Abstract
AbstractTo better understand the nature of flocs of varying organic content in estuarine surface waters, Laser in situ Scattering and Transmissometry, video settling, and pump sampling were deployed in the York River estuary. A new in situ method was developed to simultaneously solve the floc fractal dimension (F), primary particle size (dp), and primary particle density (ρp) by fitting a simple fractal model to observations of effective floc density (Δρ) as a function of floc diameter (df), while ensuring that the integrated particle size distribution was consistent with measurements of bulk apparent density (ρa). When fractal fits were statistically justified, application of the above methods showed the bulk fraction of organic matter (forg) to be well correlated to multiple floc properties. As forg increased, dp and ρa also increased, while ρp, total suspended solids (TSS), and median floc size decreased. Notably for microflocs, neither F nor Δρ was significantly related to either forg or TSS. This indicates that organic matter may partially displace water content within microflocs without fundamentally changing the flocs' inorganic structure. When pooling multiple samples, a marked decrease in F was seen at the transition to macroflocs, and most strongly for high forg cases. This suggested that settling velocities ≥ ∼1 mm/s may produce turbulent stresses that tend to tear macroflocs apart. This study also found that when the fractal theory held, ρp had a near 1:1 correlation with the bulk dry density of filtered TSS, implying that primary particles are tightly bound aggregates of combined mineral and organic components.
Highlights
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 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
Observations of particle properties were collected from surface waters of the York River estuary (Figure 1) using the Virginia Institute of Marine Science (VIMS) Coastal Hydrodynamics & Sediment Dynamics (CHSD) lab's water column profiler (Figure 2)
Summary
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. 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.
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