EXECUTIVE SUMMARY

Didymosphenia geminata, commonly referred to as “didymo” or “rock snot,” is a freshwater diatom, a type of algae that lives in streams attached to rocks and other substrates. Didymo has been present in North America for over 10,000 years and there is strong evidence that there were blooms prior to any human activity (Card, 2014). Preliminary calculations show that approximately 20 percent of Colorado mountain streams may contain didymo (Spaulding, 2007). The ability for didymo to be prolific was correlated to flow regimes of dam-influenced river systems (Spaulding, 2007). The impact of didymo on the Gold Medal fishery in the Lower Fryingpan River is currently unknown. This project analyzed the presence or absence of didymo including spatial and temporal variability, and water quality throughout the Lower Fryingpan River, from immediately below the Ruedi Reservoir dam to the confluence with the Roaring Fork. Potential management strategies for reducing the occurrence and persistence of didymo were also identified.

Results from year one of this study revealed a significant decrease in didymo after the high flow event (peak 700 cfs) in 2014. The decrease could be attributed to the scour effect from bedload migration. When compared to reference sites (upstream of Ruedi Reservoir and at the inlet), there was no visual evidence of didymo cover but presence was detected through microscopic analysis. Moving forward, more data is needed to fully understand the effect of flow regimes and the seasonality of didymo.

Results from a study of Colorado’s rivers indicate that high densities of didymo were related to a decline in total macroinvertebrate richness (Spaulding, 2007). Studies on the effects of didymo on fish are too recent with limited data (2-3 years) to determine any trends in the fish population. Fish that consume benthic prey and nest beneath or between cobbles could be most impacted because they utilize the same habitat as didymo (Larned et al., 2006). Given large amounts of non-nutritious stalk material present on stream substrate, didymo is predicted to have a deleterious effect on native fish (Spaulding, 2007).

There is much uncertainty around why and how didymo is spreading and reaching nuisance levels. The mechanisms driving the spread of didymo cannot be explained just by human transport, as didymo is being found in remote, hard to access areas of the world such as New Zealand, Colorado, and Chile (Spaulding, 2007). Some scientists speculate climate change could be causing didymo to bloom in new places on a global scale. It is not realistic or feasible for water managers of the Fryingpan River to minimize causes of climate change. It is more realistic to acknowledge and manage the impact of humans on the spread of didymo. Fishermen and other water users carry didymo from one body of water or reach of stream to another on their boots, waders, boats and/or tackle. At this time, the most feasible control of the spread of didymo is containment through educating the public on the importance of cleaning gear. A public awareness campaign, including signage and gear wash stations located in popular fishing areas could reduce the probability of the transfer of didymo to clean waters.