Understanding the Ways of the Water

Anthropogenic modifications to estuarine topography can alter hydrodynamics and affect basic processes like circulation and salinity distribution. Over the past century, the Mobile Bay ship channel has been extensively deepened and widened to accommodate larger vessels, transforming it into a pronounced channel-shoal estuary. To investigate the impact of these changes on the estuary's hydrodynamics, unstructured 3-D hydrodynamic models were developed for the system under both historical 'mild-modified' and contemporary 'severe-modified' conditions. A recent one-year period covering a wide range of forcing conditions was used to compare and contrast the hydrodynamic behavior of the two systems. Results show that the modifications have significantly altered tidal and non-tidal dynamics, circulation, and both horizontal and vertical salinity gradients. Changes in tidal constituent amplitudes and phases have modified tidal currents. Increased non-tidal circulation and associated salt flux have elevated salinity by up to 10 PSU in the upper channel, extending salinity intrusion by nearly 10 kilometers. The salt intrusion was more responsive to discharge variations in historical conditions compared to contemporary conditions. In contemporary conditions, the salt front position was found to be least variable, making salt removal more challenging. Additionally, increased salinity in the channel under modern conditions has intensified lateral salinity gradients due to differential advection between channel and shoals, enhancing lateral baroclinic circulation. Overall, the shift in Mobile Bay's hydrodynamics due to channel deepening and widening has been significant, underscoring the profound effects of extensive channel modifications on estuarine dynamics.

Understanding the impacts of river diversion operations is crucial for natural resource management. The Bonnet Carré Spillway (BCS), constructed in 1931 to prevent levee failure and flooding in New Orleans by diverting up to 250,000 cubic feet per second (cfs) of Mississippi River waters into Lake Pontchartrain, ensures the river's flow at New Orleans does not exceed 1.25 million cfs.
The Coupled Ocean Atmosphere Wave Sediment Transport based modeling system (msbCOAWST) developed by the University of Southern Mississippi (USM) is employed to hindcast BCS opening events and simulate various scenarios to investigate the operational impacts of the 2019 BCS openings on flood management and water quality in the Mississippi Sound and Bight. Using a twin experiment methodology, a pairing of hindcast simulations illuminates how an experiment with modified forcing compares with a run with observed BCS operation. The study examines the impacts of applying various operational scenarios such as: without the second opening; shorter second opening; and reducing river discharge to 1.2 million cfs and 1.25 million cfs at Baton Rouge. Additionally, different opening and closing paces, such as 10 bays per day during opening and closing, are investigated. Through these twin experiments, the effects of varied BCS operational strategies are explored.
Preliminary results indicate significant impacts on salinity levels in the Mississippi Sound and Bight, with freshwater influx affecting both surface and bottom layers. Salinity levels began to recover one-month post-closing, but Hurricane Barry caused substantial mixing of the water column. Regional model results closely aligned with field measurements, enhancing the reliability of the findings. This study provides insights for optimizing spillway operations and enhancing flood management strategies, potentially minimizing the impacts for BCS operations while reducing river discharge at Baton Rouge. Engaging with local stakeholders and policymakers will enhance the practical relevance and application of the findings.

In the spring and summer of 2019, the Bonnet Carré Spillway was opened twice,
leading to an unprecedented diversion of river discharge into the Mississippi Bight, Gulf of
Mexico. This flood event raised concerns about its potential impacts on regional water quality
and coastal dynamics. This study investigates the dynamics of dissolved oxygen and their
connection to physical processes associated with the flood, drawing on data from shelf
hydrographic surveys, a mooring site, satellite observations, and reanalysis data collected during
the event. Prolonged periods of hypoxia were detected on the shelf throughout the summer,
showing considerable variability in both spatial and temporal scales. To identify the key drivers
of this hypoxic phenomenon, analyses of river discharge from the Mississippi and local rivers,
wind patterns, stratification, and chlorophyll concentrations were conducted. The findings
revealed a large cumulative volume of Mississippi River discharge in the summer of 2019,
higher than in previous years, which brought more freshwater and enhanced salinity stratification
at the study site. Additionally, anomalous southwest winds during June and high chlorophyll
patches throughout the summer were detected. The timing of the southwest wind coincided with
a large discharge from the Mississippi River suggesting enhanced eastward circulation bringing
more nutrients and increasing productivity, which could have contributed to the large,
persistent area of hypoxia in the Mississippi Bight. Further investigation into the position of
Loop Current frontal eddies and their influence on Mississippi River discharge in the study area
is needed. This study highlights the crucial influence of physical processes in the development of
hypoxic conditions during major flood events.

Oxygen concentration is an important indicator of water quality status in marine ecosystems. The dissolved oxygen in the northern Gulf of Mexico is fundamentally connected to Mississippi River discharge, as an established driver of hypoxia. However, the impacts this river discharge on the dissolved oxygen dynamics in the Mississippi Bight are uncertain. Regional dissolved oxygen monitoring efforts involving a combination of data from hydrographic surveys and a mooring on the shelf, were used to identify and track the evolution of a large-scale hypoxic event in the western Mississippi Bight during the summer of 2024. Beginning in mid-July and lasting through the end of August, large areas of the shelf experienced near-anoxic (less than 0.5 mg/l) to near-hypoxic (less than 3.0 mg/l) conditions. These regions were generally west of Mobile Bay with shallower sites in Mississippi waters typically experiencing the lowest dissolved oxygen values. The large-scale, persistent nature of this event was surprising as May discharge from the Mississippi River, a primarily forecast indicator, suggested hypoxia in the northern Gulf would be below average. The lack of consistency with this forecast indicator suggestions that an understanding of forcing mechanisms beyond river discharge and the associated nutrient levels are required to fully understand the frequency, areal extent, and duration of hypoxic east of the Mississippi River. This work highlights the continued need for shelf monitoring of dissolved oxygen to better understand hypoxic patterns and dynamics in the Mississippi Bight

The University of Southern Mississippi (USM) modeling group has developed an annual hindcast, a daily hindcast and a short term (48 hour) forecast of the Mississippi Sound and Bight region. This 400 m resolution, 24-layer circulation model, called mcbCOAWST, is based on a regional Coupled Ocean Atmosphere Wave Sediment Transport modeling system (COAWST) application established during the GoMRI-funded CONCORDE consortium. This numerical model of water quality and hydrodynamic conditions in the Mississippi Sound (MSS) and Bight, is being integrated with in situ sampling, laboratory and field-based investigations, and remote sensing observations to provide reliable science-based information for regional restoration planning and coastal protection programs.
The MSS and Bight is a complex coastal marine system that experiences many challenges and disruptions including chemical spills, major hurricanes, freshwater influx events, low oxygen events, and harmful algal blooms. Long term ocean measurements in the Northern Gulf of Mexico, necessary to assess habitat sustainability and climate change response in the coastal environment, are rarely available and sparsely located, forcing scientists to rely on model data. Many global and continental ocean models which are historically available are low resolution and do not capture the complicated dynamics of the coastal ocean. The high resolution msbCOAWST model has temporally continuous model hindcasts (a combination of annual and daily hindcasts) with hourly resolution that extend from April 2015 onward. This data is available to the research and resource management communities through Coastal CUBEnet (http://oceancube.usm.edu/) and the USM THREDDS server. This presentation will focus on the utility of the model and the ways in which it can provide long-term, science-based guidance and be used to explore interannual and seasonal variability of water quality and environmental conditions that impact habitat suitability in the Mississippi Sound.

This study presents a comprehensive analysis of compound flooding events along the coastal regions of the Gulf of Mexico, focusing on vulnerable areas under Mississippi. We utilize hourly storm surge data from long-term tide gauges and corresponding precipitation records to investigate the joint occurrence and dependencies of these phenomena. Advanced statistical techniques, including Kendall's rank correlation coefficient, copula theory, and Bayesian Networks, are employed to capture the joint probability distributions and tail dependencies of extreme events. This approach allows us to project future maximum precipitation and storm surge events for different return periods, accounting for observed trends and potential climate change scenarios. Temporal variations in these dependencies are examined using multi-year moving windows, revealing long-term trends that may indicate climate change impacts. The influence of tropical and extratropical cyclones on compound flooding is also analyzed through examination of synoptic weather patterns, providing insights into potential future risks. For Mississippi's vulnerable areas, the study integrates statistical analysis with hydrological modeling, using GSSHA, to simulate coastal dynamics and inland hydrology. This enables high-resolution simulation of various hurricane scenarios, incorporating projected changes in storm intensity and frequency. The findings provide a detailed understanding of potential inundation scenarios to inform concerned authorities on mitigating the impacts of compound flooding in a changing climate.

Microplastic pollution is a rising environmental issue, particularly in aquatic environments and ecosystems. This study aims to identify and characterize microplastics (MPs) in wastewater from different wastewater treatment plants (WWTPs) located in Alabama (AL), Florida (FL), and Mississippi (MS), within the Gulf Coast Region. These WWTPs differ in location and treatment technology, providing a unique opportunity to study how MPs respond to varying treatment processes and other conditions. Fourier Transform Infrared (FTIR) spectroscopy was employed to analyze and identify the types of MPs present in both influent and effluent samples. Preliminary analysis of the samples shows significant variations in MPs concentrations. In Dauphin Island, AL sample, the influent contained approximately 7533.3 particles/m3, while the effluent contained 3,800 particles/m3, indicating a partial reduction during treatment. At the Cantonment Waste Reclamation Facility in FL, the influent contained 3800 particles/m3, with a more substantial reduction to 216 particles/m3 in the effluent. Analysis of the Jackson County, MS, samples are still in progress. These findings suggest that while WWTPs in the Gulf Coast region reduce the load of MPs, a significant proportion of these pollutants still make their way into aquatic environments through treated effluent. The differences in treatment technologies and locations highlight the need for tailored approaches to improve MP removal. Continued monitoring and analysis are crucial for developing strategies to mitigate the environmental impact of MPs in the Gulf Coast region.