Observations on Shallow Subsurface Hydrology at Bay of Fundy Macrotidal Salt Marshes

Hydrology is a major driver of salt marsh functions and the ecological services they support. Sediment saturation, and thus subsurface hydrology, affects salt marsh vegetation productivity, zonation, and survival. Subsurface hydrology influences rates of subsidence; concentrations of nutrients, organic matter, and oxygen; fluxes of the greenhouse gases methane and nitrous oxide; and sediment toxicity. Therefore, baseline knowledge of subsurface hydrology is crucial in predicting and managing change in salt marshes. Previous research on subsurface hydrology has mostly been conducted in organogenic, microtidal marshes that do not provide suitable models for minerogenic, macrotidal marshes such as those in the Bay of Fundy. In this study, we examine drivers of subsurface hydrology and compare two pairs of reference and recovering marshes with tidal amplitudes of 6 to 12 m. Measurements in PVC wells at various depths reveal differences between seasons, during the neap-spring tidal cycle, and between channels of differing depths. We suggest that soil characteristics, duration between inundation events, marsh geomorphology, and precipitation all are more important drivers of subsurface hydrology in Bay of Fundy marshes than tidal height. Our observations also indicate that subsurface hydrology generally can be restored to previously drained marshes. Bay of Fundy marshes are likely to be more ecologically resistant to rising sea level because changes in tidal heights will have a minimal effect on subsurface hydrology. Thus, restoration of the many Bay of Fundy marshes previously drained for agriculture is likely to be successful in offsetting losses expected in more vulnerable, microtidal marshes.