Water and fine sediment dynamics in transient river plumes in a small, reef-fringed bay, Guam |
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Authors: | Eric Wolanski Robert H Richmond Gerald Davis Victor Bonito |
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Institution: | a Australian Institute of Marine Science, PMB No. 3, Townsville MC, Qld. 4810, Australia;b Marine Laboratory, University of Guam, UOG Station, Mangilao 96923, Guam;c Division of Aquatic and Wildlife Resources, Department of Agriculture, 192 Dairy Road, Mangilao 96923, Guam |
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Abstract: | Fouha Bay is a 400-m-long funnel-shaped, 10-m-deep, coral-fringed embayment on the southwest coast of Guam. It drains a small catchment area (5 km2) of steeply sloping, highly erodible lateritic soils. River floods are short-lived and the sediment load is very large, with suspended sediment concentration (SSC) exceeding 1000 mg l−1. The resulting river plume is about 1 m thick and is pulsing in a series of 1–2 h-long events, with outflow velocity peaking at 0.05 m s−1. Turbulent entrainment results in an oceanic inflow at depth into the bay. As soon as river flow stops, the plume floats passively and takes 5 days to be flushed out of Fouha Bay. The suspended fine sediment flocculates in 5 min and aggregates on ambient transparent exopolymer particles to form muddy marine snow flocs. In calm weather, about 75% of the riverine mud settles out of the river plume into the underlying oceanic water where it forms a transient nepheloid layer. This mud ultimately settles and is trapped in Fouha Bay. Under typhoon-driven, swell waves, the surface plume is at least 7 m thick and bottom entrainment of mud results in SSC exceeding 1000 mg l−1 for several days. It is suggested that successful management of fringing coral reefs adjacent to volcanic islands may not be possible without proper land use management in the surrounding catchment. |
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Keywords: | river plume fine sediment flocculation sedimentation muddy marine snow coral guam |
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