Factors influencing the environmental quality of the Bay of Saint Louis, Mississippi and implications for evolving coastal management policies.

摘要

The Bay of St. Louis, MS is a small northern Gulf of Mexico estuary that has been identified by the Mississippi Department of Environmental Quality (MDEQ) as an impaired waterbody for its designated uses, mainly due to the presence of pollutant pathogens. A systematic study of this estuary was important to understand the behavior and responses of the bay to several natural and anthropogenic forcing factors. A 14-month long study (bimonthly sampling) to evaluate its environmental quality was undertaken from April 2003 to May 2004. Environmental quality was defined as "the health of an ecosystem characterized in terms of water clarity, ability to support aquatic life, nutrient concentrations, and phytoplankton biomass.";Water column temperature, salinity, pH, DO, and turbidity were measured. Surface samples were analyzed for dissolved inorganic nutrients (nitrate, nitrite, ammonium, soluble reactive phosphate, and silicate) and chlorophyll a concentrations. Weather parameters including air and dew point temperature, relative humidity, PAR, solar radiation, and wind speed were measured. Total precipitation, river discharge, and gage height data were also obtained. Similarly, Land use and Land cover (LULC) data from the watershed of the estuary was also included in this study. Parameters such as concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphates (DIP), dissolved oxygen (mg L-1) (DO), chlorophyll a, and turbidity were selected as indicators of environmental quality. An Environmental Quality Index (EQI) was developed for this ecosystem using the selected indicators and suitable reference values. Based on the EQI, an environmental quality report card was created as an evaluation tool for this estuary. Spatial interpolation techniques were applied to present the variability in the environmental quality graphically in the form of maps using GIS software. Data from previous studies conducted separately between 1977 and 1998 were compared to understand the factors influencing the longer-term environmental quality of this estuary.;Based on the EQI and the indicator parameters selected for this study, it was found that the environmental quality of the Bay of St. Louis was not "impaired" during the 2003-2004 study period. Precipitation, river discharge, winds, and tides were determined as the primary factors influencing changes in the environmental quality of the bay. Significant spatial and seasonal variability in the environmental quality was observed due to changes in nutrient (DIN and DIP) and sediment loads. The spatial variability was due to increased nutrient concentrations at locations close to point sources than other areas in the bay. River and bayou mouths, wastewater outfalls, and the Mississippi Sound were determined to be the point sources of nutrients to the bay. The Mississippi Sound and Bayou Portage were identified as the major sources of DIP to this estuary. Spatial variability in nutrient concentrations in the bay was also related to the extent of urban and agricultural land uses in the surrounding sub watersheds. Temporal variability in the environmental quality was due to significant differences in nutrient concentrations and turbidity observed during different seasons. Increased nutrient concentrations (particularly DIN) and turbidity were observed during periods of increased rainfall and riverflow (Spring and Summer), whereas, increased DIP concentrations were observed during periods of low riverflow and high wind speeds (Fall).;Variability in the environmental quality of the bay was also seen over different data periods from 1977 to 2004. The environmental quality of the estuary varied over the years in response to shifts in climate patterns/interannual oscillations such as the El Nino Southern Oscillation (ENSO). Although significant changes in the LULC in the watershed (due to increasing population and increases in urban and agricultural uses) were observed, a declining trend in the environmental quality was, however, not observed over the years.;A management plan for the Bay of St. Louis must be designed to include three key components: a comprehensive suite of indicators with suitable target values, effects of changes in activities and developments in the watershed, and effects of natural shifts in climate patterns. It is imperative that management programs are based on sound science, detailed study, and regular monitoring of this dynamic environment. Equally important is participation and coordination between scientists, land managers, coastal managers and user groups. Finally, effective dissemination of information (such as the use of a GIS-based Environmental Quality Report card), communication with all stakeholders, and timely review and improvisation of implemented programs is crucial.