ENVIRONMENTAL GRADIENTS IN A TIDAL CARBONATE LAGOON (PIGEON CREEK, SAN SALVADOR ISLAND, BAHAMAS) AND THEIR RELATIONSHIP TO MOLLUSCAN DIVERSITY, BODY SIZE, AND PREDATOR-PREY INTERACTIONS

Spatial gradients of physical and chemical environmental properties have been shown to shape biotic community structure and function in a variety of manners over many scales of observation. Here we investigate the relationship of environmental gradients with diversity, body size, and predator-prey interactions of bivalves and gastropods from the Pigeon Creek tidal carbonate lagoon, San Salvador Island, Bahamas. Temperature, salinity, substrate data, and surficial death assemblage bulk samples were collected at 26 locations along Pigeon Creek resulting in 3,407 bivalve individuals from 11 taxa and 3,220 gastropod individuals from 21 taxa. Canonical correspondence analysis ordinations were performed separately for the bivalve and gastropod taxon abundance and environmental variable data matrices in order to identify the relationship between environmental gradients and biotic composition of the samples. The primary ordination axes (CC1) for bivalves and gastropods explained 84.2% and 88.1% of the variation in the matrices, respectively, and in both cases CC1 values were strongly related to salinity (ranging from 36.3 to 45.7 psu). Richness and diversity for bivalves and gastropods are highest at normal marine salinity and decrease at elevated salinity values; these values for bivalves increase again at the highest salinities. The median body size of pooled taxa within a location are highest at normal marine salinity and decrease at elevated salinities for both bivalves and gastropods, however, there is no systematic variation in body size related to salinity when considering individual taxa that span the gradient. Drilling frequencies within single taxa decrease with increasing salinity. There is no significant difference in drilled vs non-drilled median prey size along the salinity gradient, however, the correlation coefficient between drill hole diameter and prey body size decreases sharply with increasing salinity for bivalve prey. The influence of environmental gradients on diversity, body size, and predator-prey interactions within an incipient fossil shell bed further illustrates the importance of considering stratigraphic and facies contexts for the proper interpretation of trends in the fossil record as called for by the stratigraphic paleobiological approach.