Projects

A circulation and transport model for fishery management in Mobile Bay and eastern Mississippi Sound

End Date: 2/1/17

In a two-pronged study, Kyeong Park of the University of South Alabama and Ruth Carmichael of Dauphin Island Sea Lab, will take a close look at water as it moves through Mobile Bay, the Mississippi Sound and the northern Gulf of Mexico to determine how it affects the movement of the larvae of oyster and similar shellfish through the area. First, the pair will create a hydrodynamic model of the Mississippi and Alabama coastal water using data for water level, current velocity, salinity and temperature, and then use this model to evaluate the flushing capacity for Mobile Bay. They will simultaneously conduct field sampling of oyster and other commercially important shellfish larvae in the area. All of the information will go toward developing a larval transport model to define the habitat and distributions of these species in their early life stages.

Abstract

All processes related to water quality and living resources require studies of both physical transport and biogeochemical processes to understand the fate and distribution of the related materials. Study of physical mass transport using hydrodynamic models, then, is an essential element in understanding and managing issues related to water quality and living resources. Currently we do not have a validated hydrodynamic model for the entire Alabama/Mississippi coastal waters, which is needed to inform adaptive and ecosystem based management of fishery and water quality. 

We propose to develop a hydrodynamic model for the region that includes the Mobile Bay/Mississippi Sound system and the Alabama/Mississippi inner shelf. We will use an existing model as a starting point, and will extend the modeling domain both westward and southward to include a greater portion of Mississippi Sound and the northern Gulf of Mexico. This hydrodynamic model will be validated using the field data collected in this study as well as those from the existing observational infrastructure. Then, as an example to demonstrate application of this model in the context of water quality, we will investigate the flushing capacity and the associated residence time and age of water for Mobile Bay. 

As another example to demonstrate application of this model in the context of fishery management, we will conduct a state-of-the-art field program for larvae of oyster and other commercially/ecologically important species with similar early life stages, improve an existing oyster larval transport model in terms of larval-specific active processes (e.g., growth and mortality), validate the model using data from this study, and examine the effects of the biological conditions on oyster larval distribution and settlement patterns. We, then, will assess using data and model results the potential for model predictions to broadly define habitat and distributions of fishery species with similar early life stages. 

The main outcome of this project will be a validated hydrodynamic model for circulation and mass transport for the Alabama/Mississippi coastal waters. The model will be accompanied by historical forcing data for freshwater discharges and winds so that it will be readily set up for any given model applications. The model will have a utility for a wide range of society-critical applications including management (e.g. effluent discharge from point sources, fishery conservation and management efforts), event-based forensic studies (e.g. hindcasts for search and rescue), and event-based forecasts (e.g. onshore intrusion of spilled oil). The proposed study addresses the Sea Grant Focus Areas of Healthy Coastal Ecosystems (develop and calibrate a hydrodynamic model that focuses on changes in development and weather patterns within the watershed(s) on estuaries; and determine the impacts of causeways and ship channels on hydrologic function of the estuaries) and Sustainable Fisheries and Aquaculture (develop a circulation model for Mississippi Sound for use by resource managers, restoration practitioners and others).

Objectives

This project has 5 objectives:

  1. to develop a hydrodynamic model for the Alabama/Mississippi coastal waters including the Mobile Bay/Mississippi Sound system and validate the model using data for water level, current velocity and salinity/temperature;
  2. to use the validated model to determine the residence time and age of water as an index of flushing capacity for Mobile Bay; 
  3. to conduct field sampling for oyster larvae (marked and unmarked) and other commercially/ ecologically important species with similar early life stages (shrimp and blue crab);
  4. to apply field data to improve and validate an oyster larval transport model to better represent biological conditions and use it to investigate the effects of the biological conditions on oyster larval distribution and settlement patterns; and
  5. to assess the potential for model predictions to broadly define habitat and distributions of fishery species with similar early life stages.

Methodology

Development and validation of a hydrodynamic model: We will develop a hydrodynamic model for the Alabama/Mississippi coastal waters. Starting with an existing model, we will expand the modeling domain to include a greater portion of Mississippi Sound and the northern Gulf of Mexico. A fine-resolution grid will be generated with a minimum grid size of ~50 m to realistically resolve narrow ship channels. The model will be validated using the data collected in this study as well as those from the existing observational infrastructure. The focus of the model validation will be the model’s capability of reproducing the target processes, including tropic-equatorial and seasonal variations in stratification, horizontal density gradient and current velocity, which have been identified by the previous studies to be important in determining physical mass transport in the study area. Quantitative measures, including mean error, mean absolute error, and model predictive skill, will be estimated in model validation.

Residence time and age of water for Mobile Bay: We will estimate residence time and age of water, two fundamental metrics for describing the exchange and transport of water and materials, for Mobile Bay to demonstrate application of the hydrodynamic model in the context of water quality. We will apply the freshwater fraction method based on the modeled spatial and temporal distribution of salinity to estimate residence time. We will estimate age of water using the modeled spatial and temporal distribution of a conservative tracer. To take into account of highly varying forcing conditions, we will conduct model simulations for a 3-year period that includes one normal, one wet and one dry year with observed wind and open boundary water level. The residence time and age of water estimated using the results of this 3-year model simulation will allow us to evaluate their spatial and temporal variations over a wide range of forcing conditions.

Improving the oyster larval transport model: We will improve the existing oyster larval transport model such that the biological conditions represented in the model will reflect the influence of changes in larval development, growth (which will affect swimming and sinking velocity), and survival rates on larval supply and recruitment. We will develop a stand-alone, vertical one-dimensional model based on the results from previous studies, test it with our data, and incorporate it into the three-dimensional oyster larval transport model. The results of the improved model then will be compared to the results of the original model to assess how much improvement the refined biological conditions may bring about in terms of reproducing the exceptional cases with no west-east gradient in oyster spat settlement intensity.

Field sampling: To collect biological data to improve and validate the larval transport model, we will conduct field sampling in Year 1 that consists of two components. The first component is to plankton net and settlement plate sampling of native larvae and spat at 10 mooring stations during the spawning season (May-September) to capture changes in settlement rate, development, growth, and relative survival through time. The second component is to release and recapture calcein marked hatchery-reared veliger oysters in the study area, which will allow us to assess the pathway for larval transport and determine the extent of larval retention in the study area. While calcein-labeling is not essential for model improvement, we will include this cutting-edge approach because it is relatively low cost and low effort (i.e., compared to genetic analyses) and, even with very low recovery of marked oysters, it will provide high quality ground-truth data on model responsiveness to environmental and biological inputs.

Broadly defining habitat and distributions of fishery species: Since many of the smallest larval stages are indistinguishable among shellfish species, much of the data collected for oyster larvae in the early weeks of the study will be broadly applicable to all species with similar early life stages (e.g., panaeid shrimps and blue crabs). Data for all species will be used along with model results to assess the potential for model predictions to broadly define habitat and distributions of these species.

Rationale

All processes related to water quality and living resources require studies of both physical transport and biogeochemical processes to understand the fate and distribution of the related materials. While biogeochemical processes are material-specific (i.e., different for different materials), physical transport processes affect all dissolved and passive particulate materials in the same manner. Then, understanding the dynamics of water movement and the resulting physical mass transport is critical for the study of the fate and distribution of materials. Because of a high level of variability in forcing conditions, it often is difficult to understand the physical transport processes solely based on data. We employ mathematical hydrodynamic models to simulate circulation and estimate information of physical transport. Hydrodynamic models for circulation and transport are, therefore, an essential element in studying and managing issues related to water quality and living resources. Only with credible hydrodynamic models, we can attempt to perform a wide range of society-critical applications including management, event-based forensic studies (hindcasts), event-based forecasts, design, and risk assessment. Currently we do not have a validated hydrodynamic model for the entire Alabama/Mississippi coastal waters, which is needed to inform adaptive and ecosystem based management of fishery and water quality. We propose to develop a hydrodynamic model for the region that includes the Mobile Bay/Mississippi Sound system and the Alabama/Mississippi inner shelf.