Abstract
Dense beds of filter-feeding mussels can exert a considerable grazing impact on phytoplankton in many marine areas depending on downmixing promoted by current, wave- and wind action. But downmixing may also be promoted by biomixing caused by the action of the strong exhalent jets of water from the mussels. Here we study the strength of biomixing exerted by large actively filtering blue mussels Mytilus edulis in stagnant water. Vertical concentration profiles of added algal cells (Rhodomonas salina) were measured (as chl a) over a 70 cm high and stagnant water column in an aquarium above a population of 48 ind.m-2 of mussels of shell length 69.5 ± 2.3 mm. Due to the intense agitation (biomixing) generated by exhalant jets of the actively feeding mussels the profiles remained nearly uniform over the full water column while decreasing exponentially with time, reaching a level of about 40% of the initial level after 120 min, which implied a population filtration rate of about 0.3 m3.h-1.m-2 in agreement with prior clearance measurements. Comparing to numerical solutions of a one-dimensional diffusion model, varying the eddy diffusivity, a value of D = 550 × 10-6 m2.s-1 was estimated. This high strength of biomixing far exceeds those of previous similar studies on the filter-feeding polychaete Nereis diversicolor (0.3 × 10-6 m2.s-1) and the ascidian Ciona intestinalis (150 × 10-6 m2.s-1) and suggests that biomixing in moderate benthic boundary layer flows past mussel beds may contribute to the downmixing of phytoplankton.
Highlights
Filter-feeding bivalves have a considerable grazing impact on the phytoplankton biomass in many shallow marine areas, and dense beds of the blue mussel Mytilus edulis can filter more than 100 m3.d-1.m-2 [13]
With reference to these magnitudes of diffusivity it was conjectured that biomixing would likely be of importance for turbulent benthic boundary flows of moderate strengths, a conjecture that was confirmed by laboratory flume studies [12] where velocity, turbulence and concentration profiles were measured above a dense bed of Mytilus edulis at 2 flow speeds (0.04 and 0.08 m.s-1) and 3 levels of mussel filtration activity
The difference in filtration rate of individuals obtained in the air-mixed clearance experiment (Find = 5.87 l.h-1, Table 1) and in the stagnant water biomixing experiment (Find = 6.40 l.h-1, Figure 2) is small, indicating that the biomixing alone was sufficiently strong to fully mix the whole water volume in the aquarium
Summary
Filter-feeding bivalves have a considerable grazing impact on the phytoplankton biomass in many shallow marine areas, and dense beds of the blue mussel Mytilus edulis can filter more than 100 m3.d-1.m-2 [13]. Comparison of measured concentration profiles to those calculated from a simple diffusion model [8] led to estimates of an effective diffusivity (of the order of 1 to 100 × 10-6 m2.s-1) ascribable to the process of biomixing With reference to these magnitudes of diffusivity it was conjectured that biomixing would likely be of importance for turbulent benthic boundary flows of moderate strengths, a conjecture that was confirmed by laboratory flume studies [12] where velocity, turbulence and concentration profiles were measured above a dense bed of Mytilus edulis at 2 flow speeds (0.04 and 0.08 m.s-1) and 3 levels of mussel filtration activity (maximal, reduced and zero). In situations with stagnant water (no currents or wind-driven mixing) the grazing impact of mussels is solely determined by the strength of their induced biomixing
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