Salt marsh connectivity and freshwater versus saltwater inflow: Multiple methods including tidal gauges, water isotopes, and LIDAR elevation models

The hydrologic connectivity between different components of a coastal marsh is a key wetland structural element and is essential in determining habitat value. Salt marshes are often thought of as being hydrologically connected via tidal creeks, though water bodies within a salt marsh may also be semipermanently disconnected. At a salt marsh in Matagorda, Texas, USA, there are many "isolated" ponds, located at varying elevations. Our objective was to quantify the hydrologic connectivity of spatially isolated ponds at this site. We sampled water for salinity and stable isotopes (δ²H, δ¹⁸O) to determine the relative contribution of tidal water and rainfall within each pond. We also quantified the water level at which each pond floods its banks and connects to other ponds, using a light detection and ranging (LIDAR)-based elevation model. We found that pond connectivity was driven by multiple factors, of which wind and rainfall were the most important. Salinity and isotopic values between any two sampled ponds were correlated with the water level elevation at which the ponds connected. We conclude that the connectivity of the network, and the similarity of water samples within it, profoundly changes when specific water level thresholds are exceeded by both fresh water and salt water.