Semester of Graduation

Fall 2021

Degree

Master of Science (MS)

Department

Renewable Natural Resources

Document Type

Thesis

Abstract

The gulf ribbed mussel (Geukensia granosissima) exists throughout the Gulf of Mexico and influences biotic and abiotic environmental attributes as an ecosystem engineer. Ribbed mussels are an important component of marsh ecosystems providing services including filtering particulate matter, depositing and transforming nutrients in the system, increasing soil strength via byssal threads and providing structure via their shells.

The spatial distribution of mussels along the marsh edge of Sister Lake, LA was investigated via a broad survey of 150 sites, in relation to elevation, exposure and vegetation percent cover. This survey was followed by a second survey at a subset of 21 focal sites exploring mussel density, vegetation and soil characteristics associations. Of the 150 sites surveyed, gulf ribbed mussels were present at 46% of the sites, and where present, surface density ranged up to 350 mussels m-2 with the highest density (mean ± standard error, 34.9 ± 62.9 m-2) and presence (64%) on the north shore, and lower densities and presence along the south (1.8 ± 4.8 m2 and 38%) and west (3.7 ±7.6 m2 and 36%) shorelines. For both likelihood of mussel presence and density across all sites, total percent vegetation cover was the only significant predictive variable retained in models. Total percent vegetation cover was lower at south and west shorelines (south: 42.8 ± 2.0 %; west: 30.7 ± 2.3%), compared to north (47.8 ± 2.6%), with Juncus roemerianus percent cover following this pattern (south: 2.2 ± 0.9 %; west:10.9 ± 2.2%; north: 16.1 ± 3.0%). Exposure and elevation parameters showed no statistical significance but trended towards the north shoreline having the lowest relative elevation and the highest relative shoreline exposure, consistent with higher inundation, compared to the south and west shorelines. At the 21 focal sites, mussel density ranged from 0 to 400 mussels m-2. Overall, mussel density decreased with increased soil water content, and increased with total live belowground biomass. These surveys characterize G. granosissima distribution and habitat associations, providing insight to inform future studies, including examining more specifically, the potential role of G. granosissima in marsh production, and shoreline stabilization.

In a separate study, survivorship and movement of G. granosissima was explored in a marsh located near Cocodrie, Louisiana. 1,375 mussels were transplanted into five replicate blocks of 4 density treatments (0, 50, 100, or 400 mussels m-2) along a 75 m Spartina alterniflora marsh shoreline. Treatments were applied to the front half of each 1 m2 plot with all mussels cleared in the back half. The plots were monitored for one year, and final mussel density and location (front, back) were examined. Total mussel density decreased on average by 50.1 ± 16.1% regardless of initial density, with greater survival in, and redistribution to, more protected inland locations with higher stem densities (i.e., back plots, protected shoreline areas).

These studies demonstrate that gulf ribbed mussels are ubiquitous in coastal Louisiana marshes, can be found in high densities, and are able to be successfully transplanted. Important environmental variables characterizing mussel habitat was explored to determine where transplantations could best occur in the marsh. Together, these studies suggest gulf ribbed mussels may play a vital role in the marsh ecosystem, and this better understanding of their preferred environment means they may be viable for potential use in future coastal restoration efforts.

Committee Chair

Roberts, Brian R.

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