Impacts of Seawalls on Saltmarsh Plant Communities in the Great Bay Estuary, New Hampshire USA

Abstract

Seawalls are often built along naturally dynamic coastlines, including the upland edge of salt marshes, in order to prevent erosion or to extend properties seaward. The impacts of seawalls on fringing salt marshes were studied at five pairs of walled and natural marshes in the Great Bay Estuary of New Hampshire, USA. Marsh plant species and communities showed no difference in front of walls when compared with similar elevations at paired controls. However, seawalls eliminated the vegetative transition zone at the upper border. Not only did the plant community of the transition zone have high plant diversity relative to the low marsh, but it varied greatly from site to site in the estuary. The effects of seawall presence on other marsh processes, including sediment movement, wrack accumulation, groundwater flow, and vegetation distribution and growth, were examined. Although no statistically significant effects of seawalls were found, variation in the indicators of these processes were largely controlled by wave exposure, site-specific geomorphology and land use, and distance of the sampling station from the upland. Trends indicated there was more sediment movement close to seawalls at high energy sites and less fine grain sediment near seawalls. Both trends are consistent with an increase in energy from wave reflection. The distribution of seawalls bordering salt marshes was mapped for Great and Little Bays and their rivers. Throughout the study area, 3.54% of the marshes were bounded by shoreline armoring (5876 m of seawalls along 165.8 km of marsh shoreline). Localized areas with high population densities had up to 43% of marshes bounded by seawalls. Coastal managers should consider limiting seawall construction to preserve plant diversity at the upper borders of salt marshes and prevent marsh habitat loss due to transgression associated with sea level rise.

Vegetation Dynamics in Rhode Island Salt Marshes During a Period of Accelerating Sea Level Rise and Extreme Sea Level Events

“…However, there is growing concern that a combination of reduced sediment supplies and rapidly accelerating rates of sea level rise (which are three to four times higher than the global average) is now leading to marsh accretion deficits throughout the region (Sallenger et al 2012;Weston 2014). In addition, extensive manmade structures such as dams, bulkheads, seawalls, or naturally steep topography will inhibit the lateral transgression of many New England marshes (Bozek and Burdick 2005;Titus et al 2009). As a consequence, researchers and managers are increasingly documenting the waterlogging and degradation of vegetated salt marsh platforms (Smith 2009;Warren and Niering 1993).…”

Seasonal changes in soil TN and SOC in a seawall-reclaimed marsh in the Yellow River Delta, China

“…Salt marshes that regenerated on abandoned land reclaimed by seawalls have been used, to certain extent, as natural analogues for salt marsh development. Particular attentions have been given to the succession of plant communities (Bakker et al 2002;Bozek and Burdick 2005;Wolters et al 2005;Garbutt and Wolters 2008) and restoration of soil nutrient pools, soil seed bank (Santin et al 2009;Fernandez et al 2010;Hung et al 2013;Zhang et al 2013;Bai et al 2014;Gilland and McCarthy 2014;Wang et al 2014). Those researches could provide general information on the design and management of coastal marsh restoration schemes (Garbutt and Wolters 2008).…”

Engineering novel habitats on urban infrastructure to increase intertidal biodiversity

“…Loss, and changes in quality or fragmentation of terrestrial habitat have received much attention (Clergeau et al 2006;Hobbs et al 2006), but these aspects have been given less attention in marine habitats, although there can be considerable loss of and alteration to coastal habitats by construction of built shorelines (Able et al 1998;Glasby and Connell 1999; can also aVect biota using adjacent habitats (Able et al 1998;Wolter 2001;Clynick et al 2008). Although one would expect major changes to diversity when built structures impinge on or replace soft sediments (Bozek and Burdick 2005;Bulleri 2005), the infrastructure itself does not 'mimic' natural hard substrata (Glasby and Connell 1999;Chapman and Bulleri 2003;, despite suggestions that it may (Thompson et al 2002). Chapman (2003Chapman ( , 2006) demonstrated that many widespread or abundant grazers and predators from natural shores were absent from, or rare on, seawalls.…”