Freshwater Flow to Estuaries
By Kellie Potucek
Both humans and animals benefit from the many functions of tidal wetlands located in estuaries. One illustration of their importance lies in the marshes’ ability to improve water quality. Marshes trap suspended sediment and help alleviate flooding problems associated with rainstorms. Furthermore, as sources of food and habitat, the wetlands are vital to the existence of dozens of avian, mammal, fish, and invertebrate species.
The balance between freshwater inflow and tidal flushing of salt water breathes life into the unique ecosystems. In many Texas estuaries, this balance may be altered due to sea level rise, subsidence, and human activities that divert fresh water from the areas. Changes in structure or function in wetlands along the Guadalupe Estuary are especially troubling because they serve as winter refuge to the endangered Whooping Crane.
Rachel E. Butzler, of the Department of Wildlife and Fisheries Science at Texas A&M University, is researching variations in freshwater flows into tidal marshes in the Guadalupe Estuary. Butzler’s research is focused on effects to vegetation, with special attention to Carolina Wolfberry, a plant that comprises 21 to 51 percent of the Crane’s energy intake early in the wintering period. The Guadalupe estuary is located along the Gulf of Mexico where the San Antonio and Guadalupe Rivers empty into San Antonio Bay. The study area was established in salt marshes of the Aransas National Wildlife Refuge (ANWR). In her research, Butzler is working to ascertain the spatial and temporal variability of vegetation, or macrophytes, in the region. Additionally, she is tracking the changes of the Carolina Wolfberry in relation to field measurements of salinity and flooding, or inundation.
“This study can help predict how variations in salinity and inundation will affect recruitment, production, and growth patterns of the Carolina Wolfberry, which provides food for marsh species, specifically the Whooping Crane,” explained Butzler. While other studies briefly reference the Carolina Wolfberry, Butzler’s research focuses on the plant because of its importance as a food resource.
In the fall of 2003, Butzler established three data collection sites along the estuarine gradient: a boat ramp close to the Guadalupe River mouth, a pump canal that serves as an intermediate site and a point in Sundown Bay, which is most influenced by the gulf. Temperature, dissolved oxygen and salinity were measured every 30 minutes by a water sampler to monitor water quality parameters along the gradient. Vegetation was harvested along transects at the three sites to characterize plant communities and estimate Wolfberry abundance across the marsh. Soil cores were taken to determine organic matter and nutrient content, which may play a role in plant distribution. Additionally, Butzler established macrophyte plots as a non-destructive method to measure the composition of vegetative species and aboveground biomass of the Carolina Wolfberry. Salinity levels of standing water in these plots were also measured.
“Of the three sites, the boat ramp seems to support the highest macrophyte diversity, followed by the pump canal and then Sundown Bay,” reported Butzler. Preliminary results suggest correlation to the salinity levels in each site, which were lowest at the boat ramp and increased at the pump canal and Sundown Bay sites, respectively. This variation also corresponds to inundation which orrcurs most frequently at Sundown Bay and least frequently at the Boat Ramp. Monthly sampling will continue until March 2005, the end of the growing season. However, sampling will continue as part of a larger ecosystem project. Furthermore, Butzler is planning a greenhouse experiment for the fall in which she will test salinity and inundation effects on Carolina Wolfberry in a controlled environment.
“This continued research will further clarify the role of salinity and inundation in governing marsh community dynamics and wolfberry production at ANWR,” explained Butzler. As part of a larger ecosystem study, these results will be combined with other components and used in making management decisions. These findings may also apply to other estuarine watersheds that are facing the same pressures as the Guadalupe Estuary.
