Quantifying nitrate dynamics in an oligotrophic lake using Δ¹⁷O

Abstract. The stable isotopic compositions of nitrate, including the ¹⁷O anomalies (Δ¹⁷O), were determined twice in 1 yr (June and August 2007) in the oligotrophic water column of Lake Mashu, Japan. These data were then used to quantify the geochemical dynamics of nitrate in the lake, by using the deposition rate of the atmospheric nitrate onto the entire catchment area of the lake. The total amount of nitrate in the lake water decreased from 4.2 to 2.1 Mmol during the period between the observations, while the average Δ¹⁷O values remained uniform at +2.5‰. The Δ¹⁷O values corresponded to an small and uniform mixing ratio of atmospheric nitrate to total nitrate of 9.7 ± 0.8%. These results indicate that 0.52 ± 0.34 Mmol of the remineralized nitrate was fed into the water column through nitrification, while 2.6 ± 0.4 Mmol of nitrate was simultaneously removed from the water column by assimilation, during the period between the observations. The lake water dissolved nitrate was characterized by rapid removal through assimilation during summer until it was almost completely removed from the euphotic layer, as well as continuous feeding into the lake through nitrification (3.2 ± 0.3 Mmol a⁻¹) and deposition (0.35 ± 0.2 Mmol a⁻¹), regardless of the seasons. The ¹⁵N-depleted nitrogen isotopic compositions of nitrate were as low as −6.5‰ in June, which also indicates that in-lake nitrification is the major source of nitrate in the lake and suggests that there is low potential for denitrification in and around the lake. Atmospheric nitrate deposited into the lake will be assimilated quickly, having a mean residence time of 1.2 ± 0.1 yr. In addition, more than 90% of the assimilated nitrate will be remineralized to nitrate and re-assimilated via active nitrogen cycling in the lake.