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Cullis, James D ¹

¹ University of Colorado

Previous studies in Boulder Creek, CO, have shown that maximum shear stress is a significant factor in determining the abundance of the nuisance diatom Didymosphenia geminata. The aim of this study was to investigate this observation further in order to determine a possible threshold for the removal of D. geminata.

Coverage was monitored at a number of study sites in Boulder Creek using a qualitative Didymo Rating Index (DRI) on a bi-weekly basis over two consecutive summers. The results indicated that a removal threshold does exist and that this is best described by Shield’s stress, which is the ratio of the maximum average bed shear stress to the average bed particle size.

In Boulder Creek the critical flow required for the removal of D. geminata is around 10 m3/s. This is approximately equal to the average annual maximum flood. The critical level of Shield’s stress is similar to that required for the initiation of bed disturbance which supports the hypothesis that physical bed disturbance is required for effective removal from the stream bed. Spatial variations due to channel characteristics and variations in bed particle size result in spatial variation in the removal of D. geminata between sites and within a stream reach for a given discharge.

These results have implications for considering the potential to used managed flood releases from reservoirs to control future growth of this nuisance species. Further studies will look at the spatial variation in shear stress and bed disturbance and how this might affect the removal of D. geminata. In addition laboratory experiments are planned that will determine the shear resistance of the mats and the removal rate due to simulated bed disturbance directly.