Effect of beaver dam analogs (BDAs) on waterborne protozoal pathogens Giardia duodenalis and Cryptosporidium parvum.

Installing beaver dam analogs (BDAs) in freshwater ecosystems is a process-based restoration technique mimicking natural beaver dams to increase stream channel complexity, floodplain connectivity, and hydrological residence time. BDAs have been applied in recent years by resource managers to improve riparian habitats with beneficial effects on ecosystem services, including improved water quality. While BDAs have been shown to reduce suspended sediments and nutrients in surface waters, research is needed to evaluate BDA effects on waterborne fecal pathogens such as Giardia duodenalis (syn. G. lamblia, G. intestinalis) and Cryptosporidium parvum. At a California riparian field site, this study used an in situ release trial of inactivated protozoal (oo)cysts upstream of BDAs to compare the concentrations and loads detected above and below the BDAs. The median percent decrease in load-based recovery rates was 78% for G. duodenalis and 80% for C. parvum at sites with BDAs. Mixed-effect linear regression analyses showed a significant reduction (by 81%) of the G. duodenalis adjusted cyst concentrations in downstream surface waters at sites with BDAs compared to similar sites without BDAs. While a reduction of C. parvum oocysts was noted, it was not statistically significant. The improved surface water quality downstream of BDAs could be because the BDA structures promote hyporheic exchange and act as a passive filter to remove pathogenic protozoa from streams. If BDAs are used to aid the restoration of riparian meadow ecosystems to slow water and raise the water table, they may also help to promote cleaner and safer waters for communities downstream.

Importance: Beaver dam analogs (BDAs) are a cost-effective, low-environmental impact technique for stream and riparian meadow restoration that provide a variety of beneficial ecosystem services; however, their impact on waterborne fecal protozoal pathogens has not been evaluated. This study used an in situ protozoal release trial to quantify the effect of BDAs on the load of Giardia duodenalis and Cryptosporidium parvum in streams in California. Results showed that G. duodenalis concentrations below BDAs were significantly reduced by 81%. The median percent decrease in load-based recovery rates below a BDA for G. duodenalis and C. parvum was 78% and 80%, respectively. This finding indicates that BDAs may promote passive filtration of waterborne pathogens, thereby improving water quality in downstream reaches and adding to the beneficial outcomes associated with using BDAs in stream and meadow restoration efforts. The potential benefit to resource managers and communities is immense.