The impact of past management practices on tidal marsh resilience to sea level rise in the Delaware Estuary

Abstract

Defining appropriate management and conservation strategies to maximize tidal marsh resilience to sea level rise requires a clear understanding of the causes of marsh degradation. While sea level rise is a well-known threat to tidal marshes, current and past management practices on marshes can also greatly influence present-day marsh condition, resilience and future persistence. Using point-intercept analysis of maps and imagery, we assessed the past and current landcover and elevation of Delaware Estuary tidal marshes in New Jersey, USA. We estimated the historic extent of tidal marsh impoundment for agriculture and determined current marsh vegetation composition and elevation in areas that were and were not historically impounded. We estimate that more than half of all tidal marsh in the 36,539 ha study area had been historically impounded. A small fraction of this area remains impounded at present (7.6%). While tidal flow has since returned to formerly diked areas, marsh recovery has been incomplete. Overall 21.6% (4048.8 ha) of formerly impounded marsh has not revegetated, becoming open water after impoundment breaches. Marsh loss as a result of impoundment is also responsible for the loss of 2.3 km of adjacent shoreline beaches. Conversely, only 0.5% of marsh that was never impounded has converted to open water since 1931. This difference is likely due to dramatic elevation deficits caused by impoundment. Marsh elevation of current and formerly impounded areas (derived from LiDAR and validated with RTK GPS) is significantly lower than the elevation of marsh areas that were never impounded. Supporting this finding, the frequency of high marsh vegetation (an indicator of higher elevation)in vegetated formerly impounded areas is half that of areas that were never impounded. Marsh edge erosion and creek expansion have added an additional estimated 3836 ha to the amount of tidal marsh loss since 1931. Marsh transgression inland into forest and agricultural areas has resulted in estimated gains in marsh area of 2815 ha, offsetting a considerable proportion of losses. Given our results, we recommend the following management actions to maximize tidal marsh persistence in the Delaware Estuary: (1) Beneficial use of sediment to offset marsh elevation deficits resulting from historic impoundment, (2) Strategic land protection to maximize the potential for inland marsh migration, (3) Tidal flow restoration to remaining impounded areas in combination with the beneficial use of sediment to address elevation deficits. Determining the impacts to tidal marshes from past management practices makes it possible parse the relative contribution of relative sea level rise and site-level management, resulting in more targeted conservation strategies.