Date of Award
Master of Science (MS)
Richard P. Hale
Margaret R. Mulholland
After years of efforts to restore the Chesapeake Bay, bacterial levels are down and species diversity has increased, however, algal blooms (primarily dinoflagellates) persist, occurring nearly every summer. Dinoflagellates produce resting cysts that accumulate in the bottom sediments and are thought to provide seed populations for future algal blooms when they are resuspended. When estuarine sediments are advected from a bed, other materials, such as pollutants, nutrients, and organic matter are also released into the water column. Thus, resuspended sediments can contribute to the degradation of water quality, habitat, and aquatic life, and impart negative impacts on local ecosystems and economies. To investigate the causes of sediment resuspension in a shallow, tidal system and the potential role of sediment resuspension on algal production, time-series measurements of current velocity, wave height, and suspended sediment concentrations were recorded using acoustic, optical, and pressure sensors, in conjunction with a temporal and spatial survey of conductivity, temperature, suspended sediment concentration profiles, benthic sediment samples, and water samples. Regional meteorological data including hourly wind speed and direction and precipitation totals were also compiled for comparison with sediment resuspension, chlorophyll a (Chl a) concentrations and dissolved nutrient concentrations. Sediment resuspension in estuaries typically results from wind-driven waves, tidal currents, or wind-driven currents. Results from this study found maximum wave orbital velocities (Ubm) to be an order of magnitude less than current velocities (Uc), however, periods of elevated Ubm, were associated with the majority of observed resuspension events. Despite surface gravity waves primarily causing resuspension, currents (tidal and wind-driven), as well as water depth, appeared to mitigate or even negate wave induced resuspension. Overall, resuspension most often resulted when Ubm >~2 cm/s, coinciding with southwesterly winds ≥ 5 m/s, during periods of relatively weak current speeds and water depth. Observation of increased nutrient concentrations and/or Chl a concentrations followed numerous resuspension events, suggesting that resuspension likely aided in the growth of algal blooms observed in the Lafayette River. The link between sediment resuspension, elevated nutrient and Chl a concentrations, was supported via observations of elevated near-bed concentrations of ammonium and nitrate/nitrite concentrations. Nutrient concentrations in bottom waters then declined as Chl a concentrations increased. The timing of this sequence of events (2-7 days) was on the order observed previously. This study suggests that sediment resuspension may be an import factor for stimulating algal production in shallow, eutrophic, microtidal estuaries.
McGill, Samantha C..
"Sediment Resuspension in a Microtidal Estuary: Causative Forces and Links with Algal Blooms"
(2019). Master of Science (MS), Thesis, Ocean/Earth/Atmos Sciences, Old Dominion University, DOI: 10.25777/1ffs-hr31