Factors controlling nitrogen cycling in freshwater ecosystems

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Human activities have seriously altered the global nitrogen (N) cycle, increasing the amount of biologically reactive N in circulation and leading to a significant increase in the amount of N transported into aquatic ecosystems. This has resulted in serious environmental problems including reduced quality of drinking water, degraded fish habitat, biodiversity loss, and the proliferation of toxic algae. Factors influencing the amount of N entering lakes are fairly well characterized however the fate of N once it enters a lake is poorly understood. Although global N models suggest that lakes may be important sites of N retention, there is surprisingly little understanding as to the fate of that retained N. Denitrification, alone or coupled with nitrification is an important bacterial transformation that converts dissolved inorganic forms of N into volatile gases that can be released into the atmosphere from aquatic ecosystems. Dinitrogen gas (N2), the end product of denitrification, is a favoured transformation while nitrous oxide (N2O) a by-product of both denitrification and nitrification is a potent greenhouse gas with a warming potential 300 times greater than CO2, is not. Factors regulating the production of N2O and the relative ratio of N2O: N2 in lakes are poorly understood. This research will focus on estimating the production of and the factors regulating N2O gas emissions from Canadian lakes. The intention is to identify "hot spots" and "hot moments" of microbial N transformations, like denitrification, that produce a greater or lesser amount of the undesirable greenhouse gas, N2O, relative to the more favorable transformation to N2. This research will characterize where within the lake N processes are taking place and compare lakes with different physical, chemical and biological characteristics to identify patterns in time and space as to the type and rates of N processing. Work will also be extended to look at the diversity of N cycling microorganisms, including N-fixing cyanobacteria. Results from this work could serve in regional and larger scale models with the objective of reducing N2O emissions and managing N retention at the landscape level thus preserving the quality and integrity of our Canadian freshwaters.