Benthic-pelagic coupling in northern Gulf of Mexico estuaries: Contribution of the suspension-feeding bivalve, Rangia cuneata (Bivalvia: Mactridae)

Benthic-pelagic coupling in estuarine ecosystems is a process mediated by suspension-feeding organisms such as bivalves. The contribution of this trophic group to phytoplankton dynamics, water clarity, and other biogeochemical processes is well documented in certain ecosystems. It has been shown that suspension-feeding bivalves can have an impact on turbidity and phytoplankton turnover. The current research investigated factors that drive a bivalve species' (Rangia cuneata; Mactridae) impact on benthic-pelagic coupling in northern Gulf of Mexico estuaries. It quantified complex interactions between cross flow and feeding currents that lead to tighter coupling between benthic and pelagic regimes using a laboratory flume and video of live bivalves' siphoning activity. These interactions move water from above the benthic-boundary layer towards the inhalant siphon where particles, normally unavailable to an infaunal bivalve, can be captured. This research also examined the food source of Rangia cuneata using HPLC and optical microscopy. Photosynthetic pigments and species composition of water-column particulate material were compared with those of sediments and gut contents. Pigments in sediments and water-column particulates were very similar, with diatoms the dominant taxon. Rangia cuneata utilizes a mixture of food including suspended benthic microalgae, phytoplankton and detritus. The final aspect of this research quantified the effect of R. cuneata populations on particle concentration in three important estuaries of the northern Gulf of Mexico: Lake Pontchartrain, LA, Biloxi Bay, MS, and Mobile Bay, AL. A model was developed to predict the density of clams necessary to completely clear the water column in 24 or 48 hr. This model predicted that the R. cuneata populations in Mobile Bay and Biloxi Bay do not affect phytoplankton or particle density while the Lake Pontchartrain population is large enough impact turbidity. Local regions of northern Gulf of Mexico estuaries that have large bivalve populations may also be controlled by suspension feeders. In general, benthic-pelagic coupling is enhanced by Rangia cuneata in localized regions of northern Gulf of Mexico estuaries that are characterized by large bivalve populations, little mixing of the water and shallow depth.