Effect of oysters Crassostrea virginica and bottom shear velocity on benthic–pelagic coupling and estuarine water quality

Abstract

Increasing the biomass of bivalve suspensions feeders has been proposed as a means to improve water quality in eutrophic estuaries such as the Chesapeake Bay. However, water quality impacts are likely to be determined by the balance of bivalve feeding and the deposition and sediment regeneration of nutrients from particulate organic matter. In shallow-water environments, benthic and pelagic processes are closely coupled and water flow can regulate the supply of seston to the bivalves. In addition, such flow may regulate benthic–pelagic nutrient fluxes through mass transfer limitation and resuspension. We studied the interacting effects of juvenile oysters Crassostrea virginica and bottom shear velocity on phytoplankton biomass and on nutrient regeneration in a series of three 4 wk long comparative experimental ecosystem experiments. All mesocosms had a 1000 l water volume, a 1 m2 sediment surface area, and a 1 m water-column depth, and the same realistic water-column mixing (turbulence intensity 1 cm s–1). The systems included a multi-component mesocosm with moderate bottom shear velocity (0.6 cm s–1) and 2 standard cylindrical tanks with an unrealistically low bottom shear velocity (0.1 cm s–1). Oysters shifted processes to the sediments by decreasing phytoplankton biomass without stimulating additional blooms and by increasing light penetration to the bottom. Through complex and indirect relationships, the interaction of oysters and enhanced shear velocity significantly affected microphytobenthos biomass. Light, as enhanced by the oyster feeding on phytoplankton, increased microphytobenthos biomass; a moderate bottom– shear velocity eroded the biomass. Microphytobenthos biomass decreased nutrient regeneration from the sediments to the water column and may have implications for water quality in low-energy parts of shallow-water estuaries such as Chesapeake Bay. Enhanced bottom shear in more energetic parts of shallow estuaries negatively affects microphytobenthos biomass and may increase nutrient regeneration from the sediments.