Ecological tradeoffs of stabilized salt marshes as a shoreline protection strategy: Effects of artificial structures on macrobenthic assemblages

Armoring shorelines to prevent erosion is a long-standing global practice that has well-documented adverse effects on coastal habitats and organisms. A relatively new form of shoreline protection, referred to as hybrid stabilization, incorporates created marsh in combination with a stabilizing structure such as a low-profile stone sill and is being implemented in many US coastal states as a means to not only control erosion but also to restore coastal habitat. However, there has been limited scientific investigation of ecological benefits and impacts associated with implementation of hybrid stabilization. We evaluated relative habitat capacity of marsh-sills by comparing plant, sediment, and benthic macroinvertebrate attributes in intertidal and subtidal zones of existing marsh-sills, natural marshes, tidal flats, and riprap revetment within two subestuaries of Chesapeake Bay, USA. Low and high marsh plant characteristics (stem count and height) of marsh-sills were similar to or greater than natural marshes. However, sediment was coarser, total organic carbon and total nitrogen concentrations were lower, and benthic macrofaunal community structure differed in marsh-sills compared to natural marshes. Marsh-sills supported lower deposit-feeding infaunal biomass than marshes in the intertidal. Epifaunal suspension-feeders were most prevalent at sites with artificial structure (riprap and marsh-sill), but highly variable among subestuaries. Infaunal abundance, biomass, diversity, and proportion of suspension/interface and deposit feeding animals were greater in shallow subtidal than in intertidal environments. Conversion of existing habitat to marsh-sills may cause localized loss of benthic productivity and sediment bioturbation and nutrient-cycling functions, with the opportunity to enhance filtration capacity by epifaunal recruitment to structures. When creating marshes that require structural support, there should be a balance of minimizing loss of existing habitats while encouraging use of suitable structural habitat for suspension-feeders. If properly implemented, the addition of structural habitat could subsidize secondary productivity particularly in areas where loss of complex biogenic habitat (e.g., oyster reefs) has occurred.