REGIONAL COHERENCE OF STREAM NITRATE CONCENTRATIONS: IMPLICATIONS FOR THEORY AND MANAGEMENT

Stream chemistry integrates plant, soil, and aquatic biogeochemical cycling at the catchment scale, and therefore, understanding the drivers of its temporal and spatial variability is a fundamental challenge for watershed biogeochemistry and water resources management. Seasonal patterns in stream nitrogen chemistry have commonly been attributed to terrestrial vegetative nitrogen demand as well as nitrogen saturation status. Therefore, because climatic conditions and atmospheric nitrogen deposition tend to vary across broad regional extents, regional coherence in seasonal nitrate (NO₃) export might be expected in watersheds that share similar land use features. Here we use a network of 11 stream and river sites instrumented with an in situ NO₃ sensor for 2-5 years to test the extent to which there is regional coherence in the seasonality and overall temporal variation in stream NO₃ concentrations across gradients of stream size, watershed topography, and land use in New England. We found that across sites, there is little regional coherence in the overall seasonal variation in stream NO₃ concentrations, although multiple watersheds displayed similar NO₃ responses to particular annual events including autumn leaf fall. Diel NO₃ variability is surprisingly common, even in heavily-shaded streams, and the magnitude of diel variation increases with NO₃ concentration across sites. These results suggest that our conceptual models of drivers of seasonal NO₃ export may not adequately account for the complex suite of catchment factors affecting stream nitrogen availability across a range of temporal scales.