Sources, fate and geochemical dynamics of nitrate in an oligotrophic lake

Abstract

Abstract. To trace the fate of atmospheric nitrate deposited into a oligotrophic lake and to clarify the geochemical dynamics of nitrate in a oligotrophic environment, the stable isotopic compositions of nitrate, including the 17O anomalies (Δ17O), were determined twice in one year (June and August 2007) in the water column of Lake Mashu, Japan, which is a crater lake with a depth of 211 m. The highest transparency of the lake (41.6 m) was recorded in 1931, although the transparency has decreased substantially in recent years. While the total inventory of nitrate in the lake water decreased from 4.2 to 2.1 Mmol (Mmol = 106 mol) during the period between the observations, the average Δ17O nitrate values were uniform at +2.5‰, which corresponded to an average mixing ratio of atmospheric nitrate to total nitrate of 9.7 ± 0.8%. Using the total mass of the atmospheric nitrate deposited onto the entire catchment area of the lake during a period of 2 months (0.047 Mmol), we estimated 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. The lake water dissolved nitrate was characterised 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 and deposition, regardless of the seasons, which corresponds to a gross annual flux of 3.2 ± 0.3 Mmol a−1 and 0.35 ± 0.2 Mmol a−1, respectively. 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. These results indicate that atmospheric nitrate deposited into the lake will be assimilated quickly having a mean residence time of 1.2 ± 0.1 years. Besides, more than 90% of the assimilated nitrate will be remineralized to nitrate and assimilated again via active nitrogen cycling in the lake.