Abstract
With changes in ice cover duration, nutrient loading, and anoxia risk, it is important to understand the mechanisms that control nitrogen cycling and oxygen depletion in lakes through winter. Current understanding is largely limited to description of changes in chemistry, with few measurements of the processes driving winter changes, how they differ across lakes, and how they are impacted by under-ice conditions. Nitrification is a process which consumes oxygen and ammonium (NH4+), and supplies nitrate (NO3–). To date, nitrification has been measured under ice cover in only two lakes globally. Here, we used 15NH4+ enrichment to measure rates of pelagic nitrification in thirteen water bodies in two ecozones. Our work demonstrates ecologically important rates of nitrification can occur despite low water temperatures, impacting NH4+, NO3– and, most importantly, oxygen concentrations. However, high rates are not the norm. When, where and why is nitrification important in winter? We found that nitrification rates were highest in a eutrophic lake chain downstream of a wastewater treatment effluent (mean: 226.5 μg N L-1 d-1), and in a semi-saline prairie lake (110.0 μg N L-1 d-1). In the boreal shield, a eutrophic lake had nitrification rates exceeding those of an oligotrophic lake by 6-fold. Supplementing our results with literature data we found NH4+ concentrations were the strongest predictor of nitrification rates across lentic ecosystems in winter. Higher nitrification rates were associated with higher concentrations of NH4+, NO3– and nitrous oxide (N2O). While more work is required to understand the switch between high and low nitrification rates and strengthen our understanding of winter nitrogen cycling, this work demonstrates that high nitrification rates can occur in winter.
Highlights
Changes to the global nitrogen (N) cycle have led to significant increases in N inputs to rivers, lakes, oceans, and the atmosphere [1]
Partitioning the nitrification rates into low (< 1.1 × 10−1 μg N L-1 d-1) and higher (> 1.1 × 10−1 μg N L-1 d-1) rates revealed that nitrogen species differed across these groups
When nitrification rates were higher, median NH4+ concentrations were higher at 516 μg N L-1, while when rates were lower median NH4+ concentrations were markedly lower (86.0 μg N L-1; wilcox.test: P = 0.015, 11 DF, Fig 4A)
Summary
Changes to the global nitrogen (N) cycle have led to significant increases in N inputs to rivers, lakes, oceans, and the atmosphere [1]. Elevated nitrogen concentrations and associated ecological effects are shown in many aquatic ecosystems, often driven by runoff from intensive agriculture [2]. Within freshwater ecosystems, some of the most acute impacts of nitrogen fertilization are seen at sewage outfalls–where high NH4+ concentrations are nitrified [3,4]. The process of nitrification is a microbially-mediated one, whereby NH4+ is oxidized to nitrite (NO2–) to NO3–, (Fig 1; [5]). Nitrification leads to consumption of oxygen, which can be associated with fish kills [6,7].
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