Citation: Bilkovic, D. M., M. M. Mitchell, R. E. Isdell, M. Schliep, and A. R. Smyth. 2017. Mutualism between ribbed mussels and cordgrass enhances salt marsh nitrogen removal. Ecosphere 8(4):e01795. 10.1002/ecs2.1795Abstract. Salt marsh ecosystems have declined globally and are increasingly threatened by erosion, sea level rise, and urban development. These highly productive, physically demanding ecosystems are populated by core species groups that often have strong trophic interactions with implications for ecosystem function and service provision. Positive interactions occur between ribbed mussels (Geukensia demissa) and cordgrass (Spartina alterniflora). Mussels transfer particulate nitrogen from the water column to the marsh sediments, which stimulates cordgrass growth, and cordgrass provides predator and/or heat stress refuge for mussels. Here, we test mussel facilitation of two functions in salt marshes that relate to N removal: microbial denitrification and water filtration. Microcosm experiments revealed that the highest rates of N 2 production and nitrification occurred when mussels were present with marsh vegetation, suggesting that mussels enhanced coupling of the nitrification-denitrification. Surveys spanning the York River Estuary, Chesapeake Bay, showed that the highest densities of mussels occurred in the first meter for all marsh types with mainstem fringing (1207 AE 265 mussels/m 2 ) being the most densely populated. The mussel population was estimated to be~197 million animals with a water filtration potential of 90-135 million L/hr. Erosion simulation models demonstrated that suitable marsh habitat for ribbed mussels along the York River Estuary would be reduced by 11.8% after 50 years. This reduction in mussel habitat resulted in a projected 15% reduction in ribbed mussel abundance and filtration capacity. Denitrification potential was reduced in conjunction with projected marsh loss (35,536 m 2 ) by 205 g N/hr, a 16% reduction. Because of the predominant occurrence of ribbed mussels at the marsh seaward edge and because the highest proportional loss will occur for fringing marshes (20%), shoreline management practices that restore or create fringing marsh may help offset these projected losses.

Aim Estuaries world‐wide have been modified or fragmented due to human stressors in their terrestrial and aquatic components. Estuary fragmentation often results in reductions in species richness, diversity and connectivity. Effects of human modification on estuaries have been well studied, but less is known about how land use alters connectivity of the terrestrial–aquatic ecotone. We studied the relationship between terrestrial–aquatic connectivity and the distribution of an estuarine turtle, diamondback terrapin (Malaclemys terrapin). Location Chesapeake Bay, Virginia, USA. Methods We conducted diamondback terrapin surveys at 165 sites from late spring to mid‐summer in 2012 and 2013. We evaluated associations between terrapin occurrence, land use, salt marsh, shoreline armouring and crabbing intensity in concentric–circular neighbourhoods ranging from 0.27 to 2 km to cover daily and annual terrapin movements. We used occupancy modelling and model averaging to identify key terrestrial and aquatic variables explaining heterogeneity in terrapin occupancy. We evaluated the final model with an independent data set and identified occurrence thresholds for key variables. Results Diamondback terrapin occupy areas with ≥ 10% of marsh within a 750‐m neighbourhood, ≤ 17% armoured shoreline within a 1‐km neighbourhood, ≤ 20% of agriculture within a 500‐m neighbourhood, ≤ 33% low‐density housing within a 270‐m neighbourhood and ≤ 9 active crab pots within a 270‐m neighbourhood. Our model performed well when evaluated with an independent data set. Main conclusions We are the first to identify thresholds and quantify negative associations between the distribution of diamondback terrapin and alterations to terrestrial–aquatic connectivity from land development, shoreline armouring, and fishing activity. Because diamondback terrapin responses are reflective of changes in coastal habitats, especially marshes, terrapin occurrence can be used to direct wetlands conservation and restoration efforts.

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Nature-based shoreline protection provides a welcome class of adaptations to promote ecological resilience in the face of climate change. Along coastlines, living shorelines are among the preferred adaptation strategies to both reduce erosion and provide ecological functions. As an alternative to shoreline armoring, living shorelines are viewed favorably among coastal managers and some private property owners, but they have yet to undergo a thorough examination of how their levels of ecosystem functions compare to their closest natural counterpart: fringing marshes. Here, we provide a synthesis of results from a multi-year, large-spatial-scale study in which we compared numerous ecological metrics (including habitat provision for fish, invertebrates, diamondback terrapin, and birds, nutrient and carbon storage, and plant productivity) measured in thirteen pairs of living shorelines and natural fringing marshes throughout coastal Virginia, USA. Living shorelines were composed of marshes created by bank grading, placement of sand fill for proper elevations, and planting of S. alterniflora and S. patens, as well as placement of a stone sill seaward and parallel to the marsh to serve as a wave break. Overall, we found that living shorelines were functionally equivalent to natural marshes in nearly all measured aspects, except for a lag in soil composition due to construction of living shoreline marshes with clean, low-organic sands. These data support the prioritization of living shorelines as a coastal adaptation strategy.