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of tubes on near-bed flow and the indirect effects of the resulting biogenic changes
(Lukenbach, 1986).
Polychaete tubes are thought to exert important and complex effects on near-bed flow.
While isolated tubes, or 'roughness elements' in hydrodynamic terms (Eckman, 1983),
destabilise sediments by creating local currents which result in sediment scour and
local deposition (Eckman et al., 1981; Lukenbach, 1986; 1987), there is a widely
accepted notion that sediments are stabilised by arrays of tubes. This opinion has
developed from both field studies (Sanders et al., 1962; Fager, 1964; Daro and Polk,
1973) and laboratory flume studies (Nowell and Church, 1979; Nowell et al., 1981;
Eckman, 1983). Tube size, geometry, and numerical density determine the properties
of near-bed flow, including the magnitude of the shear stress exerted on the bed (and
hence its susceptibility to erosion, i.e., stability), the rate of fluid transport, and the
production of turbulence (Eckman et al., 1981; Nowell et al., 1981). The shear stress
exerted on the bed affects emigration of established individuals and the composition
of the sediments, while fluid transport near the bed will affect immigration rates of
fauna dispersed passively by lateral advection. It is conceivable that these
hydrodynamic processes alone may result in the distinct faunal communities observed
in field studies. The effects of tube-beds on fluid transport have been shown to
facilitate colonisation by micro-organisms which further helps stabilise the sediments
(Eckman et al., 1981; Lukenbach, 1986) by mucus binding (Grant et al., 1986). Since
the velocity of near-bed flow generally exceeds the swimming speeds of larvae (Chia,
1989), larvae are generally advected horizontally as passive particles along the bed
(Butman, 1987) and deposition to the bottom is usually predominantly via passive
entrainment (Hannan, 1984; Pawlik and Butman, 1993; Snelgrove et al., 1993).
Eckman (1983) however, using flume experiments suggested that the velocity of near-
bed flow was reduced using seagrass mimics to such an extent that larvae would be
passively deposited. In addition to the hydrodynamic effect of their tubes, tube-
building organisms themselves significantly affect processes at the sediment-water
interface by their behaviour, e.g., feeding, burrowing, defecation and tube irrigation
(Rhoads and Young, 1970; Frithsen and Doering, 1986; Noji and Noji, 1991).
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