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Abstract Most work on how estuarine dynamics impact dissolved oxygen (DO) distributions has focused on tides, but in shallow estuaries with large fetch or small tides, wind can be the primary mixing agent and also drives advection. To investigate how these processes affect DO distributions, an observational study was conducted in the shallow, microtidal Neuse Estuary. Salinity, DO, and velocity profiles were measured at multiple positions along and across the estuary over a 6‐month period. A one‐dimensional model (General Ocean Turbulence Model) provided additional insight into the response of salinity and DO to wind. Salinity and oxygen conservation equation terms were calculated from observations and simulations. Cross‐estuary wind drove lateral circulation and tilted the isohalines, reducing stratification; lateral advection and enhanced mixing reduced vertical gradients and increased the bottom DO. Down‐estuary wind tended to increase the exchange flow and stratification, but concurrently the surface wind‐mixed layer deepened over time. The balance of these processes determined if the water column became fully mixed or remained stratified, and the depth of the pycnocline and oxycline. An expression for steady state surface layer thickness was derived by considering the competition between the horizontal and vertical buoyancy flux, and the predictions agreed well with observations and simulations. Up‐estuary wind inhibited the exchange flow and the combination of advection and mixing homogenized the water column. While these patterns generally held for purely across‐ or along‐channel wind, the response was often more complex as the wind vector varied in orientation and with time.
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Unprecedented summer hypoxia in southern Cape Cod Bay: an ecological response to regional climate change?
Abstract. In late summer 2019 and 2020 bottom waters in southern Cape Cod Bay (CCB) became depleted of dissolved oxygen (DO), with documented benthicmortality in both years. Hypoxic conditions formed in relatively shallow water where the strong seasonal thermocline intersected the sea floor, bothlimiting vertical mixing and concentrating biological oxygen demand (BOD) over a very thin bottom boundary layer. In both 2019 and 2020, anomalouslyhigh sub-surface phytoplankton blooms were observed, and the biomass from these blooms provided the fuel to deplete sub-pycnocline waters of DO. Theincreased chlorophyll fluorescence was accompanied by a corresponding decrease in sub-pycnocline nutrients, suggesting that prior to 2019 physicalconditions were unfavorable for the utilization of these deep nutrients by the late-summer phytoplankton community. It is hypothesized thatsignificant alteration of physical conditions in CCB during late summer, which is the result of regional climate change, has favored the recentincrease in sub-surface phytoplankton production. These changes include rapidly warming waters and significant shifts in summer wind direction, bothof which impact the intensity and vertical distribution of thermal stratification and vertical mixing within the water column. These changes inwater column structure are not only more susceptible to hypoxia but also have significant implications for phytoplankton dynamics, potentiallyallowing for intense late-summer blooms of Karenia mikimotoi, a species new to the area. K. mikimotoi had not been detected in CCBor adjacent waters prior to 2017; however, increasing cell densities have been reported in subsequent years, consistent with a rapidly changingecosystem.
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- Award ID(s):
- 2053240
- PAR ID:
- 10388335
- Date Published:
- Journal Name:
- Biogeosciences
- Volume:
- 19
- Issue:
- 14
- ISSN:
- 1726-4189
- Page Range / eLocation ID:
- 3523 to 3536
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.5194/bg-19-3523-2022
