Nitrogen loading from watersheds to estuaries: Verification of the Waquoit Bay Nitrogen Loading Model

World-wide eutrophication of estuaries has made accurate estimation of land-derived nitrogen loads an important priority. In this paper we verify predictions of nitrogen loads made by the Waquoit Bay Nitrogen Loading Model (NLM). NLM is appropriate for watersheds with mixes of forested, agricultural, and residential land uses, and underlain by coarse unconsolidated sediments. NLM tracks the fate of nitrogen inputs by atmospheric deposition, fertilizer use, and wastewater disposal, and assigns losses of nitrogen from each source as the nitrogen is transported through the land use mosaic on the watershed surface, then through the underlying soils, vadose zones, and aquifers. We verified predictions of nitrogen loads by NLM in two independent ways. First, we compared NLM predictions to measured nitrogen loads in different subestuaries in the Waquoit Bay estuarine system. Nitrogen loads predicted by NLM were statistically indistinguishable from field-measured nitrogen loading rates. The fit of model predictions to measurements remained good across the wide range of nitrogen loads, and across a broad range in size (10-10,000 ha) of land parcels. NLM predictions were most precise when specific parcels were larger than 200 ha, and within factors of 2 for smaller parcels. Second, we used NLM to predict the percentage of nitrogen loads to estuaries contributed by wastewater, and compared this prediction to the delta(15)N signature distinguishable from N derived from atmospheric or fertilizer sources. The greater the contribution of wastewater, the heavier the delta(15)N value in groundwater. The significant linear relation between NLM predictions of percent wastewater contributions and stable isotopic signature corroborated the conclusion that model outputs provide a good match to empirical measurements. The good agreement obtained in both verification exercises suggests that NLM is an useful tool to address basic and applied questions about how land use patterns alter the fate of nitrogen traversing land ecosystems, and that NLM provides verified estimates of the land-derived nitrogen exports that transform receiving aquatic ecosystems.