A conceptual model for predicting the ecological effects of coastal armoring in soft-sediment environments

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Shoreline armoring structures are widely used in soft-sediment environments, ranging from bulkheads and revetments in protected marshes to sea walls on open coast beaches. By fixing shoreline position, armoring constrains possible responses and evolution of soft sediment ecosystems to changes in sea level and other dynamic coastal processes. Despite their long history of use worldwide, information is lacking about the ecological effects of these widespread coastal structures. In addition, the majority of scientific studies are focused on armoring structures in only a single coastal environment type, making comparisons across soft sediment systems challenging. A cross-site working group on the responses of soft sediment coastal ecosystems to armoring in the LTER Network addressed these questions.  Here we present a conceptual model developed by the working group that contrasts the effects of shoreline-parallel armoring structures across soft-sediment environments. The model scales the predicted ecological effects of shoreline armoring based on the purpose of the armoring structure (to slow or stop water) and the energy of the water at the structure. We then identified a suite of six ecological responses to shoreline structures (alterations in habitat and species distributions, connectivity, productivity, nutrient cycling and trophic structure), and predicted that the ecological impacts of structures will vary depending on their purpose and the energy regime. We then applied our conceptual framework to a literature review of the effects of shoreline structures (including living shorelines) in differing tidal and wave conditions across a range of soft-sediment environments (open and protected beaches, estuary and harbor shores, salt marshes, and mangroves), with a focus on ecological impacts. We found a preponderance of studies examined changes in species and habitat distributions, with far fewer studies of effects on connectivity and trophic structure. Studies of ecological effects of armoring in salt marshes were more numerous than in other environment types. Addressing these research gaps and hypotheses will be critical for advancing our understanding of the range of ecological responses of soft sediment ecosystems to coastal armoring. This is particularly important given that the motivation to build additional armoring structures in vulnerable soft-sediment coastal environments is expected to increase in response to sea level rise, coastal development and other pressures.