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The sediment in reservoirs is mainly derived from the sedimentation of suspended grains, such as organic particles and inorganic minerals, carried by runoff scouring from vegetation. There are three parts in this chapter: (1) Effects of pollutants released from sediment on water quality; (2) Effects of metals released from sediment on water quality; (3) Algal blooms and their impact on water quality. The decrease of the oxidation–reduction potential (ORP) in a multiphase interface under anaerobic conditions resulted in the increase of soluble ferrous hydroxide. Phosphate combining with iron hydroxide in sediment is released to interstitial water and then diffused into the overlying water. In addition, the metabolism of microbes resulted in the decrease of the ORP in anaerobic conditions, promoting the release of soluble iron in sediment and then causing the huge release of phosphorus. The release of endogenous phosphorus from sediment under anaerobic conditions resulted in the significant increase of PO4 3− in the overlying water. The release of phosphorus from sediment depends on the conditions of the ORP.

Reservoirs are artificially formed in the bed of a river by building a dam, which intercepts the runoff. They have narrow surface but great depth, and the water level changes significantly during flooding. The mean depth of Chinese reservoirs is around 20–50 m.

Nitrogen is not only a basic constituent element of life, but it is also one of the key elements causing eutrophication. It has an important effect on nutritional status and water quality in lakes and reservoirs. Inorganic nitrogen, including ammonia (NH4 +) and nitrate (NO3 −), often occurs following nitrification and denitrification reactions in a multiphase interface with the change of dissolved oxygen (DO) and redox. In addition, the extent of heavy metals is diffused from sediments to overlying water. It may cause potential harm to the water quality of the reservoir. This chapter is separated into two parts: (1) The release of nitrogen and phosphorus from sediment is investigated in typical Chinese reservoirs; (2) The release of metals from sediment is investigated in typical Chinese reservoirs.

The concentration of inorganic phosphorus (IP) in the surface sediment of each of the studied reservoirs ranged from 208.8 to 636.7 mg/kg. The concentration of Fe/Al-P in the surface sediment in the four reservoirs ranged from 82.1 to 216.7 mg/kg, and the average value was 129.6 mg/kg, accounting for 11.54–18.1 % of the sediment total phosphorus (STP) content. The highest proportion of IP is observed in Yuqiao Reservoir, and Jinpen Reservoir had the lowest proportion. In addition, the results indicated that the Ca–P concentrations of the four reservoir sediments are in the range 180.7–303.4 mg/kg, with an average value of 247.1 mg/kg, accounting for 22.33–37.1 % of the STP. The content of organic phosphorus (OP) in the reservoir surface sediment is in the range 64.2–201.8 mg/kg, accounting for 9.85–18.52 % of STP content. The ion exchange nitrogen (IE-N) content (mg/kg) in reservoirs is significantly different. The IE-N content of Shibianyu Reservoir is much higher than that of other reservoirs, accounting for 18.35 % of the total nitrogen (TN). The IE-N in the studied reservoirs accounted for 15.33–18.35 % of the TN. IE-N exists in three different speciations: the concentration of ammonia is much higher than that of nitrate and nitrite, and the concentration of nitrite is the lowest. The cloning and sequencing results showed that microorganisms were made up of ten major categories, including shaped bacillus, Chloroflexi, Verrucomicrobia, Bacteroides, Acidobacteria, thick wall bacteria, actinomycetes, Gemmatimonadetes, Nitrospirae, and Planctomycetes, in the Jinpen Reservoir sediments.