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species, e.g., P. elegans and C. capitata, were significantly different from those in
non-patch areas. The structure of P. elegans patch communities were significantly
different from those of non-patch areas. Additionally, the physical, chemical and
physico-chemical properties of the sediments within these patches were significantly
different from those of non-patch areas. These findings are very important as marked
ecological differences within high densities of tube-builders have previously only been
documented either for species with much larger tubes, e.g., L. conchilega (Carey,
1982; Ragnarrson, 1996) and 0. fusifonnis (Fager, 1964), or for P. elegans in much
higher densities than those found in patches on Drum Sands (Dupont, 1975; Morgan,
1997). The present results suggest that tube-building polychaetes can significantly
alter their environment at much lower densities than previously thought. Since Drum
Sands is a moderately high-energy sandflat, it is possible that tube-building species
have more marked effects in high-energy sandflats compared to those in more
sheltered sandflats or mudflats.
The present study did not explicitly examine the actual processes responsible for the
differences in the fauna and sediments between P. elegans patches and non-patch
areas, this should form the basis for further research. The large numbers of tubes
emerging above the sediment surface within patches have a hydrodynamic effect,
reducing net flow and leading to sediment stabilisation (Nowell and Church, 1979;
Eckman et al., 1981). Evidence for this was given by the smooth appearance of the
patch surfaces and the presence of a golden-brown colour, assumed to be due to high
densities of diatoms. A tube-bed offers a refuge from erosion: increased sediment
cohesiveness and stability allow residents to resist resuspension by strong localised
hydrodynamic activity. Individuals of small species may otherwise have been unable
to tolerate life in such physically disturbed areas.
The findings that P. elegans patches comprised of significantly different macrobenthic
communities from surrounding sediments emphasise the importance of investigating
the processes which generate areas of high tube-building polychaete densities. The
importance of spionid tube-beds have been summarised by Noji and Noji (1991).
Basically, such beds are important in the supply of colonists for rapid colonisation of
disturbed sediments, they condition and improve sediments for future colonists and
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