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

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 m* 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 0.0 1 m* (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. Animal tubes have been implicated in both the stabilization and destabilization of seafloor sediments (Rhoads and Young 197 1; Eckman et al. 198 1). Stabilization by tubes has been suggested by field observations of reduced bed rippling (Featherstone and Risk 19 7 7) and increases in fine-grained sediments and fecal pellets around tubes (Mills 1967; Bailey-Brock 1979) and by the flume experiments of Rhoads et al. (1978). Destabilization of sediments by animal tubes is indicated by observations of scouring around single tubes (Scoffin 1970; Gage 1977). Recent flume experiments (Eckman et al. 198 1) have shown that below a certain threshold density, animal tubes may cause sediment destabilization through a sufficiently high transfer of turbulent kinetic energy to the bed. At greater densities such roughness elements reduce the flux of turbulent kinetic energy to the bed via skimming flow. The importance of biotic factors in affecting sediment stability has long been rec’ Contribution No. 6 14 from the Belle W. Baruch Institute for Marine Biology and Coastal Research. * Present address: Department of Estuarine and Coastal Ecology, Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point 23062. ognized (Ginsberg and Lowenstam 1958; Sanders 1958). P&cous binding of sediments by bacteria (Webb 1969), microalgae (Boer 198 l), meiofauna (Riemann and Schrage 1978), and macrofauna (Grenon and Walker 1980) may stabilize sediments. Alternatively, surface sediments may be destabilized by the activities of meiofauna (Cullen 1973) and macrofauna (Rhoads 1967, 1974; and many others). This destabilization may result from changes in surface microtopography, grain size, and grain exposure as well as through direct sediment .displacement by animals (Jumars and Nowell 1984). Sediments around animal tubes (and simulated tubes) can have greater abundances of bacteria (Eckman 1985), diatoms (Sanders et al. 1962), meiofauna (Eckman 1983), and macrofauna (Woodin 1978) than surrounding sediments lacking structure. Since the biota associated with tubes can alter sediment stability, the effects of tubes on sediment stability include both the direct hydrodynamic consequences of flow alterations and the indirect effects of biogenic changes. I report here the results of flume experiments on the stability of natural sediments around a small array of animal tubes. Specifically, I test whether the hydrodynamic effects of tubes alter the erodibility of nat-