Anaerobic terminal electron acceptors (aTEAs, i.e. NO3, Fe, SO4) enable anaerobic respiration, and each has a specific ability to oxidize reduced compounds. However, little is known about how seasonal and lake-specific aTEA fluxes form anaerobic oxidation capacity (AOC) to oxidize organic carbon in boreal systems. We compiled 26 years of data from two interconnected semi-pristine boreal lakes and defined mean daily imports, pools, and exports of aTEAs. In both lakes, the export of NO3 formed 2 %-3 % of the total AOC in summer and autumn, and up to 11 % in winter and spring. In a predominantly monomictic humic lake surrounded by peatlands, Fe was responsible for 15 %-31 % of the seasonal export of AOC, with a large proportion of Fe originating from the lake bottom. A dimictic clear-water lake downstream retained Fe and exported 87 %-95 % of AOC as SO4. In the humic lake, the annual SO4:Fe:NO3 export ratio for AOC was 10:3:1 and in the clear-water lake 15:0.4:1. In the monomictic lake, exports were specifically regulated by stratification; in the dimictic lake, exports were more regulated by spring flooding and the ascending and descending side of the peak flood. These events modified lake dynamics and caused lake-specific NO3, Fe, and SO4 exports which continued for months. We conclude that a catchment and lake network can cause spatial and temporal variation in exports of NO3, Fe, and SO4 affecting AOC export. Such natural variations in exports have significant potential to modify the system's capacity to oxidize C and resist changes in oxidation-reduction reactions coupled to nutrient cycling and the formation of greenhouse gases in downstream water bodies.
Keywords: Carbon; Iron; Mineralization; Nitrogen; Oxidation; Oxygen; Redox; Sulfur.
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