Study Sites

Synopsis:

We selected two main study areas in Mississippi for this project: the lower Pascagoula River and the Grand Bay National Estuarine Research Reserve, which both contain large amounts of coastal marsh (Figure 1). They were chosen as these two closely adjacent (within 20 miles) areas represent extremes in a continuum from a river dominated estuary with high amounts of freshwater and sediment inputs to a retrograding delta with high salinity and little sediment inputs. Coastal marshes are generally found in estuaries that lie between these two extremes. Therefore, these two study sites allowed us to explore the importance of sediment supply and other variables for future coastal marsh sustainability in the face of projected sea level rise of between 1-2 m by 2100 in this area.

Local climatological patterns are essentially identical across the two sites. The climate is subtropical with hot and humid summers and mild winter conditions. Average annual maximum air temperature is 24.7 ºC, with July averages reaching 32 ºC. Annual average water temperatures range from 22.5 ºC to 23.1 ºC, with summer reaching as high as 35.9 ºC. Average annual rainfall is about 1600 mm, with summer having above-average precipitation. The coast is influenced by diurnal astronomical tides with an annual average range of about 0.6 m and a maximum range during the summer months of 0.6-0.9 m.

Geologically both sites are found in the Coastal Deposits stratum in southern Mississippi, which is comprised of loam, sand, and clay that was deposited during the Holocene epoch (past 12K years approximately) of the Quarternary period. Marsh sediments at both sites can be characterized by high mud and silt/clay contents.

Pascagoula River:

The Pascagoula River is the largest (by volume) undammed river remaining in the lower 48 states. It is only 80 miles long, arising from the confluence of the Leaf and Chickasawhay rivers. The lower river (30°25'01.4" N, 88°35'27.8" W) is comprised of state wildlife management areas and the Gulf Ecological Management Site (GEMS). Nature tours and birding opportunities can be found through the Pascagoula River Audubon Center. Water levels on the river can be accessed from the USGS gauge at Highway 90 (tidally influenced) or the gauge at Graham Ferry.

The lower Pascagoula delta is comprised of the less developed western branch and more heavily industrialized and impacted eastern branch. Coastal marshes in the delta range from Spartina alterniflora in the lower saline marsh, to Juncus roemerianus in the brackish marsh between Highway 90 and Interstate 10, and freshwater marsh to the north of I-10 with Cladium jamaicense, Sagittaria lancifolia and Zizania aquatica. Cypress tree (Taxodium distichum) swamps are found north of these coastal marshes and are extensive within the Ward Bayou WMA. Submerged aquatic vegetation is prevalent throughout the delta including Vallisneria americana, Najas guadalupensis, and Potamogeton pusillus among others. Common reed (Phragmites australis) is found in disturbed areas along the river banks. Salinity increases along a gradient from north to south and has a pronounced estuarine salinity wedge that fluctuates with river flow and tidal events, but is generally fresher in the western branch. It has a median of 5 ppt (range 0 and 30+ ppt) with the highest salinity generally during the late fall (Oct-Nov).

Grand Bay NERR:

The Grand Bay National Estuarine Research Reserve (30°25'47.3" N, 88°25'39.8" W) was established in 1999 and comprises a portion of the western part of the larger Grand Bay that spans both Mississippi and Alabama. There is a small visitor's center in the main building as well as access to walking trails, educational events, and boating/fishing recreation. Water quality and meteorological data can be accessed through the SWMP program as well as data on other active research programs. Grand Bay is one of three NOAA Sentinel Sites in the Gulf of Mexico, used to better understand ecological responses to future climate and sea level around the nation.

Grand Bay NERR has a total area of about 18,000 acres that contain a variety of wetland habitats, including about 3,000 ha of extensive intertidal salt marshes dominated largely (>90%) by Juncus roemerianus with a small area of Spartina alterniflora in the lower marsh. Adjacent to these marshes is a shallow estuarine area with an average water depth of approximately 0.9 m. Seagrasses, including Ruppia maritima and Halodule wrightii, can be found in the nearshore shallows at Middle Bay, Jose Bay, and the Grand Battures. Salinity increases along a gradient from north to south and fluctuates with rainfall events. It has a median of 22 ppt (range 0 and 30+ ppt) with the highest salinity during the late summer/early fall dry season.