Long-Term Erosional Trends Along Channelized Salt Marsh Edges

James P Browne

doi:10.1007/s12237-017-0245-y

James P Browne

Open Access

https://doi.org/10.1007/s12237-017-0245-y

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Abstract

Salt marshes provide important habitats for many species in the estuaries along the east and Gulf coasts of North America. With many species dependent on these coastal marshlands and extensive documentation that these marshlands are disappearing, a clear understanding of the mechanisms causing loss is critical. Much of the salt marsh was lost to reclamation and construction before these activities were curtailed circa 1970; however, losses due to other causes have continued and multiple hypothesized causes have been proposed, not all mutually exclusive. Yet it remains unclear whether there are legacy effects from the reclamation projects. When the edges of salt marshes are cut into, and gentle vegetated slopes are replaced by sharp edges adjoining deep water of 2 m or more, erosion could accelerate and could continue for many years. One method that may help shed light on the relative importance of the various causes of salt marsh erosion would be to compare the erosion rates of specific edges within a marshland that are exposed to particular conditions. We therefore used several sets of aerial photography spanning 84 years to track the changes at specific edge locations along marsh edges and then make comparisons between anthropogenically created edges and naturally created edges, including comparisons within use and width categories of navigational channels. Erosion rates were found to remain significantly higher on channelized edges than along otherwise similar wetland edges even several decades after modification. Likely reasons include the continued exposure of underlying layers that lack reinforcing plant root systems, vertical edges that are more vulnerable to undermining from wave action, and increased erosion related to altered tidal flows.

Highlights

Many mechanisms have proposed to explain the losses of salt marshes (Mendelssohn and McKee 1988; McKee et al 2004; Edwards et al 2005; Silliman et al 2005; Mendelssohn et al 2006; Alber et al 2008; Ogburn and Alber 2006; Gedan et al 2011; Deegan et al 2007; Deegan et al 2012), and it is unlikely that any single factor is the major cause of marsh loss throughout their geographic range
Edge change data from 1966 to 2007 were collected for 30 points that fell on channelized marsh edges and 408 points that fell on natural marsh edges
This study confirmed that channelized edges of marshes can have significantly more erosion than channels retaining natural edges, even after accounting for boat traffic

Summary

Introduction

Wetlands serve as a valuable natural resource supplying may ecosystem services (Keddy et al 2009), including roles as nurseries for thousands of species, as barriers protecting human infrastructure from storm surges, in the dissipation of wave energy, in denitrification, and as water filtration systems (Möller et al 1999; Shriver et al 2004; Cooper 2005; Zedler and Kercher 2005; Costanza et al 2008; Feagin et al 2010; Barbier et al 2011, Fagherazzi 2014). The edges of marshes that were channelized retreated horizontally by 27.52 (±3.82) m while those points on natural edges of marsh retreated on average 8.38 (±0.76) m, and these differences were highly significant using a Mann-Whitney U test (P < 0.001, n1 = 408, n2 = 30; Table 2).

Results

Conclusion