Freshwater inflow requirements for the Nueces Delta, Texas: Spartina alterniflora as an indicator of ecosystem condition

Abstract

Estuarine wetlands and salt marshes are fundamentally driven by variations in freshwater inflow. However, many estuaries have been subject to a heavily modified hydrology due to flood protection engineering and the construction of upstream dams for municipal water supply. Assessment of the impacts of these activities on the health of estuarine wetlands has traditionally focused on tracking the abundance of economically important shellfish and finfish species. In this study, we examine fluctuations in the abundance of selected salt marsh plants and use this information to develop estimates of freshwater inflow needs. The impact of freshwater inflow events on 3 common emergent plants in the Nueces River Delta (Spartina alterniflora, Borrichia frutescens, Salicornia virginica) was determined from long-term monitoring of permanent census plots. Of the 3 species examined, Spartina alterniflora was determined to be the best indicator species because its abundance most closely tracked variations in freshwater inflow. For example, under low salinity conditions S. alterniflora cover approached 66%. However, when salinities exceeded 25‰, S. alterniflora cover declined rapidly. Our results provide clear evidence that the presence or absence of key plant indicator species (in this case S. alterniflora) is reflective of overall estuarine hydrological condition over time scales exceeding 6 months.