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formed patches less than 1m2 and thus had non-significant correlograms at this scale.
At a larger scale, the 8m survey also revealed patches of different sizes, e.g., less than
8m2 (P. elegans, E. cf flava and G. duebeni) and 8-24m2 (C. capitata, S. martinensis
and L. conchilega). The two bivalve species M. balthica and C. edule had significant
spatial structures at this scale but the sizes of their patches could not be estimated
because of their location at the edge of the survey area. At the largest scale
investigated, 8 of the 11 species with aggregated distributions exhibited significant
spatial patterns. Many of these formed patches between 40-60m2, P. elegans, E. cf
flava, C. edule and M. balthica, for example.
The sediments were also spatially structured within the study area. Again, mapping,
together with spatial autocorrelation analysis revealed that at the smallest scale,
patches of 1.5-2m2 of increased % organic carbon content, % silt/clay and sorting
coefficient were present while patches >3.5m 2 of increased median particle size (Md
4)) were present. These variables formed larger patches in the 8m survey but their
sizes could not be ascertained with any confidence because they were located at the
edge of the survey area. The 40m survey indicated that these sediment variables
formed patches approximately 80-100m 2 across the centre of the survey area.
High-density patches formed by tube-dwelling polychaetes on intertidal sandflats have
been well documented (e.g., Sanders et al., 1962; Daro and Polk, 1973; Dupont, 1975;
Featherstone and Risk, 1977; Noji and Noji, 1991; Morgan, 1997). Patches of spionid
polychaetes vary greatly in size from 0.04-9m2 (Marenzelleria viridis; Zettler and
Bick, 1996) to 150,000m2 (Clymenella torquata; Sanders et al., 1962). However,
comparisons of the spatial patterns identified in this study with those of tube-builders
found at other intertidal sandflats are difficult since most studies have been carried out
in either very different habitats and/or focused on defining smaller-scale patterns. The
few studies explicitly investigating spatial distributions at this scale have found that
soft-bottom macroinvertebrates generally exhibit significant spatial structuring on
intertidal sandflats, forming patches of various sizes. For example, Thrush et al.
(1989) investigated the spatial arrangements of polychaetes and bivalves in the
intertidal sandflats of Manukau Harbour, New Zealand. Of the 36 populations
studied, 30 were found to have significant spatial patterns, as assessed by variance to
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