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 processes. 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 experimentally at 2 flow speeds (4 and 8 cm s -1 ) in a laboratory flume channel at natural (low) algal concentrations in order to determine its relative importance compared to current-generated turbulence. Distributions 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 contributing to down-mixing of phytoplankton.

Feeding Behaviour of the Mussel,Mytilus edulis: New Observations, with a Minireview of Current Knowledge

“…Under optimal conditions, suspensionfeeding bivalves filter the ambient water at a maximum rate, but under suboptimal environmental conditions, including low or very high concentrations of algal cells, the valve gape is reduced, and the mantle edges retracted [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. However, it has for a long time been a matter of discussion whether the filtration rate in suspension-feeding bivalves is physiologically regulated [14][15][16][17][18][19][20][21][22][23], or if it should be conceived as a basically autonomous process [2,5,[24][25][26][27].…”

Effects of coexistence between the blue mussel and eelgrass on sediment biogeochemistry and plant performance

“…Z. ma rina beds are suitable habitats for M. edulis, providing substrate for settlement, shelter from predators and high sedimentation of particles (Reusch 1998, Gacia et al 2002, Bologna et al 2005. Being an efficient suspension feeder, M. edulis is among the most important regulators of suspended materials in the coastal zone, and is able to filter the water up to 1 m above the mussel bed (Prins et al 1998, Dolmer 2000, Lassen et al 2006. Furthermore, M. edulis ex cretes a large part of ingested material as faeces or pseudofaeces (Kautsky & Evans 1987, Hartstein & Rowden 2004, which can enhance nutrient availability in the sediments.…”

Grazing effects of blue mussel Mytilus edulis on the pelagic food web under different turbulence conditions

“…The dynamics of exhalent jets are more complicated to resolve since they can either increase mixing (biomixing) in the logarithmic boundary layer (O'Riordan et al 1995, Larsen & Riisgaard 1997 or increase refiltration, depending on bivalve density, siphon height, filtration velocity and friction velocity (O'Riordan et al 1995, Jonsson et al 2005. A recent study showed that the exhalent jets above a 2 m-long, dense bed of Mytilus edulis increased friction velocity by 56% at low flow speeds of ~0.04 m s -1 and hence increased turbulent mixing (Lassen et al 2006). Increased turbulent mixing above the mussel bed due to roughness and biomixing was included in the applied physical water column model by increasing the roughness length by a factor of 10 in comparison with the sand bed.…”