Population structure of the ribbed mussel Geukensia demissa in salt marshes in the southern Gulf of St. Lawrence, Canada

Abstract

The ribbed mussel, Geukensia demissa, is highly dependent on the cordgrass Spartina alterniflora for amelioration from environmental stress and substrate stabilization. Spartina alterniflora is a foundation species in marshes, and G. demissa is typically associated with cordgrass beds. Marshes in the southern Gulf of St. Lawrence are experiencing erosion and degradation, presumably as a result of increases in sea level, which increases salinity exposure and negatively impacts S. alterniflora. The population structure of the ribbed mussel, Geukensia demissa, was studied at nine sites in six estuaries in the southern Gulf of St. Lawrence in Nova Scotia, Canada, where marsh degradation is occurring. Mussel length was used as a proxy for age of G. demissa in three salt marsh zones characterized by density and elevation of Spartina alterniflora: (1) a lower zone in which the S. alterniflora was dead, but where the basal mat was coherent, (2) a zone of living, but low density S. alterniflora at the margin of the living marsh, and (3) a zone of dense S. alterniflora one to three meters back from the edge. Mussel length was significantly different across the three zones in seven of the nine sites. Mean length decreased as elevation increased, and small mussels (i.e., 1–3 cm) were absent at seven sites. The smallest mussels occurred in the dense S. alterniflora zone, higher in the marsh. Mussel length in the two western sites did not differ between zones, and small mussels (i.e., 1–3 cm) were present, but rare. The absence of small mussels in seven of the nine sites, and the size frequency distribution at remaining sites, suggests a lack of recent recruitment and a long-term threat to the survival of G. demissa. Salt marsh degradation and the death of S. alterniflora have negatively impacted G. demissa recruitment, and population decline is evident.