ORCID ID

https://orcid.org/0000-0003-3392-0316

Date Awarded

Summer 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Marjorie A. M. Friedrichs

Committee Member

Courtney K. Harris

Committee Member

Grace M. Massey

Committee Member

Kenneth A. Moore

Committee Member

Lawrence P. Sanford

Abstract

Water clarity is a key indicator of the ecosystem health in the Chesapeake Bay. Estuarine water clarity fluctuates due to external inputs from the watershed as well as processes occurring within the estuary itself, such as sediment resuspension and organic matter production. Therefore, water clarity requires study at multiple spatial and temporal scales and with multiple metrics. One local-scale process potentially influencing water clarity is shellfish aquaculture. One part of this dissertation examined how water quality and hydrodynamics varied among oyster farms as well as inside versus outside the extent of caged areas located in southern Chesapeake Bay. Current speed and water quality were measured within and adjacent to four oyster farms during two seasons. Results revealed minor effects of oyster farms on water quality, likely due to 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. At a larger spatial scale, another potential influence on water clarity is shoreline erosion. The second part of this dissertation examined the impact of shoreline erosion on water clarity via a numerical modeling study. Experiments were conducted to simulate realistic shoreline conditions representative of the early 2000s, increased shoreline erosion, and highly armored shorelines. Together, reduced shoreline erosion and the corresponding low seabed resuspension resulted in decreased concentrations of inorganic particles in surface waters, improving water clarity overall. However, clearer waters relaxed light limitation on phytoplankton, which often increased organic matter production, sometimes yielding opposite effects on water clarity according to different metrics. Clarity improved in mid-Bay central channel waters in terms of light attenuation depth, but simultaneously degraded in terms of Secchi depth because the resulting increase in organic matter decreased the water’s transparency. A final water clarity process considered was the long-term trend in water clarity from satellite remote sensing. The third part of this dissertation examined how remote sensing reflectance changed over time in Chesapeake Bay from 2002 to 2020 using the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on satellite Aqua. MODIS-Aqua remote sensing reflectance trends were evaluated from 2002 to 2020 at multiple wavelengths and spatial resolutions for surface waters of the Chesapeake Bay. Trends showed long-term decreasing reflectancein the upper estuary yet increasing reflectance in the lower estuary in the green wavelengths. Band ratios involving red-to-green and red-to-blue have decreased, suggesting improved water clarity, while green-to-blue ratios have increased over time, suggesting increasing contribution of phytoplankton to water cloudiness. Reflectance change over time relates well to observed decreases in total suspended solids and light attenuation, yet inconclusive trends in chlorophyll-a, suggesting a long-term change in particle properties such as size and composition that affect light scattering behavior.

DOI

http://dx.doi.org/10.25773/v5-0ex4-0667

Rights

© The Author

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