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  1. Abstract

    Freshwater mussels are dominant ecosystem engineers in many streams throughout North America, yet they remain among the world's most imperiled fauna. Extensive research has quantified the ecological role of mussels in aquatic habitats, but little is known about the interaction between mussels and their surrounding physical and hydrodynamic habitat. Here the physical interactions of mussels with near‐bed flow are investigated in an experimental channel using model mussels. The results show that (1) mussels disrupt the distributions and magnitudes of time‐averaged values of longitudinal flow velocity and Reynolds shear stress depending on mussel density, and (2) at densities of approximately 25 mussels m−2and greater, a hydrodynamic transition occurs where the maximum Reynolds shear stress is displaced from the bed to the height of the mussel canopy, near‐bed longitudinal flow velocity is reduced, and average turbulent shear stresses acting on the mussels are reduced by as much as 64%, thus markedly decreasing the dislodgement potential of the mussels by these stresses. These results provide strong empirical evidence for a positive density‐dependent effect related to flow‐organism interactions and their ecological success, such as enhancing river bed hydrodynamic habitat complexity or decreasing the turbulent shear stresses acting to dislodge mussels from the river bed. This information will improve the understanding of the long‐term persistence of mussel beds and help focus future conservation strategies.

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    In geophysical flows, the presence of sediment in suspension and transport can play an important role in modifying the turbulent properties of the carrier fluid. Much research has focused on the effects of relatively small diameter (less than 1 mm) quartz-density (2.65 g/cm3) particles on turbulent flow. Asian carp is a wide-spread invasive aquatic species in the U.S., causing severe ecological problems in rivers and lakes. Unlike sediment, Asian carp eggs are semi-buoyant particles (~1.05 g/cm3 when initially spawned and ~1.00 g/cm3 in the post-water-hardening period) whose diameter stabilizes to approximately 5 mm. This paper examines how turbulent flow is affected by the presence of particles serving as surrogates for Asian carp eggs as a function of turbulence intensity. Experiments were conducted in a mixing box with the oscillating grid placed near the bottom boundary, and 2D PIV was used to quantify the turbulent characteristics of the carrier fluid. Five paired experiments with and without Asian carp egg surrogate particles were conducted. Results show that under different grid oscillation frequencies (2 to 6 Hz), the mean kinetic energy of the carrier fluid decreased slightly in the presence of the particles, but the turbulent kinetic energy of the fluid did not change appreciably. This suggests strongly that Asian carp eggs in suspension do not modify turbulence intensity of the carrier fluid. These experimental results provide important insight into the entrainment, transport, and deposition of Asian carp eggs, which can inform models to predict the future spread of this invasive species. 
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