Sediment stability around animal tubes: The roles of hydrodynamic processes and biotic activity1

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Abstract
Experiments on the entrainment of surface sediments around polychaete tubes at natural and manipulated densities were conducted in a recirculating seawater flume. Intact cores with either 0 or 8 Diopatra cuprea tubes per 0.01 m2 plus associated assemblages were collected in the field. Sediments which contained D. cuprea tubes had natural macrofaunal densities >5 x those of sediments without tubes. To distinguish between organism‐induced sediment effects and the hydrodynamic effects of tubes on the critical entrainment velocities of these sediments I used cores with eight tubes per 0.01 m2 (tubes and high macrofaunal density), similar cores with tubes removed (no tubes and high macrofaunal density), cores with eight tubes added (tubes and low macrofaunal density), and cores without tubes (no tubes and low macrofaunal density). Critical entrainment velocities for natural cohesive sediments in these treatments were 46% lower in the cores with high densities of macrofauna than in those with low densities. The implication is that biotically mediated sedimentological changes around D. cuprea tubes, rather than alterations of near‐bed flow by the tubes, are responsible for lowering erosion thresholds in this system.