Abstract: Mitigating nonpoint pollution is the single greatest challenge to improving coastal waters in the United States. In southeastern Massachusetts, the effects of nonpoint pollution are clearly evident in the degradation of water quality in Buzzards Bay, a large (~600 km2) coastal bay where nonpoint nitrogen (N) pollution is a matter of the utmost concern. Although septic effluent is considered the largest nonpoint source of N pollution, cranberry agriculture is often implicated as a prominent source of N to the bay. For instance, the heavily agricultural Weweantic River watershed is estimated to export 107,300 kg N yr-1, or 22% of the annual N load to the bay. Although the Wewenatic River is closely connected to water quality in the bay, field-based measurements of N export from the Weweantic River are lacking. To fill this gap, we initiated a 2-yr monitoring study of N export from the Weweantic River in July of 2016. The location of our water quality monitoring station was ~3.5 km upstream of a former milldam, which eliminated the potentially confounding effects of tidal fluctuations. A stage-discharge rating curve was established for continuous measurement of streamflow, and stream water samples were collected 3 d per week to determine concentrations of total N (TN), total dissolved N (TDN), nitrate (NO3-), ammonium (NH4+), dissolved inorganic N (DIN = NH4+ + NO3-), dissolved organic N (DON = TDN – DIN), and particulate N (PN = TN – TDN). Streamflow exhibited considerable seasonal variation, ranging from 90 L s-1 in the summer (August) to 7600 L s-1 in the spring (April). Concentrations of TN were highest in the summer (mean = 0.46 N L-1), intermediate between November and February (mean = 0.34 mg N L-1), and lowest from March to April (mean = 0.28 mg N L-1). The majority of N exported by the Weweantic River was in the form of DON, which represented, on average, 77% of TN (per sample basis). Measured TN load of 1.8 kg N ha-1 yr-1 (1.7 kg N ha-1 yr-1as DON) was about half the model predicted mean TN load of 4.8 (±0.8) kg N ha-1 yr-1 (1 standard deviation in parentheses). Lower observed loading could be due to the 2016 drought, supporting the need for further monitoring, or to uncertainty in model inputs (i.e., model simulations assume N fertilizer use of 84 kg N ha-1 for cranberry agriculture, whereas grower records indicate N fertilizer use between 40-50 kg N ha-1). Answers to these questions, as well as inverse modeling to estimate cranberry agriculture N loading rates and in-stream N uptake, will be the focus of the monitoring in year 2 of the study. This poster is not available for downloading.