RESUMO
The ecological problems due to reservoir construction are causing unprecedented concern. To reveal the differences in organic carbon distribution characteristics and sediment sources of total organic carbon (TOC) between the old and new reservoirs, water samples, and sediment samples from reservoirs constructed in the three different periods of Miaowei, Gongguoqiao, and Dachaoshan were collected in November 2017. The temperature (T), dissolved oxygen (DO), TOC, redox potential (ORP), total nitrogen (TN), and total phosphorus (TP) of the water samples were measured. The isotopes 15N and 13C were used as indicators with IsoSource software to analyze the contributions of TOC sources and their source materials to the corresponding reservoir sediments, in order to explore the carbon cycle mechanism and evolution mode of reservoir. The results showed that the average concentrations of organic carbon in the waters of the Miaowei, Gongguoqiao, and Dachaoshan Reservoirs were 0.95 mg·L-1, 1.97 mg·L-1, and 4.64 mg·L-1, respectively. The range of organic carbon content in the corresponding sediments was 4.41-81.63 g·kg-1, 18.30-28.42 g·kg-1, and 9.16-14.46 g·kg-1, respectively. The cascade construction of the reservoirs resulted in a difference between the sediment sources of the new and old reservoirs and the surrounding recharge area, meaning that the TOC of the new and old reservoirs were significantly different. For the TOC of waterbodies, the difference between the thermodynamic state of water and dissolved oxygen indirectly affects the distribution trend of TOC. The sediments mainly reflect the influence of source elements, that is, the ability of the sedimentary environment to preserve organic matter was the main cause of the vertical distribution of DCS, MV, and GGQ sediments. In the evolution mode of cascade reservoir, the research shows that it can be preliminarily set as three stages. Firstly, due to the short age of MV, it is in the first stage and mainly accumulates the TOC from the upstream. GGQ is longer than the age of MV, and it is mainly used to decompose the upstream TOC, so it is defined as in the second stage. Finally, as an old reservoir, DCS mainly accumulates TOC sources around the reservoir, which can be regarded as the third stage.
RESUMO
To study the mechanism of phosphorus cycling in sediment during the redox cycle, changes in physicochemical properties of overlying water and various forms of phosphorus in sediments were investigated as a way to quantify the redistribution of phosphorus. Additionally, the effect of the release flux of phosphate from sediments under controlled redox conditions was analyzed. The results showed that the redox potential Eh and the pH system, sulfur system, carbon system, and iron-related changes exhibited periodicity and played an important role in explaining the migration and transformation mechanism in the interface phosphorus of the sediment-water phase. During the redox cycle, the phosphorus content of each species varied with the redox conditions and time. Because of this, quantitative analysis based on changes in water-sediment phosphorus could be obtained. Reducible phosphorus (BD-P) and iron-aluminum-bound phosphorus (NaOH-rP) were reversibly redistributed into weakly adsorbed phosphorus (NH4Cl-P), polyphosphorus/organophosphorous (NaOH-nrP), residual phosphorus (Rest-P), and interstitial water-soluble active phosphorus (SRP). Additionally, 93.7% of phosphorus in the sediment was not released into the water phase during the reduction reaction. The 92% of change in the overlying water total phosphorus (TP) was the SRP of overlying water, which showed that the exchange of the sediment-water phase were mainly soluble active phosphorus in this cycle. According to Fick's First Law, the maximum phosphorus flux was 0.58 mg·(m2·d)-1 during reduction and 0.16-0.22 mg·(m2·d)-1 on day seven of the oxidation phase. In the oxidation stage, the diffusion flux decreased with time, while the opposite trend occurred in the reduction reaction. This indicated that the anaerobic state accelerated the diffusion of phosphorus in sediments, and that oxygen exposure decreased the phosphorus flux in sediments.
