Ribbed mussel Geukensia demissa population response to living shoreline design and ecosystem development

Abstract

Coastal communities increasingly invest in natural and nature-based features (e.g., living shorelines) as a strategy to protect shorelines and enhance coastal resilience. Tidal marshes are a common component of these strategies because of their capacity to reduce wave energy and storm surge impacts. Performance metrics of restoration success for living shorelines tend to focus on how the physical structure of the created marsh enhances shoreline protection via proper elevation and marsh plant presence. These metrics do not fully evaluate the level of marsh ecosystem development. In particular, the presence of key marsh bivalve species can indicate the capability of the marsh to provide non-protective services of value, such as water quality improvement and habitat provision. We observed an unexpected low to no abundance of the filter-feeding ribbed mussel, Geukensia demissa, in living shoreline marshes throughout Chesapeake Bay. In salt marsh ecosystems along the Atlantic Coast of the United States, ribbed mussels improve water quality, enhance nutrient removal, stabilize the marsh, and facilitate long-term sustainability of the habitat. Through comparative field surveys and experiments within a chronosequence of 13 living shorelines spanning 2-16 years since construction, we examined three factors we hypothesized may influence recruitment of ribbed mussels to living shoreline marshes: (1) larval access to suitable marsh habitat, (2) sediment quality of low marsh (i.e., potential mussel habitat), and (3) availability of high-quality refuge habitat. Our findings suggest that at most sites larval mussels are able to access and settle on living shoreline created marshes behind rock sill structures, but that most recruits are likely not surviving. Sediment organic matter (OM) and plant density were correlated with mussel abundance, and sediment OM increased with marsh age, suggesting that living shoreline design (e.g., sand fill, planting grids) and lags in ecosystem development (sediment properties) are reducing the survival of the young recruits. We offer potential modifications to living shoreline design and implementation practices that may facilitate self-sustaining ribbed mussel populations in these restored habitats.

Living shorelines achieve functional equivalence to natural fringe marshes across multiple ecological metrics

“…One particularly important nuance of our findings is the role of the sill structure in achieving functional equivalence for bivalves. Ribbed mussel recruitment and survival in the low marsh of living shorelines lags well behind what we observe in nearby natural fringing marshes (Bilkovic et al, 2021), likely as a result of a challenging post-settlement environment. There is something of a quandary in which the lack of adult conspecifics increases predation and desiccation risk to new recruits.…”

“…Soil organic matter and nutrients in plant root zone generally accrue with living shoreline age, but the accumulation rates are non-linear. With rapid vegetation establishment (Bilkovic et al, 2021;Currin, Delano & Valdes-Weaver, 2008), however, and longer-term carbon sequestration and nitrogen and phosphorus accumulation over timescales measured in decades (Chambers et al, 2021;Davis et al, 2015), living shorelines appear to be on a trajectory to approach soil equivalence with natural reference marshes.…”

“…While the geographic scope of our work was restricted to Chesapeake Bay, Virginia, USA, the ecological processes and anthropogenic pressures in our study area are common along the US Atlantic seaboard as well as other temperate regions wherever salt marshes occur. If living shorelines are constructed according to best design practices Bilkovic et al, 2021), they can provide functional equivalence to natural fringe marshes for most of the 17 ecological metrics that we examined. Living shorelines are constructed in a way that reduces erosion and allows for landward migration with SLR, thereby making them a resilient alternative to shoreline armoring while maintaining functional equivalence to natural fringing marshes.…”

“…To standardize comparisons, all selected living shorelines were constructed using clean sand fill, cordgrass plantings, and rock sill. Pairs were distributed across a gradient from predominantly rural to predominately developed surroundings (see Bilkovic et al (2021) for greater detail on site selection protocol). High marsh was sparse to non-existent at most of the NM sites, so comparisons were limited to the low marsh.…”

“…where y i represents the net functional equivalence score for pair i, and x i is the age of the living shoreline of pair i. Parameter β 0 is the intercept and β 1 is the estimate of the slope for age. Weakly informative priors were used for the intercept (β 0 ) and standard deviation (σ ), but a modestly informative prior was set to reflect expectations of a slightly positive effect of age (β 1 ) given previously published literature (Onorevole, Thompson & Piehler, 2018;Bilkovic et al, 2021;Chambers et al, 2021) as well as our original hypothesis that living shorelines would become more similar to natural marshes over time. The model was run with four chains for 50,000 iterations and 5,000 warm-up samples, and compared to the null model using leave-one-out cross-validated information criterion (LOOIC; Vehtari, Gelman & Gabry, 2017).…”