The influence of channel bed disturbance on algal biomass in a Colorado mountain stream

The effects of channel bed disturbance on benthic chlorophyll a accrual were examined in three reaches of the Williams Fork River, CO, USA. A field calibrated multidimensional hydraulic model was used for estimation of shear-stress distributions. Sediment grain size was used to determine the critical shear stress for bed movement. Disturbance at a given location is defined as the percentage of time during the growing season that the bed sediment is in motion. This metric was compared with periphyton biomass accrual. Growth control factors including temperature, grazing intensity, nutrient concentrations, and irradiance also were measured. Disturbance mapping showed that the common concept of bed disturbance as a byproduct of high-flow events is overly simplistic. In the Williams Fork, bed movement occurs constantly over certain portions of the bed, even at low flows. There is a continuum of bed movement, expressed as percentage of the bed in motion, extending from low to high flows. Periphyton biomass accumulated exponentially in all study reaches but accumulation rates were inversely proportional to local disturbance. Periphyton biomass increased by approximately three orders of magnitude across the three reaches, but failed to reach a plateau. A combination of moderate grazing rates, low-nutrient concentrations, moderately impaired solar irradiance, and, most importantly, low temperatures explains the failure of periphyton biomass to reach a plateau. This study shows that the control of periphyton biomass may be explained in streams by bed disturbance over the growing season plus the separate, superimposed on influence of population growth rate control factors. Copyright  2010 John Wiley & Sons, Ltd.