Leveraging long term water quality monitoring data to elucidate drivers and controls on N and P loading in the Lake Winnipeg Basin

Abstract

Since the late 1920s, dams have been a common tool used in the Lake Winnipeg basin to control flooding during spring snowmelt and to supply freshwater during the summer dry season. As a result, there are now over 140 reservoirs larger than 10 ha in the watershed. Many retain a substantial fraction of the inflowing riverine phosphorus (P) load, and some also remove inflowing nitrogen (N). Yet we lack a quantitative understanding of how reservoir nutrient removal reshapes seasonal nutrient delivery and N:P stoichiometry at the watershed scale. Removal efficiency variation, both between reservoirs and interannually, and its driving factors also remain poorly constrained. Chapter 1 gives an overview of the eutrophication challenges facing Lake Winnipeg, puts the challenges into context based on the Lake Winnipeg Basin’s land use, water management and climate, and summarizes how we expect reservoirs are influencing TN and TP loads across the watershed. In Chapter 2 we leverage 13 case-study reservoirs where flow and concentrations of total phosphorus (TP) and total nitrogen (TN), have been monitored near the inlet(s) and outlet over multiple years. Using the Weighted Regression on Time Season and Discharge model with Kalman filtering (WRTDS-K) we calculate P and N loads, estimate TP retention and TN removal efficiencies, and examine daily model coefficients to quantify how reservoirs modify TP and TN concentration-discharge (C-Q) relationships throughout the year. We further investigate flow volume, intensity and timing as potential drivers of interannual variation in TP retention and TN elimination efficiencies. On average, reservoirs retain 43.9% ±35.2% of inflowing TP and eliminate 6.98%± 30.9% of inflowing TN, systematically increasing the N:P ratio of downstream nutrient loads. They also consistently suppress the C-Q slopes for both TN and TP, with most pronounced impacts occurring during the spring freshet. Multiple linear regression analysis demonstrates that metrics of flow timing can explain 10-50% of inter-annual variability in retention efficiency. Collectively, these findings demonstrate that reservoirs in the Lake Winnipeg basin systematically reshape the magnitude, timing and stoichiometry of nutrient exports, with important implications for nutrient-management strategies under a changing climate. In Chapter 2, I showed that reservoirs have a large impact on the in-stream concentrations and loads of total phosphorus (TP) and total nitrogen (TN) across the Lake Winnipeg Basin (LWB). However, only a small fraction of the 140+ reservoirs in the basin have sufficient paired inflow and outflow monitoring data to accurately determine their contributions to TP and TN retention or enrichment. Based on the thirteen case study sites considered in Chapter 2, Chapter 3 uses Generalized Additive Models (GAMs) to evaluate the relative importance of a broad range of predictor variables representing reservoir and environmental factors that potentially influence the efficiency of removal of TP and TN from reaching Lake Winnipeg. The GAM features are evaluated by stepwise adding factors to the model and determining the associated improvements of performance metrics. The best model is then further assessed by comparing the residuals and adjusted R2 values generated from a leave-one-out cross-validation. The results emphasize the added value of incorporating information on water chemistry and C-Q relationships when assessing reservoir influence in water quality models, improving the accuracy of predictions of the impact of reservoirs on TP and TN concentrations and loads, especially when considering their interannual variability. The insights from Chapter 3 also contribute to a broader understanding of how reservoirs in general control riverine TP and TN flows.