Sediment trap study in the Green Bay mass balance program : mass and organic carbon fluxes, resuspension, and particle settling velocities
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Sediment trap study in the Green Bay mass balance program : mass and organic carbon fluxes, resuspension, and particle settling velocities

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    This research is a part of a large, multidisciplinary program designed to measure and model the mass balance of congener-specific polychlorinated biphenyls (PCB) and dieldrin in Green Bay, western Lake Michigan. In this report, we document the results of our sediment trap study designed to collect representative samples of settling particulate material from five sites within the southern portion of the bay. Measuring the mass collected allowed us to calculate the gross downward flux of particulate matter and particle settling velocities. The mass balance models being applied to Green Bay explicitly require these particle settling velocities and vertical fluxes of mass and particulate organic carbon. Mass and carbon fluxes from sediment traps located 2 m above bottom and distributed throughout southern Green Bay showed that seasonal flux patterns were generally high prior to stratification, declined to minimum values during summer, and then generally reached much higher fluxes during fall overturn. In the epilimnion, seasonal patterns are similar to the near bottom samples although mass flux is approximately 10% of the near bottom flux, and carbon flux is approximately 20%. Settling velocities for epilimnetic samples are approximately 0.5 m/day, similar to open-lake values. Significantly higher settling velocities (4-6 m/day) during the stratified period were calculated for the 2 m above bottom region. These rates imply that a large recharging of the particle pool by either horizontal transport or local sediment resuspension occurs throughout the year. During the unstratified period, settling velocities throughout the water column are approximately 12-18 m/day, more than an order of magnitude higher than during stratification. At this rate, the particle residence time in the water column is only a few days, again implying frequent recharging. Sediment resuspension estimated by a steady state model required to support trap observations is about 10 g/m2/day with scale thicknesses of 5-7 m, and there is little seasonal variation until late September.
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