Stable isotopes of algae and macroinvertebrates in streams respond to watershed urbanization, inform management goals, and indicate food web relationships.

Watershed development and anthropogenic sources of nitrogen are among leading causes of negative impacts to aquatic ecosystems around the world. The δ15N of aquatic biota can be used as indicators of anthropogenic sources of nitrogen enriched in 15N, but this mostly has been done at small spatial extents or to document effects of point sources. In this study, we sampled 77 sites along a forest to urban land cover gradient to examine food webs and the use of δ15N of periphyton and macroinvertebrate functional feeding groups (FFGs) as indicators of watershed development and nitrogen effects on streams. Functional feeding groups had low δ15N variability among taxa within sites. Mean absolute differences between individual taxa and their respective site FFG means were < 0.55‰, whereas site means of δ15N of FFGs had ranges of approximately 7-12‰ among sites. The δ15N of periphyton and macroinvertebrate FFGs distinguished least disturbed streams from those with greater watershed urbanization, and they were strongly correlated with increasing nitrogen concentrations and watershed impervious cover. Nonmetric multidimensional scaling, using δ15N of taxa, showed that changes in macroinvertebrate assemblages as a whole were associated with forest-to-urban and increasing nitrogen gradients. Assuming an average +3.4‰ per trophic level increase, δ15N of biota indicated that detrital pathways likely were important to food web structure, even in streams with highly developed watersheds. We used periphyton and macroinvertebrate FFG δ15N to identify possible management goals that can inform decisions affecting nutrients and watershed land use. Overall, the δ15N of periphyton and macroinvertebrates were strong indicators of watershed urban development effects on stream ecosystems, and thus, also could make them useful for quantifying the effectiveness of nitrogen, stream, and watershed management efforts.

Quantifying Urban Watershed Stressor Gradients and Evaluating How Different Land Cover Datasets Affect Stream Management.

Watershed management and policies affecting downstream ecosystems benefit from identifying relationships between land cover and water quality. However, different data sources can create dissimilarities in land cover estimates and models that characterize ecosystem responses. We used a spatially balanced stream study (1) to effectively sample development and urban stressor gradients while representing the extent of a large coastal watershed (>4400 km(2)), (2) to document differences between estimates of watershed land cover using 30-m resolution national land cover database (NLCD) and <1-m resolution land cover data, and (3) to determine if predictive models and relationships between water quality and land cover differed when using these two land cover datasets. Increased concentrations of nutrients, anions, and cations had similarly significant correlations with increased watershed percent impervious cover (IC), regardless of data resolution. The NLCD underestimated percent forest for 71/76 sites by a mean of 11 % and overestimated percent wetlands for 71/76 sites by a mean of 8 %. The NLCD almost always underestimated IC at low development intensities and overestimated IC at high development intensities. As a result of underestimated IC, regression models using NLCD data predicted mean background concentrations of NO3 (-) and Cl(-) that were 475 and 177 %, respectively, of those predicted when using finer resolution land cover data. Our sampling design could help states and other agencies seeking to create monitoring programs and indicators responsive to anthropogenic impacts. Differences between land cover datasets could affect resource protection due to misguided management targets, watershed development and conservation practices, or water quality criteria.