Pond Dynamics Yield Minimal Net Loss of Vegetation Cover Across an Unditched Salt Marsh Landscape

Abstract

Ponds in salt marshes are often misinterpreted as a symptom of degradation, yet ponds can also be part of a cyclical process of pond formation, expansion, breaching by tidal creeks, and vegetation recovery. Pond dynamics may be altered by accelerated sea level rise, with consequences for the long-term stability of ecosystems. This study tests the prediction that ponds are in dynamic equilibrium across one of the largest expanses of unditched salt marsh in the Northeast USA by (1) examining change in pond and marsh area between 1970 and present in both ditched and unditched regions and (2) by tracking individual pond dynamics across an 87-year time series. Across all marshes, vegetated area declined by 6.3%, with losses primarily due to edge erosion. Ditched marsh had significantly less pond area than unditched marsh. In unditched marshes, we found that net pond area has remained unchanged since 1970 because the amount of marsh conversion to ponds is equivalent to the amount of pond recovery to marsh. The overall extent of ponds connected to tidal creeks is increasing relative to isolated ponds that retain water at low tide, which suggests that some rates of change may be decoupling. This may correspond to a decline in the rate of pond formation, but other factors such as an increase in pond breaching rate and a lag in vegetation recovery rate may also contribute to this pattern. A nuanced understanding of marsh ponds needs to be incorporated into marsh condition assessments and establishment of restoration priorities so that ponds are not interpreted as evidence of degradation when they are exhibiting a recovery cycle. Unditched marshes around the world are a rare resource that remains essential for advancing scientific understanding and serving as reference sites for restoration of marshes altered by past management.