Ribbed Mussels and Spartina Alterniflora Production in a New England Salt Marsh

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Abstract
The ribbed mussel, Geukensia demissa, is commonly found associated with the salt marsh cordgrass, Spartina alterniflora. Mussels attach to the basal portion of S. alterniflora stems with strong proteinaceous byssal threads and deposit fecal material on the surrounding sediment as a byproduct of their filter—feeding activity. Here I demonstrate by manipulating mussel densities in the field that the presence of G. demissa stimulates S. alterniflora growth, and examine experimentally a number of potential mechanisms of this facilitation. In the natural habitat, mussel density is positively correlated with increased grass height, biomass, and flowering, and experimental removal and addition of mussels in these habitats demonstrates that mussels stimulate both aboveground and belowground S. alterniflora production. In tall—form S. alterniflora habitat, net primary production is positively correlated with mussel density and soil nitrogen levels. Experiments in this habitat show that mussels increase soil nitrogen, and this increase in nutrients would appear to be responsible for stimulating S. alterniflora growth. On the seaward edge of the marsh, net primary production is strongly correlated with mussel density, but not soil nutrients. On the marsh edge, mussels are shown to bind sediments and prevent erosion and physical disturbance. The relationship between S. alterniflora and G. demissa appears to represent a facultative mutualism that leads to increased marsh net primary production and stability. While previous research has shown that S. alterniflora production increases G. demissa growth and reproductive success, G. demissa also has strong stimulating effects on S. alterniflora. Nonconsumer plant—animal interactions such as the G. demissa—S. alterniflora association are potentially important determinants of marsh growth, stability, and community structure that have not been previously appreciated.