The value of controlling nitrogen (N) and its impact on water quality are frequently contested, partly due to inconsistencies in the effects of N pollution depending on the form, amount, and seasonal conditions in which it is released to freshwater ecosystems. This thesis aims to shed light on some of those differential effects, and advance our understanding of the seasonal variation in the impacts and drivers of N pollution to prairie lentic and lotic environments. In my first chapter, I evaluate the seasonal and differential effects (i.e., suppression vs stimulation) of NH4

• on phytoplankton abundance, using 16 years of nutrient bioassay experiments, conducted bi-weekly during the open water season. Phytoplankton biomass was significantly affected by NH4
• amendment in 44.8% of the experiments, and generalized additive models (GAMs) demonstrated that the the seasonal patterns of phytoplankton response showed a marked rise in the occurrence of both spring suppression and summer stimulation over the study period. Binomial logit GAMs demonstrated that the likelihood of NH4
• suppression of phytoplankton growth increased with abundance of siliceous algae, cryptophytes, and unicellular cyanobacteria, when water temperatures and soluble reactive phosphorus (SRP) concentrations were low, while stimulation of phytoplankton growth was more likely when chlorophytes and non- N2-fixing cyanobacteria were abundant, and temperatures and SRP concentrations were high. In my second chapter, I present the results of a series of 22 factorial urea and phosphorus (P) fertilization experiments, conducted monthly from ice-off to ice-formation in 3000-L mesocosms. These mesocosm experiments were also run in tandem with NH4+ bioassays to compare the seasonal effects of urea pollution with those of NH4+. Results showed that addition of P alone had no significant impact on either phytoplankton abundance or community composition, but that urea, alone or concert with P, consistently increased abundance of cryptophytes, chlorophytes, and non-diazotrophic cyanobacteria in spring, and abundances of chlorophytes and non-diazotrophic cyanobacteria in the summer and early fall. Comparison of urea mesocosms with NH4

bioassays demonstrated that urea lacked the inherent toxicity of NH4

• in cool waters. In my final chapter, I identify the temporal and spatial patterns in urea export along a 250-km lotic continuum in the NGP, using three years of data collected bi-weekly from May-September, and investigate the hydrologic, land use, and instream drivers of urea concentration. I found that urea concentrations were elevated compared to previously studied lentic systems in this region, and ranged from 5.2 – 792.1 μg N L-1. Significant differences in longitudinal patterns of urea concentration emerged over the course of the open water period (F(6, 290) = 8.183, p < 0.001), and were mediated by interactions between hydrology, land use inputs, and internal processing. Contrary to expectations, I detected no significant effect of wastewater effluent discharge on instream urea concentrations during low and moderate discharge rates. Instead, average porewater urea concentrations 􁈺􀝔̅ = 528.5 μg N L-1, SD = 229.8 μg N L-1) were over five times higher than instream concentrations, emphasizing the importance of hyporheic sources. Together, these three studies provide a comprehensive assessment of how the risk of N pollution to water quality in the NGP varies across the open water season, and informs a series of recommendations, provided in Chapter 5, for future water quality management.