Hypoxia in a Coastal Embayment of the Chesapeake Bay: A Model Diagnostic Study of Oxygen Dynamics

Abstract

Two distinct hypoxic patterns were revealed from high-frequency dissolved oxygen (DO) data collected from North Branch of Onancock Creek, a shallow coastal estuary of the Chesapeake Bay, from July to October 2004. Diurnal hypoxia developed associated with large DO swings during fair weather and hypoxia/anoxia developed for prolonged 2-5-day periods following rainfall events. A simplified diagnostic DO-algae model was used to investigate DO dynamics in the creek. The model results show that the modeling approach enables important features of the DO dynamics in the creek to be captured and analyzed. Large anthropogenic inputs of nutrients to the creek stimulated macroalgae blooms in the embayment. High DO production resulted in supersaturated DO in daytime, whereas DO was depleted at night as the high respiration overwhelmed the DO supply, leading to hypoxia. Unlike deep-water environments, in this shallow-water system, biological processes dominate DO variations. High macroalgae biomass interacting with low light and high temperature trigger the development of prolonged hypoxic/anoxic postrainfall events.

Temporal responses of coastal hypoxia to nutrient loading and physical controls

“…In contrast, when shallow, clearwater coastal systems (e.g., lagoons) receive substantial inputs of inorganic nutrients, photosynthetic production (often dominated by benthic plants) represents an important O 2 source, leading to diel-scale cycling between supersaturated O 2 concentrations during the day and hypoxic conditions at night (e.g., MacPherson et al, 2007;Tyler et al, 2009). Although diel hypoxia is generally confined to the warmer summer months, its occurrence and intensity tends to vary on daily-to-weekly time-scales associated with periodic fluctuations in sunlight and tides, as well as rain and wind events (e.g., Shen et al, 2008). There are surprisingly few reports of diel-scale hypoxia in the scientific literature; however, recent evidence suggests that this phenomenon is widespread in shallow eutrophic waters (e.g., Wenner et al, 2004).…”

Metabolic Balance between Ecosystem Production and Consumption

“…For example, metabolic measurements in Chesapeake Bay were applied to estimate steady-state physical transport rates over an annual cycle, revealing seasonal shifts in the relative impor tance of benthic versus bottom plankton respiration as well as a strong linkage between respiration rates and net O 2 transport across the pycnocline and from seaward bottom layers . In these nutrient-rich shallow environ ments, rates of ecosystem metabolism tend to be very high and generally dominate the O 2 balance (e.g., Shen et al, 2008). Conversely, numerical models can be used to estimate rates of ecosystem metabolism needed to generate hypoxia under different hydrodynamic conditions (e.g., Borsuk et al, 2001;Hearn and Robson, 2001;Hetland and DiMarco, 2008).…”

Modeling dissolved oxygen dynamics and hypoxia

“…In addition, remineralization can be modeled as a function of temperature and O 2 (e.g. Shen et al, 2008). The oxygen dependence can be modeled as a linear function (e.g.…”

“…D'Avanzo and Kremer, 1994). In a coastal embayment of the Chesapeake Bay, model simulations indicate that high primary production during daytime results in supersaturated O 2 levels, while at night respiration overwhelms the O 2 supply, often leading to hypoxia (Shen et al, 2008). Infrequent episodic oxygen depletion with less than one event per year seems to be the first indication that a system has reached a critical point of eutrophication, which, in combination with physical processes that restrict water exchange, can tip the system into hypoxia (Diaz and Rosenberg, 2008).…”