Nitrogen cycling in nutrient-rich freshwaters

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Nutrient pollution is widely considered the most significant stressor impacting water quality globally. Nutrient pollution can lead to high algal productivity, fish kills, and blooms which can be toxic. Addressing nutrient pollution is challenging due to technical, economic, and social aspects of the problem, and potential lags in ecosystem recovery. Nutrient management in freshwaters is typically centered on controlling inputs of phosphorus. However, there has been growing call to also control inputs of nitrogen and debate about the potential benefits and costs of nitrogen control. Although the cities of Regina and Winnipeg have made multimillion dollar investments in nitrogen removal to help combat nutrient pollution, there are key questions about how nitrogen control will impact lakes, and about how lakes themselves may regulate their nitrogen concentrations. Much emphasis has been placed on the potential role of cyanobacteria in the nitrogen budgets of lakes - due to their role in fixing atmospheric nitrogen, making it bioavailable. Much less is known regarding the processes that recycle or remove nitrogen within sediments, particularly within lakes of the prairies - a region of relative water insecurity, significant problems of nutrient pollution, and unusual lake chemistry which is likely to affect nitrogen cycling. Our work will employ the use of tracers to assess key processes in the nitrogen cycle of shallow lakes in order to understand whether nitrogen recycling from sediments is affecting nutrient supply in biologically important periods, including blooms. Our new insights into internal cycling of nitrogen will help us understand whether nitrogen can be controlled within shallow lakes, and the timescale upon which we may see changes in nitrogen. Lakes in the prairies are embedded in an agricultural landscape. Options to manage agricultural nutrients are limited, challenging our efforts to mitigate widespread issues of nutrient pollution in the region. The small ponds that riddle the landscape could represent sites of nutrient retention, but they are rapidly being lost due to drainage. Before considering wetland construction or restoration as a beneficial management practice, we need to better understand the role of wetlands in transmitting, retaining, or releasing nutrients. Within the prairies, this means we must better understand nutrient cycling in cold conditions, and during snowmelt - the key period of water and nutrient movement. Here we will focus on understanding how shallow ponds alter their nitrogen cycling processes through the spring melt period - understanding whether winter freezing impedes nutrient retention processes in spring, and how nitrogen cycling processes vary in space. With this we will build a new understanding of how changes in the management of prairie watersheds will impact nitrogen cycling and nitrogen availability in lakes, informing wise choices to improve water quality.