Long-term performance and impacts of living shorelines in mesohaline Chesapeake Bay

Shorelines in Chesapeake Bay, and many other estuaries and coastal embayments, are rapidly eroding, with even more rapid loss expected in the future from drivers like urbanization and accelerated relative sea-level rise (RSLR). Past efforts to stabilize shorelines using approaches like riprap or bulkheads generally have resulted in negative ecosystem impacts, resulting in a rise of ecosystem-based approaches using natural and nature-based features (NNBF) such as living shorelines, defined here as narrow marsh fringes with adjacent sills. Living shorelines provide similar ecosystem services as natural marshes but are threatened by the same stressors of environmental change, raising questions about their long-term resiliency and effectiveness in reducing shoreline erosion. Questions also remain about their potential impacts on benthic habitats in adjacent waters. These questions are especially relevant to the Chesapeake, where relatively rapid rates of RSLR and declining sediment supplies have led to widespread marsh loss, and submersed aquatic vegetation (SAV) is a critical indicator for water clarity. This study addresses these questions through field observations in similar to 10-year-old living shorelines and SAV habitats adjacent to them, along with observations at nearby reference (unaltered) shorelines. In general, shoreline erosion continued at or above historical rates at reference shorelines, but living shoreline installation builds shorelines seaward and results in net shoreline accretion. While the sand and organic content of bottom sediments in adjacent waters changed at many sites after living shoreline installation, changes were site-specific and typically in the same direction at both living and reference shorelines. These changes did not appear to impact SAV distributions, which followed regional trends likely linked to water clarity. Sediment and nutrient burial in the coastal zone, which includes both intertidal marsh and subtidal SAV habitats, was highest for living shorelines due to the addition of marsh habitat. While this study did not consider direct replacement of SAV with living shorelines, these results suggest that discouraging living shoreline installation in areas with SAV may miss an opportunity to enhance nutrient burial in the coastal zone.