Abstract
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.
Highlights
Anthropogenic activities have led to increased emissions of fixed nitrogen from land to the atmosphere
All of the nitrate in Lake Mashu had low, but positive 17O values ranging from +1.6 to +4.3‰, suggesting that the lake water contained some NO−3 atm
Because the 17O value of nitrate is stable during the partial uptake of nitrate through assimilation or denitrification, ( 17Olake)0 and ( (17Olake))t can be expressed by the gross nitrate feeding rate through both atmospheric deposition ( NO−3 atm in the lake water (Natm)) and nitrification in and around the lake ( Nnit) to the initial inventories of nitrate in the lake ((Natm)0 and (Ntotal)0), (Natm)0 + Natm =
Summary
Anthropogenic activities have led to increased emissions of fixed nitrogen from land to the atmosphere. Atmospheric input of excess fixed-nitrogen nutrients could play a major role in this decline in clarity through an increase in phytoplankton in the water column (Riis and Sand-jensen, 1998; Jassby et al, 1994; Fukazawa, 2008). The average δ15N, δ18O and 17O values of atmospheric nitrate deposited onto the East Asian region have been determined through continuous monitoring at Rishiri Island (Fig. 1; Tsunogai et al, 2010), which is located in the same prefecture as Lake Mashu in Japan. Determined both the concentrations and stable isotopic compositions of nitrate in Lake Mashu, including the 17O values, to quantify the NO−3 atm/NO−3 total ratio of nitrate and clarify the geochemical dynamics of nitrate in the water column. The findings presented will help clarify the fate of atmospheric nitrate deposited onto the hydrosphere in general
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