Abstract
Rubber dams are widely used for landscaping in urban rivers and they retain large amounts of sediments. The sediments are rich in phosphorus (P) which can cause river eutrophication. Little is known about P release in rubber dams. We investigated the potential of sediment P release by isotherm experiment in an urban river with 30 rubber dams of northern China. We found that the potential of sediment P release (percentage saturation of zero equilibrium P concentration, EPCsat) was 76% at natural river part above dams, and then decreased to 67% at the 4th dam because of high deposition of fine sediments within the upper 4 dams. Between the 5th and the 30th dams, EPCsat increased to 90% because of the decrease of fine sediments and water soluble reactive P. EPCsat was also significantly higher (p < 0.05) in April and August than in November. The results suggest that the potential of sediment P release in this dammed river was mainly controlled by sediment grain size and biological effects. Therefore, management strategies for dammed rivers should focus on reducing P inputs and improving the hydraulic conditions.
Highlights
Rubber dams are widely used for landscaping in urban rivers and they retain large amounts of sediments
The Standards, Measurements and Testing Program of the European Commission (SMT) method[20] defines a range of sediment P fractions that might be involved in release processes, including exchangeable labile P, apatite P (Ca-P, calcium associated forms), non-apatite P (Fe/Al-P, the forms associated with oxides and hydroxides of Al, Fe, Mg and Mn), and organic P (OP); of these, ex-P, Fe/Al-P, and OP are considered as bioavailable P (BAP) and can release or be mineralized to water for maintaining its eutrophic state over a long term[6]
In order to further elucidate the effects of dams on sediment P release, the following questions will be addressed in present study: (1) if the potential of sediment P release varied with distance downstream along the series of rubber dams, (2) which physicochemical factors, both from the sediments and the river water, primarily controlled the potential of sediment P release in this dammed river, and (3) whether the presence of dams, by interfering with the physicochemical environment, affected the potential of sediment P release in this river
Summary
Rubber dams are widely used for landscaping in urban rivers and they retain large amounts of sediments. Several study have found that the potential of P release was higher from sediments retained by rubber dams than from sediments downstream of the dams[10], which could contribute to eutrophication of the impounded river[9]. Reservoir dams generally cause a decrease in the flow velocity by increasing the water residence time; and the P concentration in the water column would decrease when particles settle and P is assimilated within reservoirs[40,41] These variations in sediments and water column of rivers with dams may have implications for the potential of P release and cause long-term eutrophication of the impoundments. We investigated the potential of sediment P release in a typical urban landscape river with 30 rubber dams that flowed through the center of Zhangjiakou city, Hebei Province, northern China. In order to further elucidate the effects of dams on sediment P release, the following questions will be addressed in present study: (1) if the potential of sediment P release varied with distance downstream along the series of rubber dams, (2) which physicochemical factors, both from the sediments and the river water, primarily controlled the potential of sediment P release in this dammed river, and (3) whether the presence of dams, by interfering with the physicochemical environment, affected the potential of sediment P release in this river
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