ORCID ID

https://orcid.org/0000-0001-5496-433X

Date Awarded

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Carl T Friedrichs

Committee Member

Lawrence P Sanford

Committee Member

Kenneth A Moore

Committee Member

Grace M Massey

Committee Member

Courtney K Harris

Abstract

Understanding the nature of suspended particles is crucial to explaining water clarity issues in many estuaries, including the Chesapeake Bay and its tidal tributaries. Typical near surface estuarine particles are not individual sediment grains, but rather are clusters of inorganic and organic components known as flocs. Because of their fragile nature, flocs are challenging to observe in-situ, so their influence on the optical properties of the system are not well-known. This dissertation used a combination of state-of-the-art optical instrumentation, including laser scattering and transmissometry, a high-definition particle imaging camera system (PICS), and irradiance meters, along with supporting laboratory analysis techniques to investigate the surface waters of the York River estuary. This work characterized estuarine floc properties while simultaneously identifying relationships between estuarine light attenuation, absorption, and scattering due to flocs as well as other water column constituents. The relative organic fraction of suspended solids was found to be an important control on the fractal nature of estuarine flocs, including primary particle size and density, as well as bulk floc properties. A new approach is presented here that simultaneously solves for multiple floc fractal characteristics (e.g., fractal dimension, primary particle size, and primary particle density) and identifies whether simple fractal models are appropriate to describe individual suspensions. Results indicate that suspensions in the York River estuary with lower organic fraction and higher total suspended solids (TSS) are dominated by larger flocs composed of smaller, denser primary particles. In contrast, suspensions with higher organic fraction and lower TSS are composed of smaller flocs with larger, less dense primary particles. Paradoxically, the organic-rich flocs containing larger, lower density primary particles are, in terms of solids content, actually denser overall. This is because the larger, organic-rich primary particles take up more space within the flocs, leaving less room for water. Diffuse light attenuation, scattering, and absorption were related to the nature of the flocs in the York estuary, as well as to other water column constituents. It was found that as TSS increases, larger, lower density flocs containing less organic matter and more water increasingly dominate. This causes scattering to increase more quickly than TSS. In contrast, absorption increased more slowly than TSS. This is because the organics more prevalent at low TSS absorb more light per mass than the inorganic solids that dominate suspensions with higher TSS. Under most conditions, total scattering was dominated by inorganic particles. However, the combined effects of other components (the water itself, colored dissolved organic matter, phytoplankton, plus non-algal organic solids) typically dominated both absorption and attenuation. The importance of phytoplankton and organic solids relative to inorganic solids from land runoff have important ramifications for water clarity management, specifically suggesting revaluation of strategies solely focused on reducing inorganic sediment input. Even with an advanced video-settling column (e.g. PICS), there are issues resolving smaller flocs and sampling very low TSS. A major challenge in processing particle images is correctly identifying and sizing particles of varying composition and size, while correctly separating in-focus particles from out-of-focus particles. A new automated analysis approach was created that efficiently resolves particles, while rejecting out-of-focus objects, and was implemented into the automated processing algorithm for the PICS. Field- and laboratory-based experiments were conducted to evaluate video-based size, settling velocity, and density estimates, and it was found that all three parameters were adequately measured with the PICS.

DOI

http://dx.doi.org/10.25773/v5-nn75-t992

Rights

© The Author

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