Ecological effects of rising sea level on shorezone

Abstract

This study examines the ecological effects of sea-level rise on shorezone in the Neuse River estuary and western Pamlico Sound, NC. Shorezone is defined here in an ecohydrological context as the area of wetland that extends from an estuarine shoreline landward to where the hydrologic influence of sea level diminishes and terrestrial hydrology dominates. The thesis is structured into three chapters, each highlighting a particular scale of analysis (e.g., landscape, shorezone, and plant community). At the landscape scale, the first chapter investigates geomorphology, hypsography, wetland types, and average landscape slope of successive interstream divide units that are submerging relative to rising sea level. A geographic information system (GIS) was used to identify differences between units and translate them into a space-for-time framework consisting of four temporal stages of shorezone transgression: early - upstream migration, intermediate - non-migration, late - over-flat migration, and terminal - non-migration. The framework is intended to provide a better understanding of processes that have led to the current position of shorezones and to anticipate where effects of rising sea level will be the greatest. In the second chapter, species composition and abundance, soil properties and elevation were analyzed at a plant community scale. Communities were arranged into a hierarchical classification according to hydrogeomorphic wetland type (landscape scale), followed by cover type (shorezone scale), and then community type (plant community scale). A detrended correspondence analysis ordination was performed to analyze samples across an apparent salinity gradient. Analyses revealed a strong relationship between soil porewater salinity and the sequence and distance at which plant communities occur between the shoreline and the landward margin of shorezone. The results suggest that these irregularly flooded shorezones simultaneously exhibit mosaic and zonal patterns of vegetation. At the shorezone scale, changes in cover type over time were estimated for an interstream divide unit in the outer estuary. Cover type classes were ranked to detect the extent, direction (e.g., landward vs. seaward migration), and magnitude (e.g., differences in rank) of vegetation change between 1958 and 1998 using the GIS to analyze aerial photographs. Results show that seaward migration of cover types (517 ha) is more than twice that of landward migration (234 ha). This occurs in spite of an estimated 249 ha landward expansion of shorezone (i.e., transgression) caused by an approximate 15 cm rise in local sea level over the 40 yr study period. This information suggests that at shorter temporal scales, landward migration of shorezone vegetation is not aligned with sea-level rise.