Episodic Salinity Management to Counter Climate Change Effects on Tidal Brackish and Fresh Wetlands

The capacity of brackish and freshwater tidal marshes to accrete vertically in response to sea level rise is threatened where drought and salinity intrusion are being amplified by climate change. Episodic salinity management with purchased augmented freshwater is an option for two modest-sized tracts in southeast Texas, where drought and hydrologic modifications threaten wetland resiliency and the Mottled Duck. We developed a transferable methodology to assess biophysical benefits in a spatially explicit manner for these heterogeneous wetlands. Four salinity objectives reflected zonal geography of the wetland plant communities and Mottled Duck brood-rearing needs. A calibrated daily wetlands hydrologic-salinity model contrasted scenarios of severe drought with those of freshwater augmentation. The volume of freshwater available, up to 12.33 M m³ per year, could be effective at moderating salinity over significant wetland areas, but benefits were sensitive to management approach, as well as delivery rates and duration of augmentation. Additionally, fixed freshwater application rates could depress salinities to suboptimal ranges and waste a purchased resource. Feedback scenarios based on in-marsh salinity conditions elevated the ratio of benefits to delivered water volumes but would entail additional monitoring and management cost. Compared to the extremely deleterious conditions of severe drought, most freshwater augmentation approaches would greatly benefit the Mottled Duck and the productivity of the wetland vegetation within the tracts. However, portions of a fragile brackish zone dominated by Spartina patens would remain at risk from elevated salinity, suggesting a need for complementary restoration actions.