Particle dynamics of the surface, intermediate, and benthic nepheloid layers under contrasting conditions of summer monsoon and typhoon winds on the boundary between the Taiwan Strait and East China Sea

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Two shipboard surveys of hydrographics and sediment dynamics were conducted off the mouth of Minjiang (the Min River) in July 2012, under contrasting summer monsoon and typhoon wind conditions. Observations included profiles of conductivity, temperature, depth, fluorescence, and suspended-particle volume concentration and current velocity. In addition, water samples were taken for the analyses of mass concentration, POC, and chlorophyll of suspended sediment. Seafloor sediment samples were also taken for grain-size analysis. Nepheloid layers (NLs) were observed at surface, intermediate, and benthic depths (SNL, INL, BNL, respectively). The NL dynamics under the two wind conditions were mainly controlled by (1) the presence of the Minjiang river plume and terrigenous material it carries; (2) water-column stability affected by wind, current, and waves; (3) current-induced resuspension of seafloor sediment; and (4) physically coupled biological activities. The wind field was the major forcing controlling the observed flow field and the upper water column stability. In the typical summer condition, southerly winds induced offshore-directed currents, enhancing the seaward dispersal of the Minjiang river plume, facilitating water-column stratification, thereby forming 3 NLs in the water column. Phytoplankton growth was enhanced by terrigenous material and sunlight. The suspension of the seafloor sediment was the one of the major sources for suspended particles in these NLs. Furthermore, the C/N ratio of POC in the three NLs is 5.72, almost identical to the Redfield ratio, indicating their predominant marine origin. Conversely, under typhoon-related northerly winds, the offshore spread of the river plume was restricted. The reduced riverine input and enhanced mixing resulted in the disappearance of the SNL. Elevated turbidity in the INL was largely due to the presence of phytoplankton, while the BNL was comprised of sediment resuspended off the seafloor and from horizontal advection. Our results suggest that the presence of the river plume from the Minjiang at the study site sets the condition for the SNL dynamics. The dynamics of INL and BNL were dominated by the wind field, and to a lesser degree, the wave field and tidal current.