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Sediment Quality Assessment and Management: Insight and Progress Edited by M. Munawar© 2003 Ecovision World Monograph Series Aquatic Ecosystem Health & Management Society Estimation of phosphorus flux between bottom sediments and the water column in a shallow reservoir K. Amano*, J. Li, H. Suzuki, Y. Yasuda Public Works Research Institute, 1-6 Minamihara, Tsukuba, 305-8516, Japan. *amano Keywords: eutrophication, settling velocity, efflux rate Introduction The role of internal loading of phosphorus in lakes with regard to algal growth in summer has been mentioned frequently (e.g., Larsen et al., 1981), and anoxic hypolimnia are usually described as the cause of the release of phosphorus from bottom sediments (e.g., Mortimer, 1971; Nürnberg, 1988). The mass balance of phosphorus is usually difficult to measure in natural lakes, because the change in total phosphorus concentration in lake water is controlled mainly by inflow, outflow, particulate settling, and release from bottom sediments. In particular, it is difficult to make an accurate estimate of the amount of phosphorus brought by inflow. As a result it is also difficult to evaluate the role of bottom sediments as a sink and source of phosphorus throughout the year. We chose Watarase Reservoir, outside Tokyo, Japan, as an experimental field because it is isolated from nearby rivers by gates, so water exchange with the rivers is limited and intensive water quality measurements of inflow and outflow allow us to estimate the material balance in the reservoir precisely. When river water is taken into the reservoir, if the concentration of total phosphorus (T-P) and total nitrogen (T-N) is higher in the river water than in the reservoir water, the concentration of these nutrients suddenly increases and then gradually decreases as settling occurs in the reservoir. In contrast, when the reservoir water is discharged in summer, the nutrient concentration in the reservoir gradually increases. In particular, during the first half of the summer low-water-level period, the T-P concentration increases without any external loading of phosphorus. Because there 318 is no external loading, this increase in T-P concentration seems to be caused by the release of nutrients from the bottom sediments. Internal loading of phosphorus released from the bottom sediments seems to play a significant role in increasing the T-P concentration over this period. We assumed that phosphorus flux from the water column to the bottom sediments was caused mainly by the deposition of particulate phosphorus (P-P), and that flux from the bottom sediments to the water column was dominated by the release of phosphate phosphorus (PO4-P). We then estimated the rates of these fluxes to determine the role played by sediments in phosphorus dynamics in summer. Site description Watarase Reservoir (Figure 1) is located about 60 km northwest of the Tokyo metropolitan area. The reservoir serves as a water storage and flood control reservoir. Since the exchange of water between the reservoir and nearby rivers is controlled by gate operation, the material flux into and out of the reservoir can be estimated precisely. Water quality measurements are taken most days that the gates are operated. As the gates are not operated frequently, we were able to observe changes in water quality that were driven only by biochemical changes in the water column and by interactions between the water column and sediments. The surface area of the reservoir is about 4.5 km2 and the bottom is flat (plan view: Figure 1). Summer flood season operations require that the water is controlled to a maximum depth of 3 m from 1 July to 30 September so that the capacity to store flood water can be maintained. (During floods, the water level can exceed 3 m, but after the peak of the flood the reservoir water is released immediately to maintain the 3-m water depth.) Outside the summer flood season the water is controlled to a maximum depth of 6.5 m. The phosphorus and nitrogen concentrations of the river water taken into the reservoir are as follows. The T-P varies from 0.10 to 0.40 mg l-1 and the concentration of PO4-P ranges between 0.07 and 0.30 mg l-1. Total nitrogen (T-N) ranges from 2.0 to 4.5 mg l-1, and nitrate nitrogen (NO3-N) is the major inorganic nitrogen component. Since the influent concentrations of phosphorus and nitrogen are high, the reservoir is highly eutrophic. Changes in water quality in the reservoir are closely associated with...


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