Disturbance by floods is believed to be 1 of the fundamental controllers of temporal and spatial patterns in stream periphyton. However, the exact causes of biomass losses are still poorly understood and discharge measures of disturbance often only explain limited variance in periphyton development. We investigated the effects of 2 of the main mechanisms of flood disturbance to periphyton--frequency of high-velocity events and frequency of bed sediment movement--in an effort to better understand disturbance processes and improve the quantification of flood disturbance regimes for studies of stream periphyton. Three sites were selected in headwater streams in each of 4 groups according to a 2-way factorial design of frequency of high-velocity events and sediment stability, giving a total of 12 sites. Periphyton were sampled monthly for 15 mo and analyzed for chlorophyll a. Maximum photosynthetic rates (P_max), chlorophyll-specific P_max, community respiration (CR), and P_max:CR ratios were determined seasonally. Nutrient concentrations were generally low and did not vary as a function of disturbance regime. Peaks in chlorophyll a were usually low reflecting the low nutrients. Chlorophyll was 2-10× higher where bed sediments moved <15×/y and with seasonal maxima most often in autumn. Frequency of bed movement, soluble reactive P, and the frequency of velocity perturbations were significant predictors of mean monthly chlorophyll a (r² = 0.88). Chlorophyll a and water temperature were major correlates of P_max, specific P_max, and CR, and thus the metabolic variables partly reflected changes in biomass among the disturbance regimes. With chlorophyll and temperature removed as covariates, the main factor influencing all metabolic parameters was season. P_max was 7× higher in summer than in spring when minima occurred, chlorophyll-specific P_max was 10× higher in summer than in spring, and CR was 4× higher in autumn than in spring. P_max:CR ratios indicated that the communities were generally autotrophic at times of maximum photosynthesis with the highest ratios in summer (3× higher than winter). The frequency of velocity perturbations also had a significant effect on P_max:CR ratios with highest ratios at sites where there was a low frequency of high-velocity events. Our results suggest that sediment instability greatly increases disturbance intensity for periphyton. It is therefore essential to assess not just the frequency of floods, but also the degree of bed movement when quantifying disturbance regimes for periphyton in headwater streams.

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