Climate-induced changes in streamflow and nitrogen loading to Long Island Sound

Climate-induced changes in streamflow and nutrient loading have important implications for estuarine water quality. We present an empirical approach for estimating changes in the magnitude, timing, and duration of streamflow based on statistically downscaled projections from global climate models. Two generalized additive models (GAMs) fitted using precipitation (Pr) as a predictor and either mean air temperature (Pr + T) or potential evapotranspiration (Pr + ET) produced similar estimates of daily streamflow for 2003-2016. However, projected changes in streamflow for 2030-2050 differed between models, which suggests that T should not be used to represent climate-induced changes in evaporative demand. For the Pr + ET model, streamflow increased between past and future periods during February-May (28.6 %) and July-September (12.0 %) and decreased during October-January (24.8 %). Projected changes in mean annual streamflow varied between -14.4 and 8.69 % across drainage basins. For the Connecticut River basin, the largest and most inland basin in the Long Island Sound (LIS) watershed, annual streamflow decreased by 12.6 %. Total nitrogen (TN) loading from the Connecticut River would decrease by approximately 10 % due to decreased streamflow. Although TN loading from near-coastal basins would increase by 5.23 ± 5.66 %, future TN riverine loads to LIS are 7.43 ± 9.01 × 10⁵ kg y^-1 less (-4.66 ± 5.41 %) compared to 2008-2016. Projected shifts in nutrient loading would likely lead to oligotrophication in eastern LIS, while eutrophication and increased hypoxia may occur in western LIS where water quality impacts due to nutrient pollution are currently most severe. These climate-induced changes may have implications for water quality management programs, including possible changes in mitigation actions needed to attain water quality objectives.