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Dzialowski and Jessie (Dzialowski and Jessie, 2009) report that phytoplankton biomass (measured as relative fluorescence) significantly decreased in phosphatefertilized 18-L experimental mesocosms containing zebra mussels (0.5 mussels L), relative to those without mussels (see their Fig. 1). The authors present this laboratory response (i.e. decreased fluorescence) as evidence that mussels “negate or mask” the effects of nutrient stimulation of phytoplankton growth in reservoirs or natural lakes. However, like many previous laboratory (MacIsaac et al., 1992; Bunt et al., 1993) and field (Nicholls and Hopkins, 1993) studies of mussel effects on phytoplankton communities, Dzialowski and Jessie’s experiment ignores the importance of hydrodynamic coupling between the pelagic and benthic zones on the rate of delivery of pelagic phytoplankton to benthic mussels relative to the reproductive rates of the algae. In particular, experimental containers were mixed only every 3–8 days leaving ample time for phytoplankton to sink to the bottom of the containers and be consumed, whereas in natural ecosystems mixing rates are much higher. Further, in lakes, dreissenids’ effects are most apparent in the 1-m thick concentration boundary layer at the lake bottom (Ackerman et al., 2001; Edwards et al., 2005), except under conditions of a completely quiescent water column (as occurred here), high mussel clearance rates and/or low refiltration rates (MacIsaac et al., 1999; Boegman et al., 2008b). Boegman et al. (Boegman et al., 2008b) have very clearly shown the difference between restricting mussel effects to the benthos (as occurs in nature) and distributing their effects throughout the water column, even in a relatively shallow ( 7-m mean depth) and well-mixed system such as the western basin of Lake Erie. Mussels, then, obviously depend on hydrodynamics (both mixing and phytoplankton sinking) to supply food from the pelagic to the benthos. Dzialowski and Jessie’s (Dzialowski and Jessie, 2009) proposal that dreissenid mussels can provide a “buffer” from system eutrophication by “masking” the effects of nutrient pulses through their grazing on phytoplankton does not agree with published examples of post-invasion increases in phytoplankton (Conroy et al., 2005b; Raikow et al., 2004), benthic algae (Higgins et al., 2008) and cyanobacterial toxins (Knoll et al., 2008); hence, it is inappropriate to propose that highly invasive dreissenid mussels could serve to ameliorate eutrophication. The ways in which mussels cause ecosystem impairments are complex (Vanderploeg et al., 2001; Conroy et al., 2005a), although again, hydrodynamics play an essential role here because diffusion and turbulent mixing rates mediate the balance between stimulation of phytoplankton growth through mussel nutrient excretion and mussel consumption of phytoplankton (Boegman et al., 2008a; Zhang et al., 2008). Thus, determining the role of benthic dreissenid mussels in any

Mussel grazing and the importance of hydrodynamic coupling