50% of its biomass after a 70-fold increase in shear stress. Shear stresses required for 50% loss of biomass for the different communities were as follows: 3.6 Newtons . m(-2) for the Melosira varians/Gomphonema parvulum community, 10.0 N . m(-2) for the Spirogyra sp./Gomphoneis herculeana/Ulothrix zonata community, 50.6 N . m(-2) for the Fragilaria construens/Cymbella minuta/Achnanthes minutissima community, and >90.0 N . m(-2) for the Fragilaria vaucheriae/Cymbella minuta community. These results show that spates without bedload movement can potentially have widely differing disturbance effects on periphyton loss among streams depending on the initial taxonomic composition of resident communities. These results have important implications for stream ecosystem analysis and modeling.