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Monitoring a Freshwater Tidal Wetland in the Delaware Estuary: Flooding Influence on Accretion and a Potential for Priority Planning Under Vulnerability to Sea Level Rise

Tidal freshwater wetlands provide numerous important ecological services including carbon and nutrient sequestration, sediment trapping, and natural flood control. Urban and agricultural development, nutrient loading, and sea level rise continue to threaten these ecologically and economically valuable ecosystems. Rates of sea level rise are accelerating with global climate change, which has implications for wetland loss. Near Philadelphia, the rate of local sea level rise has been 0.29 cm per year over the last century (NOAA 2012), however, recent analyses have found that rates of sea level rise have accelerated over past decades.

Our objectives for this study were three-fold: (1) to continue monitoring wetland elevation and plant community composition at Tinicum Marsh; (2) to determine the relative magnitude of tide and storm deposition of sediment to the marsh surface, including assessment of upstream inputs, and (3) to evaluate the effects of increasing storm intensity and/or frequency on near-term wetland survival.

We report on marsh elevation change, sub-surface consolidation, plant community composition, and biomass. We measured event-based sediment transport to the marsh from upstream sources, through installation of a stream gauge and development of rating curve for Darby and Cobbs Creek. We measured wetland sediment deposition during storms and baseflow conditions. We developed an empirical model to determine marsh survival with sea level rise. We analyzed changing storm intensity patterns, and determined whether increased storminess might encourage more rapid sediment deposition, and therefore higher resilience to sea level rise with climate change.

The results of our study suggest that wetlands at Tinicum are currently stable. If past rates of sea level rise (historic rates) were to continue into the next century we would expect only modest reductions in wetlands. However, if sea level rise acceleration occurs, as is expected under even the most conservative emissions scenarios, significant reductions in marsh area will likely occur. Wetlands at Tinicum should continued to be monitored to help detect the early stages of wetland drowning. Within the project area, there was little undeveloped land available for wetland upslope migration with sea level rise due to abrupt slopes, although a parcel of undeveloped land upstream of Tinicum may comprise a potential marsh migration zone. Our analysis of plant communities overall suggest the wetlands are currently healthy and robust, although we did notice significant loss in plant cover in 2016 as the result of a beetle infestation. The waterlily beetle was often found at extremely high densities, and the presence of this beetle and potential damaged caused by herbivory should be tracked over the coming decade.

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