Generalizing Ecological Effects of Shoreline Armoring Across Soft Sediment Environments

J E Dugan,S E Sojka,J E Byers,M Alber,A M Gehman,K A Emery,C R Alexander,N Mclenaghan

doi:10.1007/s12237-017-0254-x

Abstract

Despite its widespread use, the ecological effects of shoreline armoring are poorly synthesized and difficult to generalize across soft sediment environments and structure types. We developed a conceptual model that scales predicted ecological effects of shore-parallel armoring based on two axes: engineering purpose of structure (reduce/slow velocities or prevent/stop flow of waves and currents) and hydrodynamic energy (e.g., tides, currents, waves) of soft sediment environments. We predicted greater ecological impacts for structures intended to stop as opposed to slow water flow and with increasing hydrodynamic energy of the environment. We evaluated our predictions with a literature review of effects of shoreline armoring for six possible ecological responses (habitat distribution, species assemblages, trophic structure, nutrient cycling, productivity, and connectivity). The majority of studies were in low-energy environments (51 of 88), and a preponderance addressed changes in two ecological responses associated with armoring: habitat distribution and species assemblages. Across the 207 armoring effects studied, 71% were significantly negative, 22% were significantly positive, and 7% reported no significant difference. Ecological responses varied with engineering purpose of structures, with a higher frequency of negative responses for structures designed to stop water flow within a given hydrodynamic energy level. Comparisons across the hydrodynamic energy axis were less clear-cut, but negative responses prevailed (>78%) in high-energy environments. These results suggest that generalizations of ecological responses to armoring across a range of environmental contexts are possible and that the proposed conceptual model is useful for generating predictions of the direction and relative ecological impacts of shoreline armoring in soft sediment ecosystems.

Highlights

Soft sedimentary shores composed of mud, sand, and gravel make up the majority of the world’s coastlines (Reise 2001)
The conceptual model could theoretically be applied within a single soft sediment habitat, in this study we focused on differences in hydrodynamic energy
We did not have an a priori expectation as to which of these axes would be more important and so predicted a general upward increase in ecological impacts commensurate with intensification of both factors (Fig. 2). To investigate these predictions for ecological impacts, we identified six categories of ecological responses that we expected could be altered by the presence of shoreline armoring in soft sediment ecosystems

Summary

Introduction

Soft sedimentary shores composed of mud, sand, and gravel make up the majority (two thirds) of the world’s coastlines (Reise 2001). Soft sediments are associated with a variety of ecosystems including beaches, dunes, coastal bluffs, marshes, estuaries, bays, and inlets (Nordstrom 2000). These areas provide a range of ecosystem functions and services, ranging from storm protection to wildlife habitat to carbon sequestration (e.g., Snelgrove 1999; Piersma 2009). Soft sedimentary shores are inherently dynamic, and this has led to the installation of coastal armoring structures built for the purpose of protecting upland areas and slowing or halting erosion and migration of the shoreline (Nordstrom 2000; Rippon 2001; Charlier et al 2005; Griggs2005a, b). The extent of armoring is expected to increase as a result of expanding coastal populations and cities interacting with growing threats from climate change, storm surges, and sea level rise

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Discussion

Conclusion