Citation:

Hagy, Jim, B. Jarvis, AND Rick Greene. Evaluating Aquatic Life Benefits of Reducing Nutrient Loading to Remediate Episodic and Diel Cycling Hypoxia in a Shallow Hypereutrophic Estuary. Restore Americas Estuaries National Summit on Coastal and Estuarine Restoration, New Orleans, LA, December 10 - 15, 2016.

Impact/Purpose:

This is an abstract for a conference poster presentation. The research was conducted to identify the potential aquatic life benefits of reducing nutrients to Weeks Bay as a restoration strategy that could be pursued under the Deepwater Horizon NRDA

Description:

Theoretical linkages between excess nutrient loading, nutrient-enhanced community metabolism (i.e., production and respiration), and hypoxia in estuaries are well-understood. In seasonally-stratified estuaries and coastal systems (e.g., Chesapeake Bay, northern Gulf of Mexico), hypoxia is predominantly seasonal, such that the spatial extent indicates potential aquatic life impacts. However, in relatively small and shallow Gulf of Mexico bays and bayous, hypoxia frequently occurs episodically or on a diel basis. This study utilized continuous DO monitoring and 3-D hydrodynamic (Environmental Fluid Dynamics Code) and water quality (Water Quality Analysis Simulation Program) models to examine physical and biological controls on DO dynamics and ecosystem metabolism in Weeks Bay, AL. Observed vertical DO gradients varied on a diel basis, with larger amplitude variations at depth relative to the surface, underscoring the importance of benthic production and respiration as a driver of ecosystem metabolism in shallow estuaries. Hydrodynamic and water quality models simulated seasonal and event-driven dynamics, but struggled to resolve the amplitude of daily DO fluctuations, particularly in bottom waters. Using these data in conjunction with the 10-year continuous O2 record from Weeks Bay, we applied empirical relationships and simple scaling relations to predict how reducing nutrient loading may affect the frequency, severity and duration of hypoxia. We further applied species sensitivity data to evaluate how the existing DO status may impact aquatic life, while recognizing the possible impacts of behavior on exposure to hypoxia, population dynamics, and trophic interactions. This analysis could be used as a screening tool to better understand nutrient effects via eutrophication and hypoxia, thereby informing development of numeric nutrient criteria. The analysis could also inform restoration of shallow coastal systems subject to complex patterns of eutrophication and hypoxia. The complexity of interactions between DO status, population dynamics, and trophic interactions highlight the ongoing need for direct monitoring of hypoxia effects to strengthen the scientific basis for management.

Record Details: