LAUSR

Phragmites growth in a marsh watershed due to increased salinity has been a crucial issue across the world. The objective of this study was to investigate the salinity movement in the ungagged Mentor Marsh of Ohio, USA, where the salinity had increased due to a number of potential sources causing a decline in the native vegetation and leading to the increased invasive phragmites growth. In this study, we conducted a detailed bathymetric survey and established several monitoring stations to record hourly environmental data in Mentor Marsh. Since Mentor Marsh has a complex hydrologic characteristic, which interacts with Lake Erie due to the backwater effect, a hydrodynamic model, Environmental Fluid Dynamic Code (EFDC +), was developed to simulate the western Mentor Marsh wetland’s salinity distribution. We evaluated the model performance by comparing water level, temperature, and salinity using statistical measures for a duration from December 2019 to March 2020. The model was calibrated using the measured time-series data of water temperatures, water levels, and water salinity from monitoring stations in the western basin. The model performance for salinity calibration (R2 = 0.82, RMSE = 0.041, and Pbias =  − 1.05%) and validation (R2 = 0.84, RMSE = 0.066, and Pbias =  − 2.05%) was good. In the next step, the calibrated model was utilized to investigate the salinity distributions under different inflow and lake level rise conditions. Our analysis suggested that during high-flow conditions, the advection of the saline water from Marsh Creek was vigorous in comparison to the diffusion of salinity mixing by tidal influence pushing the salinity towards Mentor Marsh and resulting in the lower salinity distribution within the model domain. Similarly, when the lake level rise occurred, the model predicted a significant decrease in the salinity of Mentor Marsh near Lake Erie. The average decrease of salinity from the salinity during the base run was − 45.8% near Lake Erie, − 29.7% at the junction of Mentor Marina and Mentor Marsh, − 21.2% in Mentor Marsh, and − 4.4% in Marsh Creek. The analysis further suggested that under high-flow conditions from Marsh Creek, the salinity moved towards Mentor Marsh, especially when lake level rise conditions were considered. This is mainly because the high water level of Lake Erie pushed March Creek towards Mentor Marsh. However, the salinity moved towards Lake Erie from Marsh Creek during low-flow conditions. Presumably, the phragmites growth in the western section seems to be due to the road salt used in winter for deicing purposes.