Spatial considerations for linking watershed land cover to ecological indicators in streams

Watershed land cover is widely used as a predictor of stream-ecosystem condition. However, numerous spatial factors can confound the interpretation of correlative analyses between land cover and stream indicators, particularly at broad spatial scales. We used a stream-monitoring data set collected from the Coastal Plain of Maryland, USA to address analytical challenges presented by (1) collinearity of land-cover class percentages, (2) spatial autocorrelation of land cover and stream data, (3) intercorrelations among and spatial autocorrelation within abiotic intermediaries that link land cover to stream biota, and (4) spatial arrangement of land cover within watersheds. We focused on two commonly measured stream indicators, nitrate-nitrogen (NO3-N) and macroinvertebrate assemblages, to evaluate how different spatial considerations may influence results. Partial correlation analysis of land-cover percentages revealed that simple correlations described relationships that could not be separated from the effects of other land-cover classes or relationships that changed substantially when the influences of other land-cover classes were taken into account. Partial Mantel tests showed that all land-cover percentages were spatially autocorrelated, and this spatial phenomenon accounted for much of the variation in macroinvertebrate assemblages that could naively be attributed to certain classes (e.g., percentage cropland). We extended our use of partial Mantel tests into a path-analytical framework and identified several independent pathways between percentage developed land and instream measurements after factoring out spatial autocorrelation and other confounding variables; however, under these conditions, percentage cropland was only linked to nitrate-N. Further analyses revealed that spatial arrangement of land cover, as measured by areal buffers and distance weighting, influenced the amount of developed land, resulting in a threshold change in macroinvertebrate-assemblage composition. Moreover, distance-weighted percentage cropland improved predictions of stream nitrate-N concentrations in small watersheds, but not in medium or large ones. Collectively, this series of analyses clarified the magnitude and critical scales of effects of different land-cover classes on Coastal Plain stream ecosystems and may serve as an analytical framework for other studies. Our results suggest that greater emphasis should be placed on these important spatial considerations; otherwise, we risk obscuring the relationships between watershed land cover and the condition of stream ecosystems.