Shorescape‐level factors drive distribution and condition of a salt marsh facilitator (Geukensia demissa)

Abstract

Ribbed mussels (Geukensia demissa) are a highly abundant bivalve filter feeder throughout the salt marshes of the U.S. Atlantic Coast. These mussels form a mutualistic relationship with smooth cordgrass Spartina alterniflora wherein the grass provides habitat and shade to the mussels, and the mussels stabilize the sediment and fertilize the grass. Salt marshes are, however, rapidly changing and eroding as humans modify the coast, and the rate of sea level rise is accelerating. In order to understand how ribbed mussels may respond to their changing habitat, we collected mussel density and distribution data from 30 marshes covering the range of geomorphic settings found in lower Chesapeake Bay. We used a combination of in situ and GIS-derived spatial variables to develop spatially applied models of ribbed mussel density and physical condition. Of the estimated 1.06 billion ribbed mussels in Virginia, we found that mussels were most abundant along the front edge of marshes in wide creeks, rivers, or bays with dense Spartina and minimal proximal forest, set in agriculturally dominated areas. In contrast, mussel condition was highest in fringing marshes located in narrow tidal creeks. Ribbed mussels responded to factors at a variety of scales, ranging from extremely local (0.5 m) to larger shorescapes (≥300 m). The methods that we used to create models linking both aquatic and terrestrial variables to explain the variation in ribbed mussel populations along the shoreline provide a valuable tool for identifying baselines and assessing potential for change across estuary-level spatial scales not only for ribbed mussels in the Chesapeake Bay, but also for other sessile, intertidal species in other systems.

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

“…Ribbed mussels have a mutualistic relationship with cordgrass (Angelini et al 2016, Bilkovic et al 2017 b ) and higher densities of this marsh plant are correlated with mussel density (Bertness 1984, Honig et al 2015, Isdell et al 2018). We also observed this pattern for both living shorelines and paired reference marshes when mussels were present at our sites.…”

“…Within the low marsh (cordgrass dominated) area, we placed two 0.25 m 2 quadrats along each transect at 1‐m intervals representing distances of 0–1 and 1–2 m from the seaward edge and we counted the number of adult and juvenile (<25 mm) mussels and cordgrass stems in each quadrat. Previous work within Chesapeake Bay marshes has shown that the vast majority of mussels (~85%) are found within 2 m of the seaward edge (Bilkovic et al 2017, Isdell et al 2018). We averaged mussel densities across all quadrats ( N = 12) for a given site and compared marsh types using paired t ‐tests.…”

“…Ribbed mussels are most likely to thrive in areas that reduce desiccation stress (lower in the tidal frame in sediments that retain moisture under the shade of cordgrass), maximize feeding opportunities (also lower in the tidal frame and closer to the front (waterward edge) of the marsh), and lower predation pressure (in and among clumps of adult conspecifics and cordgrass roots). These factors culminate in the typical density profiles observed for ribbed mussels throughout their range, that is, high mussel density on the front edge of the marsh, and decreased density with perpendicular distance up and into the marsh (Bertness and Grosholz 1985, Nielsen and Franz 1995, Isdell et al 2018, Moody and Kreeger 2020).…”

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

“…Ecological studies have consistently found that shoreline armoring negatively impacts the intertidal and nearshore benthic and nekton communities relative to unmodified sections of shoreline via habitat fragmentation (Peterson & Lowe, 2009), changes in nearshore erosion processes (Bozek & Burdick, 2005), increased depth of nearby waters (Toft et al, 2013), reduced species abundance and diversity (Bilkovic et al, 2006;Bilkovic & Roggero, 2008;Kornis et al, 2017;Seitz et al, 2006) at both local and landscape scales (Isdell et al, 2015), and prevention of landward migration of intertidal habitats (Bilkovic, 2011;Titus et al, 2009). The ecological and social benefits of coastal wetlands (e.g., Mitsch & Gosselink, 2015) typically center around storm surge protection (Spalding et al, 2014;Shephard & Grimes, 1983), water quality enhancement (Bilkovic et al, 2017a;Erwin, 2009;Nelson & Zavaleta, 2012;Zedler & Kercher, 2005), habitat provision (Angelini et al, 2015;Isdell, Bilkovic & Hershner, 2018;Rozas & Minello, 1998), and carbon sequestration (Davis et al, 2015;Mcleod et al, 2011). Owing to the extensive ecosystem services provided by natural marshes and the unique challenges to protect coastal communities under changing conditions while supporting nearshore and intertidal ecosystems, nature-based shoreline protection is the preferred alternative to shoreline armoring where suitable.…”

Within-marsh and Landscape Features Structure Ribbed Mussel Distribution in Georgia, USA, Marshes

“…The differences in the local distribution across their range may be a result of variability in tidal heights, which can range from as low as 0.12 m in Maryland to as high as 3 m in the state of Georgia (Stiven and Gardner 1992), and the associated elevation gradients in these marshes. While previous studies have focused on factors that may influence ribbed mussel distribution throughout their range, typically focusing on factors at fine scales, such as vegetation type/density (Keunzler 1961;Watts et al 2011;Schalles et al 2013;Honig et al 2015) and distance to marsh edge (Lin 1989;Stiven and Gardner 1992;Nielson and Franz 1995), more recent studies have considered both fine-and broad-spatial scale factors (Isdell et al 2018;Julien et al 2019;, although these recent studies usually only examined two or three factors (but see .…”