Introduction

Estuarine Turbidity Maxima (ETMs) are zones of elevated suspended sediment concentration and reduced light availability typically found near the limit of salt intrusion in the upper reaches of estuaries. ETMs often are characterized by sharp gradients in physical, biological, geological, and chemical properties, as freshwater rivers with their suspended sediments, plankton, and nutrients transform into brackish, tidal estuaries. Trapping of riverine sediment and detritus in ETMs is thought to contribute to the maintenance of unique and important zooplankton and fish populations1-5 and to promote rapid rates of sedimentation6 and reduced phytoplankton productivity.7

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Flocculation has long been recognized as an important contributing factor to ETM particle trapping8-11 through associated increases in settling velocity. However, flocculation in early ETM studies was largely interpreted as a one-time transition between small riverine particles and large estuarine flocs bound together by electrochemical attractive forces,12 a view now recognized as incomplete.

The ETM of the Chesapeake Bay, USA was first studied in the late 1960s and 1970s. The most extensive work was carried out by Schubel and coworkers,13-16 followed by the work of Zabawa8 and Gibbs17 on the behavior of suspended particles and agglomerates. The results of these studies indicated several consistent characteristics of particles in the Chesapeake ETM. Particle populations were divided into two groups: large flocs (or aggregates or agglomerates, depending on individual preference) between 20 and 250 ^m in size settling at rates between 0.1 and 1 mm sec-1, and smaller background particles consisting of primary particles or very small flocs. This distinction was operational with the large flocs making up the pool of tidally resuspended and deposited bottom sediments and the background particles remaining essentially unchanged over the tidal cycle. Zooplankton fecal pellets were identified as an important component of the large agglomerate population. Note that these studies necessarily relied on a limited number of point samples obtained with sampling techniques that may have disrupted weaker flocs.

Subsequent work carried out by some of the present authors in upper Chesapeake Bay confirmed the apparent two-component nature of the particle population,6,18-21 though settling velocities measured with bottom withdrawal settling tubes were higher than the 1 mm sec-1 upper limit observed previously. Recently, settling speeds of the large flocs were observed to vary seasonally, with maxima at the end of the summer and minima in winter. Models of tidal resuspension and deposition assuming that the two particle populations were independent and that the settling speed of the large flocs was constant over tidal cycles compared quite reasonably to observations. However, these studies were focused on sediment transport patterns and parameterizations and did not attempt to distinguish details of the size and settling velocity distributions of the particles.

The research reported here was carried out during a large, interdisciplinary investigation of the coupled physical and biological dynamics of the Chesapeake Bay ETM ("BITMAX," Bio-physical Interactions in the Turbidity MAXimum). This research was carried out during seasonal cruises to the upper Bay in 2001 and 2002, employing a new suite of instrumentation that for the first time allowed a detailed, simultaneous examination of the changing sizes and settling speeds of particles in this region, in relation to physical structure and dynamics. We concentrate here on data collected during 2002, primarily because our methods were better developed in the second year of the study.

Though obviously of interest for local, Chesapeake Bay research, this study also explores environmental conditions that have not received a great deal of attention in the marine and estuarine flocculation literature. Eutrophic estuarine transition zones with moderate physical energy, moderate turbidity, and moderate biological activity are, in a sense, midway between the oceanic, oligotrophic environments studied by Alldredge, Jackson, and coworkers,22-27 the coastal and continental shelf environments studied by Hill, Milligan, and colleagues,28-30 and the highly energetic, turbid estuaries studied by Dyer, van Leussen, and colleagues.31-34 Interested readers are referred to work in similar estuarine environments to this study by Kranck and Milligan35 and Fugate and Friedrichs,36 among others.

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