|Abstract or Summary
- Initially, components of an aquatic food web were examined to study impacts of recreational use on the aquatic ecology of Quartzville Creek, Oregon in 1995 and 1996. Measurements of the food web components consisted of observations of harlequin ducks (Histrionicus histrionicus), visual counts of the larval caddisfly Dicosmoecus gilvipes, benthic macroinvertebrate samples, and chlorophyll a biomass of epiphytic algae. In February 1996, a major flood forced closure of the study area to the general public throughout the 1996 season, providing a "natural experiment" situation. The difficulty was how to distinguish between effects of flood damage and effects of the presence/absence of humans. By accounting for various measured abiotic and biotic factors using multiple regression, distinctions between disturbance effects were made with regard to abundances of D. gilvipes and benthic invertebrates. Regression analysis also was used to account for annual site differences in average depth, habitat types and substrates. Generally, recreational impacts were apparent at more localized spatial and temporal scales than were effects from extensive flooding. However, scales at which these disturbances affected components of
the food web varied. Impacts of the flood included an 81% reduction in overall
D. gilvipes densities, a 37% decrease in benthic abundance, reduced chlorophyll a biomass, fewer numbers of ducks and changes in site substrates and habitats. These impacts occurred across all study sites and throughout the 1996 season, except for the recovery of benthic abundance to 1995 levels by July 1996. Recreational impacts were apparent at the site scale and appeared to be seasonal in duration. In 1995, sites without human use had significantly higher densities of D. gilvipes than those sites impacted by recreation. In 1996, no significant differences in D. gilvipes densities among sites were observed after accounting for flood effects and site differences. There were no direct significant relationships to human use among sites with regard to total benthic invertebrates, chlorophyll a biomass or H. histrionicus. Despite the lack of a direct spatial relationship between H. histrionicus and D. gilvipes at the local scale of this study, there were strong temporal correlations between harlequin brood development and D. gilvipes maturation.