Implications of eutrophication for biogeochemical processes in the Three Gorges Reservoir, China
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Although the Three Gorges Reservoir (TGR) is the largest man-made lake in the Changjiang River, it traps only a small fraction of the nitrogen (N) and dissolved silicate (DSi) inflows. Internal dissolution processes of exogenous biogenic silica (BSi) to DSi within TGR may control the overall silica (Si) retention, while the primary diatom production plays a minor role in DSi removal. Transformations of reactive N caused an increase of the dissolved inorganic nitrogen (DIN) load by 3% during transport through the TGR, while retention of dissolved inorganic phosphorus (DIP) is enhanced by biological production. As a result, the TGR causes an increase of the molar DIN/DSi, DSi/DIP, and DIN/DIP ratios, and a decrease of DIN/RSi (reactive Si, the sum of DSi and BSi), leading to an enhanced phosphorus limitation downstream of the TGR. The overall impact of the changing stoichiometry as expressed by the Index for Coastal Eutrophication Potential (ICEP) is an excess production of 27 Tg C/year of non-diatom, potentially harmful phytoplankton. More intensive monitoring is thus needed to better understand the biogeochemical processes in the TGR and to support policy development aimed at improving the water quality in the Changjiang River.
KeywordsChangjiang River (Yangtze River) Nitrogen Nutrient limitation Phosphorus Silicon Stoichiometry Three Gorges Reservoir
We thank P.X. Liu, H.T. Chen, Q.Z. Yao, B.C. Xu, Y. Qiao, H.T. Zheng, X. Wang, and J.P. Tao for their help in the laboratory work and field sampling.
X.B.R. and Z.G.Y. designed the research; X.B.R. A.F.B., and J.L. wrote the paper; J.L. worked on the figures.
This study was supported in part by the National Natural Science Foundation of China (41776089) and the Basic Scientific Fund for National Public Research Institutes of China (2017S03 and 2017Q10). A.F.B. received support from PBL Netherlands Environmental Assessment Agency through an in-kind contribution to The New Delta 2014 ALW projects no. 869.15.015 and 869.15.014.
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