Quantifying nitrate dynamics in a mesotrophic lake using triple oxygen isotopes as tracers

Abstract

Vertical distributions of both concentrations and stable isotopic compositions of nitrate, including the 17 O-excesses (D 17 O), were determined four times during 1 yr within the mesotrophic water column of Lake Biwa in Japan. By using both the deposition rate of atmospheric nitrate onto the entire surface of the lake and the influx/efflux of both atmospheric and remineralized nitrate via streams reported in the literature, we quantified the annual dynamics of nitrate (gross production rate of nitrate through nitrification and gross metabolic rate of nitrate through assimilation and denitrification), together with their seasonal variations, based on the D 17 O method. The results revealed that 642 6 113 Mmol (Mmol 5 10 6 mol) of the remineralized nitrate was supplied into the water column through nitrification in the lake on an annual basis, while 810 6 120 Mmol of nitrate was metabolized in the lake through assimilation and denitrification. In addition, it turns out that nitrification was active, not only in the hypolimnion, but also in the epilimnion and upper thermocline in this lake. Furthermore, the total metabolic rates of nitrate varied seasonally, with the highest rates in summer and the lowest in winter. Because the difference between the annual metabolic rate of nitrate estimated based on the D 17 O method and the annual assimilation rate of nitrate estimated based on the traditional 15 N incubation method was only 20%, we concluded that the D 17 O method reliably estimates the dynamics of nitrate in mesotrophic lakes. Nitrate (NO 23 ) is a key nutrient in aquatic environments that often limits primary production. Nitrate dynamics in an aquatic environment, i.e., gross production rate of nitrate through nitrification (F nit ), gross metabolic rate of nitrate through assimilation (F assim ), and gross metabolic rate of nitrate through denitrification (F denit ), are important parameters to be quantified when evaluating both the present and future state of the aquatic environment. In most studies that have been conducted to date, nitrate dynamics has been estimated via incubation experiments using 15 N tracer techniques. To quantify F assim , for instance, 15 N-labeled NO 2 3 is added into bottles that simulate in situ conditions of the aquatic environment studied, which leads to the production of particulate organic-15 N (PO 15 N) through assimilation over a known incubation period of several hours to several days (Dugdale and Goering 1967;Knap et al. 1996). The PO 15 N is then gathered and quantified using mass spectrometry.The experimental procedures using 15 N tracer, however, are generally costly and complicated. Besides, while the obtained nitrate dynamics is an instantaneous assimilation rate at the point of observation, such a value may be temporally variable in response to various factors such as changes in temperature, light intensity, nutrients, and community structure. As a result, tedious and time-consuming time series observations are needed to estimate long-term nitrate dynamics (s...

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Export flux of unprocessed atmospheric nitrate from temperate forested catchments: A possible new index for nitrogen saturation

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Stable isotope tracers: Enriching our perspectives and questions on sources, fates, rates, and pathways of major elements in aquatic systems

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