The properties of particles in surface waters are especially important for the fate of incident light, with direct ramifications for primary production, habitat and water quality, and optical remote sensing. The rate of absorption and scattering of light depends directly on the particle size, composition, and density (Bowers et al., 2011) and indirectly on the particle settling rate, which controls how long and how high in the water column particles remain in suspension (Friedrichs et al., 2008). In the York River estuary and the adjacent Chesapeake Bay, the period of 1985-2016 was marked by a significant long-term decrease in water clarity as measured by the Secchi depth (Murphy et al., 2019). Gallegos et al. (2011) showed via optical modeling that the systematic decrease in Secchi depth in the Chesapeake Bay since the 1980s was likely due to an increase in the abundance of small, organic-rich suspended particles in the estuarine surface waters. The study of the York River estuary presented in the present paper was motivated in large part by a need to better understand the effects of organic matter content on the properties of particles in surface water due to the key role of organic-rich suspensions in affecting the water clarity in estuarine environments.

Coastal sediments are an important site for transient and long-term mercury (Hg) storage and they foster a geochemical environment optimal for Hg methylation. Therefore, efforts have been taken to constrain the role of sediments as a source of methylmercury (MeHg) to the estuarine water column. This study employed the Gust Microcosm Erosion Core system capable of quantifying particle removal from undisturbed cores under measurable shear stress conditions to assess particulate Hg and MeHg exchange between sediments and the water column. Samples were collected from organic rich and organic poor sediment types from the mid- and lower- Delaware Bay. It was found that bulk sediment samples from organic rich systems overpredict total Hg and MeHg release to the water column whereas organic poor sediments under-predict the exchange. In general, organic rich sediments in shallow environments have the most impact on surface particle dynamics. There is little evidence to suggest that MeHg formed in the sediments is released to the water column via particulate exchange, and therefore non-sedimentary sources likely control MeHg levels in this estuarine water column.

In the classical theory of the Secchi disk depth, diffuse sunlight falling on the disk is reflected back to the observer's eye along the most direct route, as a beam. The disappearance depth, Z SD , of the disk is then expected to vary inversely with the sum of the beam and diffuse attenuation coefficients: c + K D. Observations presented here show that, in the most turbid waters sampled, the Secchi disk is visible at greater depths (by a factor of up to 4) than predicted by this theory. In these conditions, the disk appears blurry, and it seems likely that some of the light reflected by the disk returns to the eye as diffuse light, photons being scattered one or more times on their journey from the disk surface to the observer. We have modified the theory of the Secchi disk in turbid water to allow for a mixture of beamed and diffuse light contributing to disk visibility. The modified theory corrects the underestimate of Secchi depths in turbid waters and gives good agreement with observations over a wide range of turbidity. The insight gained allows a more informed interpretation of Secchi disk measurements in turbid water.

Aggregation state significantly influences the size, density, and transport characteristics of fine sediment. Understanding sediment transport and deposition processes in the nation's navigable waterways is a primary mission for the US Army Corps of Engineers (USACE), particularly when it comes to infilling of navigation channels. In this study, a newly developed camera system was used to evaluate the aggregation state of eroded sediment from cores collected in the tidal James River, VA. Results showed that bed sediments were composed mostly of mud, but that erosion predominately occurred in the form of aggregates with median sizes 50-270 times larger than the disaggregated sediment. Aggregate size weakly correlated to shear stress at levels <2 Pa, as well as sand content and bed density. A numerical simulation demonstrated that mud aggregates were predicted to transport in incipient suspension or bedload, while disaggregated fines were predominately maintained in full suspension. This difference in transport mode has significant implication for channel infilling and sediment transport within the system.

As the oyster aquaculture industry grows and becomes incorporated into management practices, it is important to understand its effects on local environments. This study investigated how water quality and hydrodynamics varied among farms as well as inside versus outside the extent of caged grow-out areas located in southern Chesapeake Bay. Current speed and water quality variables (chlorophyll-a fluorescence, turbidity, and dissolved oxygen) were measured along multiple transects within and adjacent to four oyster farms during two seasons. At the scale of individual aquaculture sites, we were able to detect statistically significant differences in current speed and water quality variables between the areas inside and outside the farms. However, the magnitudes of the water quality differences were minor. Differences between sites and between seasons for water quality variables were typically an order of magnitude greater than those observed within each site (i.e. inside and outside the farm footprint). The relatively small effect of the presence of oysters on water quality is likely attributable to a combination of high background variability, relatively high flushing rates, relatively low oyster density, and small farm footprints. Minimal impacts overall suggest that low-density oyster farms located in adequately-flushed areas are unlikely to negatively impact local water quality.