Nutrient Dynamics in Prairie Stream Systems: Patterns, Drivers, and Stoichiometric Implications Across Spatial and Temporal Scales

Abstract

Riverine nutrient dynamics influence water quality, ecosystem processes, and nutrient export to downstream ecosystems, yet the processes that shape nitrogen (N) and phosphorus (P) concentrations and their stoichiometry remain poorly understood in many regions. Prairie rivers are particularly understudied, as their strong seasonality, intensive agricultural land use, and episodic hydrologic connectivity may generate stoichiometric patterns that differ from those observed in more studied temperate systems. This thesis examines how nutrient concentrations, loads, and N:P ratios vary across spatial and temporal scales in Prairie stream ecosystems, and how landscape characteristics, hydrologic conditions, and episodic nutrient inputs shape these patterns. To address this goal, I conducted three studies spanning basin-, tributary-, and segment-scales. In Chapter 2, I analyzed 25 years of water quality data from river monitoring stations across the Red-Assiniboine Basin to determine long-term trends in total and dissolved nutrient fractions and their stoichiometry, and to quantify the relative influence of changing catchment characteristics. In Chapter 3, I assessed seasonal spatial patterns in nutrient concentrations and stoichiometry across Red River tributaries, evaluated whether interannual variation in nutrient ratios coincided with shifts in dominant crop types, and examined how long-term seasonal discharge patterns influenced total and dissolved nutrient ratios. In Chapter 4, I quantified the magnitude and distribution of nutrient capture during a 15-day, high concentration wastewater effluent release in Devils Creek, using a mass balance approach to determine the amount of N and P captured and exported along a 17-km stream segment. In Chapter 2, long-term monitoring across the Red-Assiniboine Basin showed that N and P concentrations and stoichiometry were changing through time at most stations, but trends were spatially heterogeneous with no consistent directional shifts across the basin. Localized nutrient sources were important drivers of these patterns, and changes in catchment characteristics contributed to the observed trends to a greater extent than changes in streamflow conditions. In Chapter 3, total N:P ratios were near the Redfield mass ratio across 24 tributaries, whereas dissolved N:P were strongly N-depleted and declined from spring through autumn. Crop type did not consistently influence nutrient ratios, but long-term seasonal discharge was a driver of spring and summer stoichiometry. In Chapter 4, despite elevated nutrient loading from a wastewater effluent release, capture of N and P in Devils Creek was substantial, with 80 % of P captured and 100% of N captured. The findings of this thesis demonstrate that nutrient dynamics in Prairie rivers are influenced by landscape characteristics, hydrologic connectivity, nutrient sources, and episodic inputs. Across multiple spatial and temporal scales, dissolved and total nutrient fractions differed in their absolute values, ratios, and trends, demonstrating that the choice of nutrient fraction is central to interpreting stoichiometric patterns and nutrient depletion in Prairie rivers. The spatial heterogeneity observed across rivers, tributaries, and stream segments indicates that stoichiometric patterns in Prairie rivers emerge from localized nutrient sources, variable connectivity, and strong in-stream processing. Overall, this thesis underscores the need for management approaches that incorporate the sources of variability that govern nutrient dynamics in Prairie rivers.