Benthic-pelagic coupling in estuarine ecosystems is often mediated by suspension-feeding organisms. The contribution of this trophic group to phytoplankton dynamics, water clarity, and other biogeochemical processes is well documented in certain ecosystems, but the magnitude of the contribution is variable within and between ecosystems and may have implications throughout an ecosystem. It has been demonstrated that suspension-feeding bivalves have an impact on turbidity and phytoplankton turnover in several estuaries, while in others they have little effect. Many factors contribute to the effects of suspension feeding, including community composition, physiological processes, and hydrodynamic regime of the ecosystem. The purpose of this study was to determine the potential effect of suspension-feeding organisms on primary productivity and turbidity in several northern Gulf of Mexico estuaries. I examined three main factors that drive Rangia cuneata’s (Bivalvia; Mactridae) impact on benthic-pelagic coupling in northern Gulf of Mexico estuaries.

Suspension feeding by bivalves has a complex effect on the hydrodynamic regime around them and can influence the ultimate source of the particles that are captured in the incurrent. Both incurrents and excurrents dramatically alter the flow that passes over and around a clam, creating vertical and horizontal mixing around the siphons that can supply particles to the benthic-boundary layer. The turbulence caused by siphoning currents leads to increased mixing between the boundary layer and water column. I quantified these complex interactions and showed that they lead to a tighter coupling between benthic and pelagic regimes by moving water from above the benthic-boundary layer towards the inhalant siphon where these particles, normally unavailable to an infaunal bivalve, can be captured.

I examined food sources of Rangia cuneata using reversed-phase HPLC and optical microscopy to determine whether the particles had a benthic or suspended origin. I compared the photosynthetic pigment and species composition of water-column particulate material with that of sediments and R. cuneata gut contents collected from three estuaries (Lake Pontchartrain, LA, Biloxi Bay, MS and Mobile Bay, AL). I found that pigments in both sediments and water-column particulates were very similar, with diatom pigments being dominant. Microscopic examination of water column particulates, sediments and clam gut contents corroborated pigment data. I concluded that R. cuneata feed on a mixture of water column particles and resuspended benthic material.

The final aspect was to determine effects of Rangia cuneata on phytoplankton turnover in three northern Gulf of Mexico estuaries. I developed a model to predict the density of clams necessary to completely clear the water column in 24 and 48 hr. Factors incorporated into the model were average depth and productivity of the estuary and particle removal rate of the clam. The model predicted that the density of bivalve populations in Mobile Bay and Biloxi Bay are too low to have a significant effect on phytoplankton and particle density. The population in Lake Pontchartrain is large enough to have an effect on turbidity although not large enough to completely clear the water column. The effect of Rangia cuneata on northern Gulf of Mexico estuarine phytoplankton levels is, in general, low although it may be locally important and is dependant on many factors including bathymetry, uniformity of the bivalve population and water-column mixing.