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likely that the tube structure of dead worms remain intact for some time after the
occupant has died. In this way, the direct hydrodynamic effect of P. elegans tubes
within patches in this study may have been far greater than that estimated from worm
densities alone.
Sediment stabilisation by P. elegans has been previously demonstrated by shear
strength measurements (Meadows and Hariri, 1991; Morgan, 1997). Although such
measurements were not taken in this study there is strong evidence to suggest that the
sediments within P. elegans patches on Drum Sands were stabilised, i.e., the lack of
ripple marks in an otherwise rippled area, the increased silt/clay fraction, and the
presence of a golden hue, assumed to be diatoms, on the sediment surface. Eckman et
al. (1981) suggested that these observations, especially the latter, provided strong
evidence of sediment stabilisation. In the studies by Sanders et al. (1962) and Fager
(1964), sediment stabilisation was also assumed by the observations of a golden hue,
confirmed to be diatoms by chlorophyll analysis in the former study, and the absence
of ripple marks where highest tube densities were found. The polychaete densities
producing these effects in their studies were also far below those expected for
sediment stabilisation using the empirical formula of Nowell and Church (1979), i.e.,
observed densities being 600 C. torquatalm2 and 500-1,000 0. fusiformislm2 in the
studies by Sanders eta!. (1962) and Fager (1964), respectively. For example, Eckman
et al. (1981) calculated that a density of at least 14,500 0. fusiformislm2 was
necessary for sediment stabilisation.
A number of studies have experimentally shown that extracellular mucus films
produced by macrofauna during tube construction (Meadows and Tufail, 1986), and
diatoms can stabilise sediments (Frostick and McCave, 1979; Grant et al., 1986;
Tufail eta!., 1989; Paterson, 1994). Sediments around tubes have been shown to have
increased numbers of diatoms (Sanders et al., 1962) and meiofauna (Eckman, 1983;
Noji, 1994). The dense films of diatoms observed on the P. elegans patch surfaces
during this study support the contention that indirect sediment stabilisation was
possibly appreciable. Therefore, the observed differences in colonisation between
patches and non-patches in this study probably resulted from differences in sediment
stability.
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