Utilization of Δ¹⁷O for nitrate dynamics in a subtropical freshwater reservoir

Feitsui Reservoir, a freshwater body in Taiwan with minimal anthropogenic stress, meets the water demand for the population of more than five million living in Taipei city. In view of the biogeochemical processes controlling the long-term trophic status of this socio-economically and ecologically important aquatic system, probing the nitrogen cycle and its dynamics is essential. Here, we monitored the concentration and stable isotopic compositions (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) of nitrate in the Feitsui Reservoir and in the atmospheric wet deposition at intervals of 1-2 weeks for a year, along with measurements of environmental data such as chlorophyll a, dissolved oxygen, and community respiration. Emphasis was laid on Δ¹⁷O (= δ¹⁷O - 0.52 × δ¹⁸O) because of the mass-conservative behavior of Δ¹⁷O during partial assimilation and denitrification. The present approach offered an effective method to quantify the gross nitrification and removal/uptake rates of nitrate in the reservoir. The atmospheric nitrate exhibited elevated Δ¹⁷O values ranging from 12.6‰ to 30.1‰ (23.3 ± 5.0‰), compared to the lower Δ¹⁷O values of ~0 to 4.6‰ (1.1 ± 0.7‰) recorded in the reservoir nitrate. Utilizing Δ¹⁷O for dissolved nitrates, we observed a seasonal trend of higher nitrification and removal rates during the summer than in the winter. Our estimates showed annually-averaged nitrification rate of 55 ± 11 mmol m^-2 d^-1 and removal/uptake rate of 57 ± 11 mmol m^-2 d^-1 (or a nitrate turnover time of ~2.5 months), representing the active nature of nitrogen cycling in this preserved subtropical reservoir.