Landscape effects of nitrogen on aquatic ecosystems: Achieving a new paradigm for freshwater eutrophication

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Human population has doubled since 1950, in part due to a 10-fold increase in agricultural fertilization that has nearly doubled food production. However, farming practices have also raised soil phosphorus (P) content by >200% and created conditions in which agricultural soils are often saturated with P, will remain so for centuries, and supply freshwaters with excess P. Under these conditions, supplies of nitrogen (N) may regulate lake production, degrade water quality, and favour toxic algal blooms. Unfortunately, because N fertilization has also increased 15-fold since 1970, and because more than half of this N is added as urea, a chemical form of N which promotes toxic cyanobacteria, there exists the possibility that continued agricultural development will lead to continental-scale outbreaks of toxic and carcinogenic algae. Furthermore, disproportionate growth of urban centres has increased supplies of chemically-reduced N (as NH4+) creating blooms in many marine and freshwater systems. Given that ~1 billion people lack clean water, we seek to develop and apply a predictive understanding of the consequences of N pollution for freshwaters in Canada and the world. This program will combine ecosystem manipulation, long-term ecological research (LTER), and innovative paleoecology within 3 component projects to quantify how N fluxes regulate the structure and function of prairie freshwaters. First, we will conduct a unique whole-basin fertilization experiment to quantify how urea regulates production, food-web composition, and biogeochemistry of prairie wetlands. Urea represents 50% of all global N fertilizer, yet little is known of its ecosystem-scale impacts on surface waters. Second, we will apply our multi-decadal LTER research in the 52,000 km2 Qu'Appelle River catchment to quantify for the first time the effects on P-rich lakes of reducing the influx of N alone. Here we take advantage of the 2016 upgrade of the Regina sewage treatment plant which has reduced total N influx >50% (>95% of NH4+), but not P (~5% decline), to three P-rich downstream lakes. Comprehensive limnological analysis will be combined with whole-lake mass balances of N, P and C as part of LTER studies at 7 reference and sewage-impacted sites to test the hypothesis that diversion of N alone can improve water quality in P-rich lakes. Finally, we will apply our advanced statistical approaches (e.g., generalized additive models; conditional probability analysis) to new and older paleolimnological time series to determine whether nutrient fertilization initiates different types of catastrophic state change in N-limited (Qu'Appelle lakes) and P-limited lakes (Yahara lakes chain, Madison, WI). Together these projects will provide managers, scientists, governments, and the public with a transformative understanding of the controls of water-quality degradation in P-rich lakes under diverse development scenarios.