Physical factors affecting the spatial distribution of infaunal bivalve assemblages and species along the continental shelf of the Pacific Northwest

Graduation date: 2014The spatial distribution and abundance patterns of benthic infauna result from interactions with a host of environmental variables including sediment characteristics (percent silt-clay, grain size, total organic carbon), depth, temperature, and dissolved oxygen. This thesis focuses on the association of bivalve assemblages and species with potentially influential environmental variables along the continental shelf of the Pacific Northwest. Data for this research comes from two surveys funded by the Bureau of Ocean Energy Management (BOEM), conducted in 2010 and 2012. Across the six sites from northern California to Washington sampled in 2010, eight distinct bivalve assemblages were identified using non-metric multidimensional scaling and SIMPER analysis from the ecological software PRIMER 6. Environmental characteristics associated with each assemblage were determined. Percent silt-clay was the most useful environmental variable for distinguishing bivalve assemblages by habitat, with major differences between sandy and silty areas. Within predominantly sandy habitats, changes from one bivalve assemblage to another were associated with small differences in the remaining percentage of silt-clay in the sediment, whereas within predominantly silty habitats, changes in assemblage were associated with changes in depth, rather than exact percent silt-clay. Sediment was also associated, to varying extent, with the distributions of the most abundant bivalve species observed -- Axinopsida serricata, Nutricola lordi, Ennucula tenuis, Macoma carlottensis, and Acila castrensis. While Axinopsida serricata was a soft-sediment generalist, Nutricola lordi was a specialist, restricted to very sandy (