Effectiveness of living shorelines in the Salish Sea

Abstract

In human-impacted coastal ecosystems, living shorelines are becoming a common restoration technique. However, we lack a comprehensive understanding of the ecological and physical benefits, and how they could inform management needs. To address this, we studied effectiveness of living shorelines at a broad spatial scale within the Washington State boundaries of the Salish Sea, USA, with restored site ages spanning 1–11 years. We surveyed 30 beaches at ten locations, each with three strata of: (1) living shoreline beaches with armor removed, (2) armored control beaches altered by seawalls or riprap, and (3) un-armored reference beaches with natural conditions. We sampled eight physical and biological attributes: beach wrack, wrack invertebrates, sediments, terrestrial insects, riparian vegetation, logs, beach profiles, and stable isotope signatures of talitrid amphipods – generating 27 metrics focusing on upper intertidal and supratidal elevations affected by armoring and targeted by living shoreline actions. These metrics spanned the functions of beach stability, ecological diversity, and food web support for juvenile salmon. Statistical tests showed that 19 of the 27 metrics had significant strata differences, indicating that some beach metrics restore quickly (e.g., wrack accumulation), while others take longer (e.g., log accumulation). Terrestrial-associated metrics were higher at reference beaches, but insect taxa richness and logs with plant growth increased at beaches restored for four or more years (the average age of the living shoreline sites). This implies that certain living shoreline functions increase through time, providing improved food web support. Globally, trajectories of restoration have shown a range of functional improvement with time, and will be important to monitor for nature-based solutions to coastal defense given the increasing rate of shoreline stressors from global change and sea level rise.