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Comprehensive Status and Trends of Nitrogen Loads to Estuaries in the Conterminous United States: Pacific Coast Results
Citation:
Detenbeck, N., M. You, AND D. Torre. Comprehensive Status and Trends of Nitrogen Loads to Estuaries in the Conterminous United States: Pacific Coast Results. Regions 8/9/10 Western Numeric Nutrient Criteria Workshop, San Francisco, CA, May 01 - 03, 2018.
Impact/Purpose:
This poster provides an overview of ongoing work on the status and trends of nitrogen loads to estuaries in the conterminous United States with a focus on results for the Pacific Coast. Interim results are being shared with EPA Regions 8/9/10 and associated states as part of the R8/9/10 Western Nutrient Criteria meeting.
Description:
We applied regional SPARROW (SPAtially Referenced Regressions On Watershed attributes) models to estimate status and trends of potential nitrogen loads to estuaries of the conterminous United States. The original Regional SPARROW models predict average detrended loads by source based on average flow conditions and 2002 source terms. We have developed time series for nitrogen sources (1987 – 2016) and for landscape features affecting land-to-water delivery ratios to enable an evaluation of recent trends in potential loads, factoring in both regional variation in land-to-water delivery ratios and instream- and reservoir attenuation between source and river mouth. Dominant sources (median %) varied among marine ecosystems along the Pacific coasts, with agricultural sources dominating for Puget Trough estuaries, natural biological N-fixation from alders dominating in the Oregon/Washington/ Vancouver systems, forests dominating in N California systems, and urban runoff dominating fro S California Bight systems. For estuaries in most marine ecoregions, delivered N loads from atmospheric deposition declined between 2000 and 2012, with the lowest median 2012/2000 ratio (0.60) observed for Floridian ecoregion estuaries. Delivered loads from atmospheric deposition declined for most estuaries in all marine ecoregions except the Oregon/Washington/Vancouver marine ecoregion, in which direct deposition increased for 49 systems and delivered loads from deposition to watersheds increased for 19 systems. Interannual variability also was greater for Gulf and West Coast models, which relied only on adjusted NADP inputs, as compared to the East Coast models, which used CMAQ inputs. Delivered loads and information on factors influencing estuarine nutrient sensitivity will be delivered in an updated spreadsheet model based on the tidal prism model developed by Abdelrhman (2007).