Ribbed mussel (Geukensia demissa) demographics and interactions with cord grass (Sporobolus alterniflorus) in the southern Gulf of St. Lawrence, Nova Scotia

Abstract

Climate change and sea level rise have impacted coastal plant communities in estuaries of the southern Gulf of St. Lawrence and resulted in conspicuous marsh erosion. The ribbed mussel (Geukensia demissa) has as its primary habitat the lower marsh fringe where it is attached to the rhizomes of the cordgrass Sporobolus alterniflorus. In numerous sites, the salt marsh is eroding, resulting in the death of the cordgrass and thereby a loss of the primary mussel habitat. I investigated population density and mussel size in 16 populations in the southern Gulf of St. Lawrence from Pugwash Harbour in the west to Cheticamp Harbour in the northeast. Population metrics were measured in three marsh elevation zones: (1) a dead, low elevation zone with intact peat but no living Sporobolus shoots, (2) an intermediate zone with low density of living Sporobolus, and (3) a higher elevation, dense zone of Sporobolus. Mussel size was related to position, with the youngest mussels occurring in the intermediate zone and the oldest mussels occurring in the dense zone. Higher mussel density occurred in intermediate zones primarily at the western end of the sampled salt marshes in Nova Scotia. Marshes with extensive dead zones had the oldest mussels and lower population densities. Low mussel density and absence of relatively small mussels, i.e. < 3 cm, occurred at the most degraded marsh sites. I conclude that as sea level rise impacts the lower marsh fringe that populations of the ribbed mussel are becoming less able to recruit and maintain healthy populations. A symbiosis was described in New England in which Geukensia and Sporobolus have a mutualistic interaction. A 12-month long field experiment tested this facultative mutualism at Fox Harbour, where Geukensia is the most abundant in northern Nova Scotia. I found no evidence to support the mutualism and suggest that mussel density at ~100 m-2 may be too low to allow for the mutualism to persist in this region.