Nitrogen removal in the Chaohu Lake, China: Implication in estimating lake N uptake velocity and modelling N removal efficiency of large lakes and reservoirs in the Changjiang River network

Abstract

Lakes and reservoirs are important inland waterscapes in linking nitrogen (N) transport and removal from the terrestrial to marine ecosystems. N removal processes eliminate N permanently from lakes and reservoirs, including denitogen, permanent burial and biomass harvest. The N removal efficiency (R_removal, i.e. the fraction of N removal amount against total N loads to the lake) is an important parameter for the quantification of N removal from lakes and reservoirs. Intrinsically, R_removal depends on hydrologic load and total N uptake velocity (Vf_removal). However, it is a challenge in determining Vf_removal which can be used to model R_removal of lakes and reservoirs. Furthermore, there is less studies in distinguishing the N uptake velocity of individual processes from each other. Here, we estimated the magnitude of each uptake velocity of N removal processes with field observed data, and further presented an integrated Vf_removal to estimate R_removal of a typical eutrophic lake (the Chaohu Lake) in Eastern China. The mean dinitrogen emission rate was 1.23 ± 0.83 mg N m−2 h−1 in Chaohu Lake in 2016. The uptake velocity of denitrogen, permanent N burial and biomass harvest was 2.67 m yr−1, 0.25 m yr−1 and 0.23 m yr−1, respectively. The removal efficiency of denitrogen, permanent N burial and biomass harvest was 26.14%, 2.76% and 2.56%, respectively. The integrated value of Vf_removal was 3.33 m yr−1. We finally compiled hydrological data of 566 large reservoirs in Changjiang River network, and modelled the N removal by combining our estimated value of Vf_removal with hydrological data. We found a significant power relationship between N removal and the reservoir discharge, which can predict the variation of N removal of each reservoir in Changjiang River network. Our research helps to understand N removal in lakes and reservoirs at an entire river network scale.