Our previous work in the Emerald Lake watershed indicates that inorganic N inputs during snowmelt are a mixture of NO3- produced in soils and atmospheric NO3- (Sickman et al. 2003b); sources of nitrate during the summer and autumn are not well understood. We recently developed an ion-exchange resin method to extend NO3- isotope measurements into the growing season when concentrations are near the analytical detection limit (Fig. 8). Nitrate collected on resin columns is eluted with KCl and the eluates used in the microbial denitrifier method of Casciotti et al. (2002). Coupling of the isotope data with frequent bioassay experiments (Heard 2013) suggests that periodic rain events and dry deposition may ameliorate N-limitation of phytoplankton during the growing season. At Emerald Lake, isotope samples are collected biweekly during snowmelt and monthly in other months. The isotope measurements during July to October, will be augmented with bioassays to determine phytoplankton nutrient limitation. Over several growing seasons, these measurements will provide understanding of how summer air quality impacts the N biogeochemistry and phytoplankton nutrient limitation.