Following a state mandate to determine freshwater inflow needs of minor estuaries, the Texas Water Development Board is extending an open bay hydrodynamic and salinity transport model to include tidally influenced areas of coastal wetlands. Wetlands serve as nursery habitat for estuarine species, as well as essential habitat for many other species. However, our existing hydrodynamic models, which are used to verify suitable salinity conditions for fish and shellfish in open bays, do not include wetting and drying of surrounding wetlands. In an effort to extend the models to better understand the importance of freshwater inflows to wetland habitats, we are monitoring soil pore water and surface water salinities. Our focal study questions are: (1) How does freshwater inflow impact salinity in the open bay and therefore inundation salinity in the lower and middle marsh zone? (2) How do inundation salinities influence soil salinities? The overall goal is to determine a freshwater inflow regime which will protect marsh plant diversity, growth, and production by maintaining acceptable salinity levels in the wetland.
Results/Conclusions
The study is being conducted in the tidally influenced wetlands at San Bernard National Wildlife Refuge, Brazoria County, Texas, USA. Low marsh areas are dominated by Spartina alterniflora. However, the majority of area wetlands are mid-elevation marshes primarily composed of Spartina patens/Distichlis spicata nearest to tidal influence and Paspalum vaginatum/Schoenoplectus americanus farther upland. A previous study in this wetland documented annual mean surface water salinities ranging 12 to 49ppt, while interstitial salinity ranged 15 to 68ppt (Miller et al. 2005). Although our hydrodynamic models can simulate surface water salinity, they cannot simulate pore water salinity which is critical to plant production and survival. In order to correlate surface and pore water salinity, we are collecting in situ salinity data in open water areas (seven sites) and in soil (four sites). Collected field data show spatial variation in the correspondence between water level within paired soil and open water sites. Soil salinities remain relatively stable despite salinity fluctuations occurring in open water sites. To our knowledge, the relationship between interstitial salinity and surface water salinity is not well known. Moreover, we know of no studies which have recorded in situ soil salinity measurements of coastal wetlands. As such, this study provides a unique high resolution dataset which is being used to calibrate a hydrodynamic model and reveal patterns in the characteristics of salinity in coastal wetlands.
