Relationships between land use and biological conditions in lowland streams of western Washington

Background/Question/Methods

Streams in northwestern Washington are directly and indirectly threatened by anthropogenic changes in land use at multiple spatial scales. The effectiveness of forested riparian buffer zones at mitigating negative influences from upstream land use is unclear. Within well-forested stream reaches, we used benthic macroinvertebrates to test for differences among biological communities in watersheds that differed in their dominant land use. Higher relative abundances of intolerant taxa and lower relative abundances of tolerant taxa were used to indicate higher quality stream conditions. Because riparian conditions directly adjacent to measured reaches were similar across all streams, we also assessed relationships between biological communities and land use at three different spatial scales (local, whole stream, and watershed).
Results/Conclusions

The relative abundance of intolerant taxa was positively correlated with increasing forested land in the watershed, and negatively correlated with increasing development in the watershed. Chironomidae, generally considered a tolerant taxon, increased with increasing development in the watershed. Mechanisms limiting intolerant taxa in developed and cultivated watersheds were likely related to increased specific conductance, increased stream temperature, and decrease in substrate size. Because macroinvertebrate communities in watersheds of each type ordinated similarly along principal component axes of abiotic conditions, topographic differences were not likely driving differences in community composition. We were unable to determine the spatial scale most associated with biological conditions because land use covaried across all spatial scales investigated. Forested stream reaches alone may not be sufficient to maintain or improve biological conditions in heavily cultivated or developed watersheds.