Dissolved oxygen (DO) microprofiles of prepared sediments from 24 sampling sites in the Fuyang River were measured using a gold amalgam microelectrode in this study. The measured microprofiles can be divided into four types. In type I profiles, DO kept constant in the overlying water and decreased smoothly in the pore water; in type II profile, DO showed fluctuation in the pore water; in type III profiles, DO showed peak in the SWI; in type IV profiles, DO decreased obviously in the overlying water. Type I profiles indicated the absence of benthic organisms and thus the degradation of the sediment habitat. Type II and III profiles indicated the activity of benthic animal and epipelic algae, which is common in the healthy aquatic sediment. Type IV profiles indicated that the excessive accumulation of pollutants in the sediment and thus the serious sediment pollution. There are nine sites showing type I profile, three sites showing type II profile, nine sites showing type III profile, and three sites showing type IV profile in the Fuyang River. The dominance of type I and appearance of type IV indicated that sediment oxygen consumption processes in the Fuyang River were strongly influenced by the sediment pollutants release and the vanish of benthic organisms. The pharmacy, metallurgy, and curriery industries may contribute to the sediment deterioration and thus to the occurrence of type I and type IV oxygen profiles in the Fuyang River.
Oxygen penetration is a key determinant of sediment nitrification rates. In this study, we analyzed the effect of oxygen penetration on the sediment nitrification rate based on sediment oxygen profiles. Six sediments were designed to produce different oxygen profiles by adding different amounts of silica gel to the collected river mud. The oxygen profiles in the sediment were detected using a voltammetric microelectrode. With increased mud content, the sediment oxygen penetration depth decreased from 8.3 to 2.6 mm, and the oxygen concentration in the overlying water and at the sediment-water interface also showed a decreasing trend. The measured nitrification rate displayed a quadratic pattern that changed with the increase in mud content. Based on the detected oxygen profiles, the nitrification rate at each depth was calculated and summed to obtain the bulk sediment nitrification rate. The bulk sediment nitrification rate showed a consistently changing pattern with the measured rate. Oxygen profiles used to calculate nitrification rates could be approximated by the penetration depth (δ). The resulting nitrification model based on δ could explain the limiting role of oxygen penetration in sediment nitrification.
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