Clean air act policies reduced stream nitrogen concentrations over time in deposition dominated watersheds of the conterminous US (2000-2014)

Abstract:

Clean Air Act regulations have reduced the total amount of nitrogen (N) deposited to US watersheds during the last two decades. However, few studies have clearly linked the decline in deposition to changes in water quality across large areas, in part, because of the difficulty in untangling the aquatic response from the backdrop of other land use and climatic changes. We combined the recently-assembled National Nutrient Input Inventory (NNI) with the National Rivers and Streams Assessment (NRSA) surveys to explore whether streams with atmospheric deposition as the largest N source to their watersheds have responded to these regulatory changes. Three NRSA surveys took place in 2000-2004, 2009-2008, and 2013-2014, respectively, across the conterminous US; Using the NNI, we identified 1967 NRSA watersheds that have deposition as the largest N source. In these streams, TN concentrations declined from 2000-2014, consistent with a decrease in N inputs. The decline in stream TN primarily occurred between 2000 and 2009; the leveling off in improvement in stream TN from 2009-2014 may reflect the rising share of reduced N deposition over time. Our results suggest that reductions in stream TN are associated with Clean Air Act regulations that changed N deposition over time. Furthermore, using NNI to describe and categorize watersheds improved our ability to identify this trend in the data and assess the response to changes in policy.