Longitudinal Variability of Phosphorus Fractions in Sediments of a Canyon Reservoir Due to Cascade Dam Construction: A Case Study in Lancang River, China

Qi Liu,Shikui Dong,Li Deng,Cong Wang,Haidi Zhao,Shiliang Liu,Qinghe Zhao,Syuhei Ban

doi:10.1371/journal.pone.0083329

Abstract

Dam construction causes the accumulation of phosphorus in the sediments of reservoirs and increases the release rate of internal phosphorus (P) loading. This study investigated the longitudinal variability of phosphorus fractions in sediments and the relationship between the contents of phosphorus fractions and its influencing factors of the Manwan Reservoir, Lancang River, Yunnan Province, China. Five sedimentary phosphorus fractions were quantified separately: loosely bound P (ex-P); reductant soluble P (BD-P); metal oxide-bound P (NaOH-P); calcium-bound P (HCl-P), and residual-P. The results showed that the total phosphorus contents ranged from 623 to 899 µg/g and were correlated positively with iron content in the sediments of the reservoir. The rank order of P fractions in sediments of the mainstream was HCl-P>NaOH-P>residual-P>BD-P>ex-P, while it was residual-P>HCl-P>NaOH-P>BD-P>ex-P in those of the tributaries. The contents of bio-available phosphorus in the tributaries, including ex-P, BD-P and NaOH-P, were significantly lower than those in the mainstream. The contents of ex-P, BD-P, NaOH-P showed a similar increasing trend from the tail to the head of the Manwan Reservoir, which contributed to the relatively higher content of bio-available phosphorus, and represents a high bio-available phosphorus releasing risk within a distance of 10 km from Manwan Dam. Correlation and redundancy analyses showed that distance to Manwan Dam and the silt/clay fraction of sediments were related closely to the spatial variation of bio-available phosphorus.

Highlights

Dam construction can change rivers’ configurations and flow regimes [1,2,3], which will have conspicuous direct effects on nutrient loading in the rivers [4,5]
Analytical Methods For P fractionation of the sediments from the reservoir, we used the chemical sequential extraction method of Psenner et al [45], slightly modified by Hupfer et al [46]. This method fractionates the phosphorus of the sediments into four fractions–loosely bound P, reductant soluble P (BD-P), metal oxide-bound P (NaOHP), calcium-bound P (HCl-P; Table 1) and residual-P–which was the difference between total phosphorus and the four P fractions extracted
In Manwan Reservoir, BD-P constituted a minor part of total phosphorus (TP) of the sediments (4.9% on average) which was relatively lower than in other reservoirs

Summary

Introduction

Dam construction can change rivers’ configurations and flow regimes [1,2,3], which will have conspicuous direct effects on nutrient loading in the rivers [4,5]. While the external P is reduced, the sediments still release P into the water, which is called internal phosphorus (P) loading [8,9,11,12,13,14,15,16]. In river systems influenced by hydropower dams, the spatial grain size distribution of the sediments might be affected by the formative reservoirs and prolonged water renewal time, which eventually has an effect on the heterogeneity of different P fractions in the sediments. The study of the mechanism of the longitudinal variability of phosphorus fractions in the sediments of canyon reservoirs affected by cascade dams is insufficient and needs further research

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Conclusion