Biomixing generated by benthic filter feeders: a diffusion model for near-bottom phytoplankton depletion

Transient concentration distributions of flagellate cells (Rhodomonas sp.) previously measured by Riisgård and co-workers in laboratory experiments have been examined to develop a diffusion model for the process of phytoplankton depletion in stagnant seawater above populations of benthic filter feeders, the polychaete Nereis diversicolor and the ascidian Ciona intestinalis, respectively. The model is based on sinks located at inhalant openings and Fickʹs law with an effective diffusivity that decreases with distance above the bottom due to the biomixing generated by exhalant and inhalant feeding currents. For N. diversicolor, having inhalant and exhalant openings flush with the sediment surface and a moderate exhalant jet velocity of ∼0.01 m s−1, concentration boundary layer growth is retarded and limited by the low values of diffusivity prevailing at heights greater >∼0.05 m above the bottom. For C. intestinalis, having inhalant and exhalant openings situated ∼0.05–0.1 m above the bottom and a higher and inclined exhalant jet velocity of ∼0.1–0.2 m s−1, the concentration distributions show a nearly uniform depletion over a layer reaching a thickness of 0.2–0.3 m above the bottom due to high biomixing in this layer. Numerical predictions of concentration distributions reproduce essential features of experiments, and suggest near-bottom values of effective diffusivity of 0.3 x 10−6 and 150 x 10−6 m2 s−1, for N. diversicolor and C. intestinalis, respectively. It is suggested that the latter value is so large that the induced mixing should be accounted for in modelling benthic concentration boundary layers under flow conditions.