Down-mixing of phytoplankton above filter-feeding mussels—interplay between water flow and biomixing

Abstract

Filter-feeding bivalves may have a pronounced grazing impact on the phytoplankton biomass in many shallow marine areas. The blue mussel Mytilus edulis, which lives in dense beds, can filter more than 100 m 3 m -2 d -1 , but the grazing impact is highly influenced by hydrodynamic pro- cesses. Without externally generated currents or turbulent mixing, only a thin layer of near-bottom water would be subject to the down-mixing that causes an important supply of food to the mussels. Here, food-depleted jets of water expelled through the exhalant opening of a mussel may not only prevent the water, once filtered, from re-entering the animal, but the substantial speed of the jet may also help to mix the near-bottom water. The extent of such biological mixing—'biomixing'—caused by a dense population of M. edulis (200 cm long bed, filtering at 147 m 3 m -2 d -1 ) was studied experi- mentally at 2 flow speeds (4 and 8 cm s -1 ) in a laboratory flume channel at natural (low) algal concen- trations in order to determine its relative importance compared to current-generated turbulence. Dis- tributions of algal cells a distance of 162 cm from the start of the bed showed a near-bottom depletion of about 58 and 45%, respectively, for the 2 flow speeds, which indicate the degree of refiltration. In addition, flow structures were quantified in terms of distributions of velocity, turbulent shear stress and turbulent kinetic energy in the benthic boundary layer at 3 levels of mussel filtration-activity (maximal, reduced and zero). A description is given of this filtration activity of M. edulis with and without added suspensions of algal cells, which influence its valve-opening degree and filtration rate. It is concluded that biomixing enhances the flow-induced down-mixing of phytoplankton and can be identified as peaks in profiles of turbulent kinetic energy and turbulent shear stress. The associated increase in friction velocity over the length of the mussel bed at maximal filtration activity amounted to 56 and 49% for the 2 flow speeds studied. This shows the functionality of biomixing to be most helpful at low speed, where it is most needed due to the low levels of flow induced turbulence con- tributing to down-mixing of phytoplankton.