Particle depletion above experimental bivalve beds: In situ measurements and numerical modeling of bivalve filtration in the boundary layer

Suspension feeders may deplete the near-bed layer of food particles, limiting growth of downstream individuals. In a field experiment, we examined food depletion above a bed with bivalves (Cerastoderma edule) compared to beds devoid of suspension feeders and how depletion depended on boundary-layer flow.

Water above the test plots was sampled with an array of artificial siphons mimicking bivalve inhalant flow. Along the 3-m bed with bivalves, chlorophyll a (Chl a) in the near-bed layer was depleted by 5-30%. Contrary to expectations from turbulent mixing, Chl a depletion increased with friction velocity.

To explore the possibility that the bending of the exhalant jet in a strong boundary-layer flow could lead to this depletion, we studied the advection and turbulent diffusion of exhalant water by the injection of fluorescent dye through artificial siphons. The plume of fluorescent dye indicated that the interaction between the exhalant jet and horizontal water flow strongly affected the near-bed mixing of depleted water.

At high ratios between jet and friction velocities (VR), the vertical momentum of the exhalant jet reduced the proportion of exhalant water reaching downstream neighbors. A hydrodynamic model incorporating inhalant and exhalant flows in the boundary layer predicted that exhalant jet flow lines reach the bed immediately downstream when the VR ratio is <20, potentially increasing refiltration at higher flow speeds due to jet bending.

However, the model could not reproduce the observed increase in refiltration with increasing friction velocity in simulations of aggregated filtration in a bed of bivalves.