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.

• The N 2 emission rate was 1.23 ± 0.83 mg N m−2 h−1 in Chaohu Lake. • The upper and lower limits of uptake velocity were 3.33 m yr−1 and 2.67 m yr−1. • The N removal efficiency via denitrogen was around 26.14% in Chaohu Lake. • The total N removal efficiency of the Chaohu lake was 31.45%. • N removal efficiency of reservoirs has a power relationship with water discharge. 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 (V f_removal). However, it is a challenge in determining V f_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 V f_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 V f_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 V f_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. [ABSTRACT FROM AUTHOR]