Date Awarded
2023
Document Type
Thesis
Degree Name
Master of Science (M.Sc.)
Department
Virginia Institute of Marine Science
Advisor
Piero L.F. Mazzini
Committee Member
Marjorie A.M. Friedrichs
Committee Member
Carl T Friedrichs
Committee Member
Mark J Brush
Abstract
Extreme temperature events known as Marine Heatwaves (MHW), akin to atmospheric heatwaves, have only recently received attention by the estuarine scientific community. Thus far, studies have focused solely on surface events due to scarcity of long-term subsurface data. This study investigates, for the first time, the subsurface temperature and dissolved oxygen (DO) anomalies associated with surface MHW events in a large, temperate, partially mixed estuary: the Chesapeake Bay (CB). Using over three decades (1986-2021) of in-situ data from several long-term monitoring programs in the CB (including sub daily moored measurements and monthly/bimonthly cruises along the main stem) and a global atmospheric reanalysis product (ERA5), I were able to 1) characterize the spatiotemporal structure of subsurface temperature and DO anomalies during surface MHW events on seasonal time scales, 2) identify the vertical extent of warming before and after events, and 3) examine the relative role of atmospheric heat flux in driving temperature changes during the onset and decline of events. I found that subsurface temperature anomalies associated with surface MHWs had two distinct regimes: a thermally stratified spring-summer regime, when positive temperature anomalies were only present in the upper water column capped by the surface mixed layer; and a thermally homogenous fall-winter regime, when temperature anomalies extended throughout the water column. This seasonal variability in temperature anomalies was largely consistent with a simple 1-D response to heat, sourced primarily through the air-estuary interface, and with downward heat transport and diffusion controlled by seasonally variable stratification and mixing. Moreover, DO anomalies during MHW events presented a more complex spatiotemporal structure, with notable DO decreases (1-4 mg L-1) primarily occurring in the winter and spring. While negative DO anomalies were present across the main stem of the CB, the greatest DO decreases (~5 mg L-1) were observed in the upper region of CB below the mixed layer depth. During the hypoxic season (May to September), and in April and October, negative DO anomalies were often associated with an expansion of the hypoxic zone. Additionally, I observed that subsurface temperature anomalies were elevated 5-10 days before and up to 20 days after MHW events, while surface temperature anomalies were elevated for up to 2 months before and after events. This indicates that the timescales of elevated temperatures are typically much longer than the duration of individual MHW events, and therefore should be carefully taken into consideration when assessing the impact of these extreme events in the estuarine ecosystem. Using a simple 1D surface mixed layer heat budget, I identified air-estuary heat flux as the largest driver of the onset and decline of MHW events, with latent heat flux being the dominant constituent. In the CB, concurrent low DO during MHW events and persistent high temperatures before, during, and after events can compound the impacts of MHWs on habitat, which will be further exacerbated by climate change, severely impacting this valuable estuarine ecosystem.
DOI
https://dx.doi.org/10.25773/v5-r4jt-nf10
Rights
© The Author
Recommended Citation
Shunk, Nathan P., "Subsurface Structure And Impacts Of Marine Heatwaves In The Chesapeake Bay" (2023). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1697552678.
https://dx.doi.org/10.25773/v5-r4jt-nf10
