Utilizing wetland analysis and detailed elevation surveys to evaluate spatial patterns of sedimentation and potential marsh sustainability in Delaware's tidal and impounded wetlands

Date
2017
Journal Title
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Publisher
University of Delaware
Abstract
The purpose of this dissertation is to provide better understanding of the processes that control the evolution of natural tidal and impounded wetlands. The evaluation of coastal resources is necessary for conservation and management efforts, especially as sea-level rise and anthropogenic alteration impact tidal wetlands. ☐ This research project consists of three separate studies in the vicinity of the Delaware River and Bay coastline within the Delaware Estuary. They include: • The determination of the optimal (“Goldilocks”) vertical growth range and above-ground and below-ground biomass production of Spartina alterniflora (S. alterniflora) with respect to mean low water (MLW) and mean high water (MHW) tidal datums within six watersheds (Blackbird Creek, Bombay Hook Complex, St. Jones River, Murderkill, River, Prime Hook Creek, and Broadkill River). In these watersheds, S. alterniflora has an optimal growth range between -0.07 and 0.18 m relative to MHW elevation and between 1.25 and 1.72 m, relative to MLW. The results can be used to assess the ability of a marsh to combat changing conditions associated with sea-level rise, by determining whether or not the marsh platform is within the optimal growth range of S. alterniflora. This assessment method can be used to determine whether a tidal wetland area is in need of restoration for longer-term sustainability, through the optimization of below-ground biomass production. • An assessment of water level management actions on accretion rates and wetland platform elevations of impoundment marshes using neighboring un-impounded tidal wetlands as reference sites. Nine impoundments (three in northern Delaware (in the vicinity of New Castle, Delaware), four in Central Delaware Bay, and two in lower Delaware Bay) and four reference tidal wetlands (two in Northern Delaware, one in Central Delaware Bay, and one in the lower Delaware Bay) were studied. Regular tidal inundation is vital in the development and evolution of natural marsh platforms. The study results show that only three of the nine impoundments having statistically significant differences (at the p<0.05 and/or p<0.01 levels) in mean accretion rate. Only two of the nine impoundments had significant differences (at the p<0.05 level) in mineral mass accumulation rates. Seven of the nine impoundments have significant differences (at the p<0.05 and/or p<0.01 levels) in organic mass accumulation rates. The effect of water level management on a wetland’s ability to produce below-ground biomass and retain organic material appears to drive the differences in accretion and marsh platform elevation between impoundments and natural wetlands. It is paramount to manage water levels for species of concern, but in a way that below-ground biomass is optimized and decomposition rates are not enhanced through long-term marsh platform exposures. • The utilization of surface elevation table (SET) monitoring within two watersheds, one relatively un-impacted (Blackbird Creek) and the other a heavily impacted mixed urban, suburban, and agricultural watershed (St. Jones River), to evaluate recent trends in tidal marsh surface elevation and short-term vertical accretion. The monitoring revealed that six of the eight sites showed a loss in marsh surface elevation (both shallow and deep) that has not recovered. Deep and shallow elevation changes occurred independent of accretion. It was observed that two (Eagles Nest and Beaver Branch) of the eight monitoring sites experienced rapid gains in elevation (and losses not experienced at the other six sites). The exact cause of large- scale wetland elevation losses that did not recover as of the end of the sampling are difficult to determine. However, moderate El Niño and high sea-level anomalies occurred along the East Coast during the sampling period. The lack of recovery may denote the overall stress of sea-level rise on the tidal wetlands and, as higher-level anomalies occur, vegetation cannot rebound in a manner that some marshes have demonstrated or been theorized to do under un-impacted conditions.
Description
Keywords
Earth sciences, Biomass, Impoundments, Sea-level rise, Spartina alterniflora, Surface elevation tables (sets), Tidal wetlands
Citation