A Bayesian changepointethreshold model to examine the effect of TMDL implementation on the flowenitrogen concentration relationship in the Neuse River basin Ibrahim Alameddine*, Song S. Qian, Kenneth H. Reckhow Nicholas School of the Environment, Duke University, Durham, NC 27708, USA article info Article history: Received 29 April 2010 Received in revised form 28 July 2010 Accepted 3 August 2010 Available online 11 August 2010 Keywords: TMDL Changepoint Threshold Floweconcentration Neuse River Bayesian model Water quality Nitrogen Load reduction abstract In-stream nutrient concentrations are well known to exhibit a strong relationship with river flow. The use of flow measurements to predict nutrient concentrations and subse- quently nutrient loads is common in water quality modeling. Nevertheless, most adopted models assume that the relationship between flow and concentration is fixed across time as well as across different flow regimes. In this study, we developed a Bayesian change- pointethreshold model that relaxes these constraints and allows for the identification and quantification of any changes in the underlying floweconcentration relationship across time. The results from our study support the occurrence of a changepoint in time around the year 1999, which coincided with the period of implementing nitrogen control measures as part of the TMDL program developed for the Neuse Estuary in North Carolina. The occurrence of the changepoint challenges the underlying assumption of temporal invari- ance in the floweconcentrations relationship. The model results also point towards a transition in the river nitrogen delivery system from a point source dominated loading system towards a more complicated nonlinear system, where non-point source nutrient delivery plays a major role. Moreover, we use the developed model to assess the effec- tiveness of the nitrogen reduction measures in achieving a 30% drop in loading. The results indicate that while there is a strong evidence of a load reduction, there still remains a high level of uncertainty associated with the mean nitrogen load reduction. We show that the level of uncertainty around the estimated load reduction is not random but is flow related. ª 2010 Elsevier Ltd. All rights reserved. 1. Introduction Anthropogenic nitrogen reaching rivers, lakes, estuaries, and coastal areas has been linked to eutrophication, acidification, adverse human health effects, the disruption of ecosystem functions, as well as the lowering of biodiversity in affected water bodies (Kelly, 2008 and references therein). High profile events such as the dramatic fish kills in the Neuse River and Estuary in mid 1980 and early 1990s as well as the development of extensive hypoxic zones in the Gulf of Mexico have been linked to excessive nitrogen release and delivery (Turner and Rabalais, 1994; Paerl et al., 1995; Paerl, 1997; Alexander et al., 2000, 2008; Scavia et al., 2003; Stow and Borsuk, 2003; Borsuk et al., 2004). Such events have stimu- lated an impetus towards the implementation of aggressive management and mitigation measures to limit the amount of nitrogen reaching the aquatic environment. While some successes have been made in some water bodies, water * Corresponding author. Tel.: þ1 919 613 8054; fax: þ1 919 681 5740. E-mail address: ima4@duke.edu (I. Alameddine). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 45 (2011) 51 e62 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.08.003