RESUMO
To investigate the vertical distribution characteristics of phosphorus in the main stream sediments of the Three Gorges Reservoir (TGR), column sediment samples were collected from 5 sites in October 2010, and sediment particle size, organic matter contents, and mineral compositions were analyzed. Investigation of the contribution rate of phosphorus released from column sediments in the TGR was also conducted. The results show that the sediment pH is between 7.3-7.8. The mainstream column sediment is mainly constituted by silt and clay, which account for 49.4%-78.6% and 20.6%-50.6% of total sediments, while sand represents less than 4.4%. Median grain size of each sampling site presented a phased increase or decrease trend. The organic matter content was between 12.94 g·kg-1 and 53.43 g·kg-1, and it tended to slightly increase from upstream to downstream. The C/N ratio in the sediment was between 4.00 and 11.64, and organic matter content was mainly affected by terrigenous input. Total phosphorus (TP) content was between 861.86 mg·kg-1 and 1024.54 mg·kg-1, and it exhibited negligible change in vertical distribution. There is no obvious enrichment phenomenon of phosphorus for column sediment. The major component of sediment TP is calcium bound phosphorus (Ca-P, 47.83%-73.90%), and there are various trends for phosphorus distribution in different sampling sites. Exchangeable phosphorus (Ex-P), aluminum-bound phosphorus (Al-P), and iron-bound phosphorus (Fe-P) in the 0-4 cm surface sediment of each sampling sites was relatively high. For most sampling sites, no obvious change of phosphorus fractions in 16-20 cm of sediment was detected. Bioavailable phosphorus (the sum of Ex-P, Al-P, and Fe-P) accounted for 2.78%-7.05% of TP, indicating that bioavailability of phosphorus in the column sediments is low. The contents of bioavailable phosphorus and organic matter were significantly and positively correlated (P<0.05, N=50). The distribution and transformation of organic matter will affect the migration and transformation of sediment phosphorus in the TGR.
RESUMO
In order to understand the characteristics of the distribution of sediment total phosphorus (TP) and phosphorus fractions in the mainstream sediments in the Three Gorges Reservoir (TGR) after impounding the water level to 175 m, 13 surface sediment samples were collected from the Wujiang to Maoping sections in October 2010. The physico-chemical properties, including organic matter content, particle grain size distribution, and major mineral analysis, as well as total phosphorus and its fractions in the sediment, were determined. Moreover, the relationships among phosphorus fractions, organic matter contents, and particle grain size were discussed, and the effect of the impoundment on sediment phosphorus accumulation and bioavailability was also evaluated. Results indicated that the sediment organic matter content of the TGR was between 7.79 g·kg-1 and 55.63 g·kg-1, and the main mineral components were chlorite, illite, and quartz. The sediments were dominated with clayey silt with a median diameter (d50) ranging from 3.84 µm to 23.65 µm. The measured total phosphorus content of the sediments were between 557.06 g·kg-1 and 837.92 g·kg-1, and the total phosphorus enrichment index of each sampling site is greater than 1, demonstrating a potential risk for phosphorus pollution. The calcium bound phosphorus (Ca-P) and the reductant soluble phosphorus (Oc-P) were the dominant sediment phosphorus fractions, while the exchangeable phosphorus (Ex-P), the iron bound phosphorus (Fe-P), and aluminum bound phosphorus (Al-P) content were relatively low. Bioavailable P only accounts for 2%-8% of the total phosphorus content. When referring to previous studies, the sediment particle size tended to be smaller and the content of comparatively easy-to-weather minerals slightly increased with the increase of the impoundment water level. However, the increase in the impoundment water level did not result in a significant increase tendency in sediment TP content. In the future, a reduction in sediment input and a decline in sediment particle size may facilitate the accumulation of phosphorus in the sediments in the broad valley section of the TGR. Moreover, large scale dry-wet alternation in the water level fluctuation zone and resuspension of floating mud near the dam both potentially impact the bioavailability of phosphorus in the sediments.
