Filter-Feeding Zoobenthos and Hydrodynamics

This chapter summarizes recent years’ studies on zoobenthic filter feeding in the sea. General principles are extracted based on experiments and mathematical modeling, mainly from own studies in shallow temperate Danish waters, in order to present primary characteristics of the sophisticated interplay between benthic filter feeders and hydrodynamics. Starting from the general concept of grazing potential and typical data on benthic population densities its realization is considered, first at the level of the individual organism through the processes of pumping and trapping of food particles for ingestion which relies on hydrodynamics. Studies have shown the importance of biomixing giving increased vertical seston flux due to mixing induced by exhalant jets of filter feeders, particularly in stagnant water but likely also in benthic boundary layers over mussel beds at moderate flow velocities. Mathematical models for such flows are discussed. At the scale of benthic boundary layers, mussels experience flows that are usually turbulent, but at the smaller scale of sublayers, colonies of bryozoans experience viscous-dominated flow that needs modeling. Finally, a case study from a particular shallow water area illustrates the effects of tide, current, and wind on vertical mixing, growth rates, and ecological implications. The main biophysical processes that may allow or prevent dense populations of filter feeders to control the phytoplankton biomass in shallow waters are presented along with remaining challenges for development of improved models for the benthic boundary layers, including effects of wall roughness, biomixing, and oscillating flows caused by waves.