An Evaluation of the Efficacy of Whole-Stream Phosphorous Enrichment to Reduce Coverage of Didymosphenia geminata in An Oligotrophic Stream

The aquatic nuisance species, Didymosphenia geminata, was first documented in Rapid Creek of South Dakota’s Black Hills during 2002 and since then has been associated with changes to invertebrate and fisheries resources. Because phosphorous (P) has been associated with D. geminata blooms, altering P was considered as a possible method to reduce bloom severity. We conducted a whole-stream P enrichment experiment in a D. geminata impacted section of Rapid Creek to determine if enrichment would cause decreased D. geminata coverage. Using a before-after-control-impact (BACI) study design and asymmetrical analysis of variance to evaluate the impact of P enrichment, we found that addition of inorganic P to Rapid Creek in amounts 60% above ambient levels had little overall effect on periphyton composition and density as chlorophyll-a, soft-algae, diatom, and D. geminata cell densities increased similarly from before to after P enrichment in sites upstream and downstream of the enrichment source. However, a short-term decrease in D. geminata coverage was attributed to the addition of P, although only for a short distance (0.6 km) immediately downstream of the enrichment source. Although it seems possible to reduce D. geminata coverage with inorganic P, the potential benefits of enrichment may not outweigh the costs.