Observations of nepheloid layers in Kin Bay, Okinawa Island

概要

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Spatial and time-series distributions of nepheloid layers (layers of suspended sediment) were investigated in Kin Bay of the Okinawa Island, using light-scattering type nephelometers and moored observation stations equipped with current meters and other oceanographic recorders. Spatially, three types of nepheloid layers were observed in the bay: (1) frontal nepheloid layers at the estuary of the inflowing rivers; (2) bottom nepheloid layers formed by resuspension of bottom sediments; and (3) intermediate nepheloid layers in the thermoclinal intermediate water columns of the offshore waters. Cross profiles of temperature, salinity, and turbidity in the frontal nepheloid layers suggest that inflowing suspended reddish soils are depositing massively at the frontal estuary region. The formation of thermoclines was observed in rather offshore region where colder offshore deep waters may enter and mix with warmer coastal waters. The formation of thermoclines in the intermediate water colums was also seasonal and influenced the formation of intermediate nepheloid layers which may be formed due to the slowing of settling velocity of sinking particles.The turbidity in the bay increases with the increased discharge of river waters which are also influenced by precipitation. The turbidity was increased by the tidal resuspension processes which occurred usually at the turning phase from high (flood) tide to low (ebb) tide.
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