The influence of sediment mobility and channel geomorphology on periphyton abundance

Summary River managers currently have difficulty predicting the combined effects of changes in flow regime and nutrients on periphyton biomass. Biomass accumulation is known to be promoted by increasing nutrients, light and temperature, and its loss has been related to invertebrate grazing and hydrological disturbance. However, biomass predictors that are reliable across a range of rivers have proven elusive. One possible contributing reason is that disturbance thresholds used in predictive models are typically linked to flow metrics, whereas the mechanisms for periphyton removal (current drag, abrasion and molar action by mobile sediment) relate more directly to hydraulic and geomorphic conditions. We explored this possibility by relating periphyton removal events to hydraulic thresholds for sediment entrainment in 18 gravel- to boulder-bed river reaches at which periphyton cover and nutrient concentrations had been regularly monitored. We converted observed threshold discharges for periphyton removal into shear stress thresholds using hydraulic models. Our results demonstrate that: (i) abrasion by finer fractions of the bed material (2–16 mm) was the dominant physical mechanism removing periphyton; (ii) the frequency of mobility of this fine bed material was the dominant control on periphyton abundance and (iii) growth-promoting variables, such as nutrient concentrations, tended to only become important to periphyton abundance when the frequency of sediment movement was low. These findings highlight the importance of geomorphic differences between sites and explain why a single flow metric may be a poor predictor of periphyton abundance across geomorphically different rivers. Our analysis suggests that partitioning sites based on frequency of sediment mobility (either sand or the D50) could improve predictability of sites at which there is potential for nuisance levels of periphyton to develop.