BEYOND WATER DEPTH: ENVIRONMENTAL CONTROLS ON SHALLOW SUBTIDAL MOLLUSCAN DEATH ASSEMBLAGE COMPOSITIONS ON SAN SALVADOR ISLAND, THE BAHAMAS

Gradient analyses of modern and fossil macrobenthic invertebrate communities reveal that water depth is the most common environmental factor associated with the primary variations in community composition. It has been hypothesized that other variables become apparent when water depth is held constant. As such, this study aims to identify the environmental variables controlling the distribution of mollusks in shallow subtidal environments on San Salvador Island, The Bahamas. Twenty-nine bulk samples were collected from four shallow-water (1.8–5.0 m) settings representing variable wave energy regimes and distinct habitats including seagrass meadows, open sand flats, and sand flats proximal to reefs. Seagrass and alga abundances, sediment for grain size analyses, and measurements of dissolved oxygen, pH, temperature, and water depth were also collected. Bulk samples were sieved using a 1 mm mesh. Fifty molluscan individuals were identified to the species level from a random subsample of the sieved material. Faunal counts were then analyzed using multivariate analyses (cluster analysis and Nonmetric Multidimensional Scaling [NMDS]) to identify environmental gradients associated with differences in faunal composition. Cluster analysis identified two distinct groups of sites: vegetated samples dominated by Eulithidium thalassicola, Cerithium spp., and Lucina pensylvanica and unvegetated samples dominated by Finella adamsi and Transenella spp. NMDS identified a gradient of samples with high density of seagrass to unvegetated samples along Axis 1, similar to earlier studies on San Salvador Island (Reich, 2014) and the U.S. Virgin Islands (Ferguson and Miller, 2007). A depth gradient was also strongly correlated with NMDS Axis 1. A gradient of increasing waves and dissolved oxygen was identified along NMDS Axis 2. Dissolved oxygen grades from approximately 18 to 21 percent and may be correlated with stronger waves through increased mixing in the water column. Although sampling was constrained to shallow subtidal environments to control for the effects of water depth, depth remained as a principal source of compositional variation. Intensified sampling within a single depositional environment also identified other controls on compositional, specifically seagrass cover, waves, and dissolved oxygen.