St. Louis Bay, Mississippi, is a direct tributary estuary to the Gulf of Mexico. It is bounded by the Jourdan River, Wolf River, and small direct tributary watersheds on the north and the Gulf of Mexico on the south. The primary objective of this study was to evaluate the sources, fate, transport, and effects of nutrients in support of developing protective nutrient criteria for St. Louis Bay. A secondary objective was to standardize a regional approach for developing numerical nutrient criteria that could be used at locations throughout the Gulf of Mexico to protect coastal and estuarine ecosystems from nutrient enrichment.
Stressor-response and mechanistic models were used to meet the study objectives. The bulk of the data used in model development was obtained through a comprehensive sampling program conducted in 2011 by the Mississippi Department of Environmental Quality (MDEQ). A total of 51 stations were established throughout St. Louis Bay and a large set of parameters was measured, including TN, TP, chlorophyll a, total suspended solids (TSS), water clarity, DO, salinity, pH, and temperature. In addition, samples of benthic macroinvertebrate assemblage were taken at 35 randomly selected stations and expressed as a biological index score. Samples also were collected monthly and quarterly from several boundary sites for characterizing nutrient loading.
Stressor-Response Approach
The stressor-response approach was used to establish empirical relationships among nutrient concentrations, nutrient-related stressors, and biological conditions and to develop predictive numerical stressor-response models to help establish nutrient thresholds for the protection of aquatic life. A site classification process resulted in five strata within the estuary—stream, tidal, bayou, estuary, and outer bay—which were used as the spatial framework for evaluating nutrients in the bay.
Data from the 2011 MDEQ survey were explored and characterized spatially and temporally using several statistical visualization techniques, including daily curves, box-and-whisker plots, scatter plots, and time-series decomposition. Stressor-response relationships were examined using Spearman correlation and regression analysis. Hierarchical and Bayesian techniques were used to develop predictive models.
Using the 2011 dataset, the stressor-response models indicated only a limited linkage between TN and TP and chlorophyll a concentrations in the bay. In addition, there was a lack of observed relationships between benthic macroinvertebrate health and DO and between DO and chlorophyll a levels. Because no evidence of clear nutrient-related impacts was found, an empirical current condition approach was applied to the monitoring data to derive proposed chlorophyll a and nutrient concentration thresholds to protect the estuary from the impacts of excessive nutrient input. The annual geometric mean threshold concentrations, based on the 90th percentile of empirical monitoring data, are 0.70 mg/L for TN, 0.08 mg/L for TP, and 13 μg/L for chlorophyll a.
Mechanistic Modeling Approach
A series of linked mechanistic models was developed to simulate the sources, fate, transport, and effects of nutrients in the St. Louis Bay. The Loading Simulation Program in C++ (LSPC) was used to represent hydrological and water quality conditions in the watersheds and to calculate nutrient loads to the bay. The Environmental Fluid Dynamics Code software was used to simulate hydrodynamics, and the Water Quality Analysis Simulation Program (WASP) was used to simulate spatial-temporal dynamics of nutrients, DO, and other water quality constituents. A geometric representation of the estuary consisting of a curvilinear orthogonal grid composed of 1,259 horizontal grid cells, each one divided vertically into two sigma grid layers, was used for the analysis.
Using the 2011 MDEQ dataset and data from the U.S. Geological Survey and the National Oceanic and Atmospheric Administration, the study authors performed a comprehensive calibration-validation process to evaluate the predictive capacity of the models. They subsequently concluded that the models could be reliably used as a predictive tool to support the development of nutrient criteria and modeled and assessed the processes governing phytoplankton production, DO, and nutrients in the estuary.
A set of four loading scenarios—current condition, natural conditions, 50-percent reduction, and 50-percent increase—were defined to evaluate the effects anthropogenic nutrient load reduction or increase might have on phytoplankton productivity, chlorophyll a, DO, and water clarity in the estuary. Analysis of loading scenario outputs revealed only a moderate response of the estuary to modeled changes in loads. Thus, similar to the conclusions derived from the stressor-response approach, the authors settled on the empirical current condition nutrient approach to deriving proposed chlorophyll a and nutrient concentration thresholds. Using the 3-year (2009–2011) average of annual 90th percentiles from the existing condition scenario as the current condition, they derived TP, TN, and chlorophyll a numeric thresholds as 0.065 mg/L, 0.66 mg/L, and 17 μg/L, respectively (Tetra Tech 2013).
Reference:
Tetra Tech (Tetra Tech, Inc.). 2013. Sources, Fate, Transport, and Effects (SFTE) of Nutrients as a Basis for Protective Criteria in Estuarine and Near-Coastal Waters – Saint Louis Bay, Mississippi Pilot Study. Prepared for Gulf of Mexico Alliance, under the direction of Mississippi Department of Environmental Quality. Accessed October 2016. http://www.gulfofmexicoalliance.org/documents/pits/nur/sfte_report_slb.pdf Exit.