|Abstract or Summary
- Metals are a persistent form of freshwater pollution and have been shown to bioaccumulate in aquatic macroinvertebrates through direct contact with contaminated water, sediments, and through consumption of contaminated organisms. This research explored the longitudinal bioaccumulation patterns of 5 common trace metals (Cr, Cu, Zn, Cd, and Pb) in the signal crayfish, Pacifastacus leniusculus, and stonefly Pteronarcys californica along the mainstem Umatilla River. The Umatilla River drains a largely agricultural basin in northeastern Oregon and spans 140 kilometers from its headwaters in the Blue Mountains to its confluence with the Columbia River near Hermiston.
Crayfish concentrations of Zn and Pb, and to a lesser degree Cr, were significantly correlated with river distance, but contrary to expectations, concentrations decreased in the downstream direction. Cu and Zn were unrelated to river distance. Concentrations of Cr, Pb, and to a lesser degree Cd, were also significantly, negatively associated with crayfish carapace length, which was used as a proxy for age. Larger, older crayfish had lower concentrations, suggesting growth dilution. ANCOVA was used to test for effects of gender and mesohabitat on crayfish metal concentration after accounting for river distance and carapace length as covariates. Male crayfish had significantly higher Zn, Cd, Pb, and Cr concentrations than females, but absolute differences were fairly small. A significant difference in crayfish Pb concentrations between mesohabitats was found, with slightly higher concentrations in runs than riffles. Crayfish Cu concentrations were not correlated with any of the morphological, physicochemical, or spatial variables measured in this study. Due to their small size, metal concentrations in stoneflies were only measured in a single composite sample at each site, and stoneflies were only found at 7 of the upstream river sample sites. As a result, this small dataset was suggestive, but not conclusive, of relationships between stonefly metal concentrations and other variables. For both crayfish and stoneflies, none of the sampled metals appeared to clearly correlate with suspected point source or potential diffuse metal inputs.
This study also examined habitat associations of P. leniusculus by life history stage. Microhabitat scale differences in collection methods were found, with nonrandom methods (kick sampling and boulder flipping) resulting in a significantly higher number of crayfish captured than the random method (kick sampling). Using PCA, an exploration of microhabitat scale characteristics associated with P. leniusculus by size group found that crayfish with carapace length >20 mm were significantly associated with deeper water, higher percentages of organic matter and riparian cover, and higher water velocity. Distinct differences in habitat association also existed among three juvenile size groups. The smallest, youngest juvenile size group (carapace length <10 mm) showed a low association with velocity, percent organic matter, and percent riparian cover; the middle juvenile size group (carapace length 10-15 mm) was negatively associated with water depth; the largest juvenile size group (carapace length 15-20 mm) was comparatively less negative in its association with water depth, and more positively associated with velocity, percent organic matter, and percent riparian cover.