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sediments containing large numbers of S. cf. wigleyi. The possible explanation for the
lack of any measurable reduction in redox in patches during the present study is not
certain, it may have been that the P. elegans densities were not high enough to
produce any discernible effect on such a spatially mutable variable, or that deposit-
feeding spionids have different effects on sediment redox potentials (cf. Featherstone
and Risk, 1977; Noji, 1994; Morgan, 1997) in different environments. In August
1998, however, the sediments of P. elegans patches were significantly more reducing
compared to non-patch sediments at 1 and 2cm depths. This result was very
unexpected since the sediments of P. elegans patches at this time contained very high
numbers of C. edule and M. balthica. These bivalves, especially the former (Reise,
1985; Flach, 1996), are bioturbators of surface sediments and their bioturbation
activity oxygenates the top few centimetres of the sediment. Therefore, one would
have expected that the patch sediments at this time would have been less reducing
than non-patch sediments. Consequently, the reasons why patch sediments at 1 cm and
2cm depths were more reducing during August 1998 remain unclear based on the
information obtained from this study.
Demise of P. elegans patches on Drum Sands.
In the few weeks preceding the August 1998 sampling, it was noticeable that the
visual appearance of the P. elegans patches at Drum Sands became less marked and it
became more difficult to distinguish them from non-patch areas. The golden brown
coloration due to diatoms was much less obvious and some of the patches had ripple-
marks across them. The samples taken during August 1998 showed that although the
numbers of P. elegans adults were still significantly higher in patch compared to non-
patch areas, there was a large decline in their numbers from the previous December
when an upward trend in P. elegans densities seemed to have been occurring
throughout 1997. By August 1998, the numbers of C. edule had significantly
increased. Although silt/clay fraction and the organic content of the sediments were
still significantly higher than those of non-patch sediments, their levels appeared to
start falling from December levels.
Populations of opportunistic species are unstable (Whitlatch and Zajac, 1985) and
dense beds are generally replaced by subsequent colonisers (Grassle and Sanders,
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