HYDROLOGIC RESPONSES OF A COASTAL MARSH ALONG A SALINITY GRADIENT: A CASE STUDY IN GRAND BAY NATIONAL ESTUARINE RESEARCH RESERVE, MISSISSIPPI

Coastal marshes offer a large range of ecosystem services. Climate change and relative sea level rise (SLR) is already and will continue to result in salt water intrusion into these vital coastal systems affecting healthy ecosystem function and causing potentially severe deviations from current hydrologic conditions. The Grand Bay National Estuarine Research Reserve (GNDNERR) is a characteristic marsh on the Gulf Coast near Pascagoula, Mississippi.

Emphasis is placed on identifying factors including tidal fluctuations and storm events that directly affect hydrologic and sedimentary dynamics. Shallow groundwater hydrologic trends between July 2015 and present are established by monitoring water levels, water temperature, and conductivity in piezometers at depths of 0.75m, 1.5m, and 2.25m along a salinity gradient. Surface water is monitored adjacent to the piezometer stations and also by NOAA Surface Water Monitoring Program and Sentinel Program stations.

Barometrically corrected records show strong signals of tidal fluctuations in the shallow groundwater at all stations. The range of the tidal signal at each station is locally consistent but varies across the marsh system. The variation between stations is likely controlled by sediment composition or depositional structures. Conductivity decreases with depth, distance from open ocean, and seasonal precipitation. In late October 2015, observations included the impacts of a hybrid weather system formed from the remnants of Hurricane Patricia and a low pressure system over Louisiana. The combined system was accompanied by heavy precipitation and high winds along the Gulf Coast causing storm surge coincident with spring tides. Average tidal range is 0.6m; the surge was approximately 0.5m above the average high. Groundwater also increased nearly as much, but low tide signals were weakened likely as a result of the prolonged precipitation and inundation. Following the storm peak, surface water returned to average levels within 48hrs, and groundwater took up to approximately 72hrs.

Tidal fluctuations and storm events offer short-term insight to the hydrologic processes. Storms of significant strength and multiday duration have the greatest potential for inundation to the marshlands creating conditions similar to a small change in relative sea level.