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

Abstract

World-wide eutrophication of estuaries has made accurate estimation ofland-derived nitrogen loads an important priority. In this paper we verifypredictions of nitrogen loads made by the Waquoit Bay Nitrogen LoadingModel (NLM). NLM is appropriate for watersheds with mixes of forested,agricultural, and residential land uses, and underlain by coarseunconsolidated sediments. NLM tracks the fate of nitrogen inputs byatmospheric deposition, fertilizer use, and wastewater disposal, and assignslosses of nitrogen from each source as the nitrogen is transported throughthe land use mosaic on the watershed surface, then through the underlyingsoils, 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 differentsubestuaries in the Waquoit Bay estuarine system. Nitrogen loads predictedby NLM were statistically indistinguishable from field-measured nitrogenloading rates. The fit of model predictions to measurements remained goodacross 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 whenspecific parcels were larger than 200 ha, and within factors of 2 for smallerparcels.

Second, we used NLM to predict the percentage of nitrogen loads toestuaries contributed by wastewater, and compared this prediction to theδ¹⁵N signature distinguishable from N derived fromatmospheric or fertilizer sources. The greater the contribution ofwastewater, the heavier the δ¹⁵N value in groundwater. Thesignificant linear relation between NLM predictions of percent wastewatercontributions and stable isotopic signature corroborated the conclusionthat model outputs provide a good match to empirical measurements. Thegood agreement obtained in both verification exercises suggests that NLMis an useful tool to address basic and applied questions about how land usepatterns alter the fate of nitrogen traversing land ecosystems, and thatNLM provides verified estimates of the land-derived nitrogen exports thattransform receiving aquatic ecosystems.