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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 The effects of sediment resuspension on phosphate release from sediment into the water column J. Li*, K. Amano, Y. Yasuda Public Works Research Institute, Asahi-1, Tsukuba-Shi, 305-0804 Japan. * Keywords: reservoir, sediment resuspension, nutrients, laboratory study Introduction Aquatic sediments as sources of nutrients in the water column have been studied extensively for more than half a century (Mortimer, 1941, 1942; Vollenweider, 1968; Matty et al., 1987). Most studies have shown that the maintenance of high nutrientlevelsinoverlyingwatersisduetointernalloadingfromtheaquaticsediments (Rossi and Premazzi, 1991; Appan and Ting, 1996). The mechanism of nutrient release from redox-induced fluxes of dissolved nutrients from sediments to the anoxic hypolimnion is well known (Fillos and Swanson, 1975; Schladow and Hamilton, 1995), but few studies have investigated the effects of sediment resuspension on soluble reactive phoshate (SRP) fluxes. Sediment resuspension is widely postulated to favour phytoplankton dominance in shallow eutrophic lakes; however, it is very difficult to study the mechanism directly, because SRP can be obscured by rapid sorption on to tripton particles (Søndergaard et al., 1992) and by rapid uptake by phytoplankton (Hamilton and Mitchell, 1997). Watarase Reservoir, a part of the Watarase retarding basin, is located north of Tokyo at lat 36°13' N, long 139°40' E. It has a surface area of 4.5 km2 and an average depth of 6.0 m, with seasonal changes in depth of about 3.0 m for flood control. In recent years, eutrophication has become a problem in the reservoir. High nutrient loading leads to excessive phytoplankton growth in spring and summer (Amano et al., 2001). One reason for the summer bloom is believed to be internal nutrient loading due to frequently occurring wave-induced sediment resuspension 338 during periods when water levels are low and the DO in the overlying water remains at a relatively high level. The purpose of this study was to investigate the release mechanisms of SRP during sediment resuspension in a simulated laboratory study and to discuss the contribution of the sediment resuspension to the total SRP load. Materials and methods Sediment investigation Sediment core tubes (inner diameter = 10.2 cm, length = 50 cm) that could also be used as reaction chambers were used in the field investigation. The tubes were made of acrylic material with 2 removable lids fitted with o-ring gaskets. Sediment cores were collected, in year 2000, near the centre of the south block in Watarase Reservoir with a HR type core sampler (RIGO Co., Ltd., Tokyo) equipped with the specially developed sediment core tubes. The bottom water overlying the sediment was carefully collected by siphoning, and the sediment cores were cut into 3-cm-thick slices. The collected bottom waters and the sliced sedimentcore segments were transferred in a cool box for chemical analysis. The sediment cores were transferred to the specialized cool containers for the resuspension simulation experiments. Both samples were transported to the laboratory. Sediment resuspension simulation tests Resuspension experiments were conducted using an apparatus consisting of reaction chambers (the specialized core tubes), electrode sensors, water bath, stirring system, sampling system, digital controller, and PC monitor (Fig. 1). Sediment resuspension was simulated by using MAZELA Model Z-1200 stirrers (EYELA Instruments, Tokyo), which were adjusted with a regulator to continuously spin in the water column of the reaction chambers. Water quality (DO, ORP, etc.) was monitored in the centre of the mixed water to obtain representative values of the water column. The overlying water was collected with a rotary pump. After the simulation, sediment pore water was obtained from 3-cm-interval sediment-core segments by centrifugation at 10 000 rpm (9600 × g) and 4 °C for 10 min, followed by filtration of the supernatant through a pre-combusted GF/F filter. Experimental conditions are shown in Table 1. The reaction chambers were set up in a water bath and covered with a shutter to simulate the dark conditions occurring in the bottom of the reservoir. To create a completely mixed system and to avoid stratification in the overlying water in the reference chamber (control), either stirring (50 rpm, Type 1 simulation) or aeration (Type 2 simulation) was Fig. 1. Schematic of experimental apparatus. Table 1. Experimental conditions for sediment resuspension simulations. Simulation type Description Stirring intensity (rpm) Stirring time Exp. temp. Suspension Reference (min) (°C) 1) chamber chamber Type 1 Duplicate spin 500 2) 50 3...


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