Hydrodynamic shear removal of the nuisance stalk-forming diatom Didymosphenia geminata

The removal of benthic algae during periods of high flow is critical in maintaining the biodiversity of stream ecosystems. Here we determine a shear removal function for the nuisance, stalk-forming benthic diatom Didymosphenia geminata by using samples collected from Rocky Mountain streams subjected to increasing bed shear stress in a laboratory flow chamber experiment. A linear shear removal function was observed to apply over the range of the shear stress obtained in the flow chamber. The overall removal of biomass was low. Less than 25% of the biomass was removed at a shear stress similar to that which would result in widespread bed disturbance in the stream. These results support the hypothesis that physical abrasion during periods of bed disturbance, rather than simply elevated shear stress, is the primary control on the removal of benthic algae such as D. geminata that are well adapted to the high-shear environments of mountain streams. The results also indicate that the shear removal function generally decreases with increasing biomass and mat thickness, the potential consequence of a positive feedback with near-bed hydrodynamics as the mats develop in the streams. The shear removal function was also influenced by the health and condition of the mats. Greater biomass removal was observed for mats in poorer condition. These mats had higher sediment content and were collected at the end of the growing season and from sites at lower elevations.