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and positions of patches. Consequently, hierarchical sampling designs are ideal pilot
surveys for field experiments and for more detailed investigations into spatial patterns
(McArdle and Blackwell, 1989).
The detection of small-scale patches of increased numbers of P. elegans was
invaluable for the present study since it provided unequivocal information as to the
scale the processes generating increased numbers of P. elegans operated.
Furthermore, their detection supported the observation that Drum Sands was a
particularly good sandflat within which the processes affecting the spatial distribution
of a tube-building polychaete could be investigated. Firstly, these patches occurred in
high numbers. Secondly, these patches were all approximately the same size which
suggested that the same process(es) were responsible for their generation. Thirdly,
these patches appeared as areas of smooth, raised sediment within a moderately high-
energy, wave-rippled sandflat, which implied that they had different hydrodynamic
properties from surrounding sediments and that they represented distinct ecological
habitats.
The processes affecting P. elegans densities and the possible role of these
processes in the formation and maintenance of small-scale patches.
Chapters 3, 4, 5 and 6 investigated the roles of population dynamics and reproductive
strategy (Chapter 3), macroalgal mat establishment (Chapters 4 and 5) and sediment
disturbance (Chapter 6) in the generation of the small-scale patches of P. elegans on
Drum Sands. Although sediment heterogeneity, sediment disturbance and life history
characteristics have been proposed as being important in the formation of patches of
increased spionid density (Noji and Noji, 1991; Zettler and Bick, 1996; Morgan,
1997), explicit investigations as to the processes creating them have not previously
been carried out. Moreover, these investigations were conducted with a good
understanding of the population dynamics and reproductive biology of the species
under study.
Chapter 3 indicated that the P. elegans population on Drum Sands exhibited a
planktonic mode of development with two main recruitment phases during the year,
April/May and November/December. This life history provides a massive larval
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