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present in high numbers around sewage outfalls (Anger, 1977) and other organic
discharges (see Pearson and Rosenberg, 1978) and, therefore, is not likely to decline
in abundance due to the reducing sediments under weed mats. Other studies have
suggested that the low abundances of P. elegans under higher weed mat densities have
been an indirect response: M. fuliginosus increased in the study by Hull (1987) and it
was suggested that M. fuliginosus competed with P. elegans. Unfortunately, these
studies have not compared the sizes of P. elegans between weed and control plots and,
therefore, it is not possible to determine whether adult mortality or recruitment failure
was responsible for the negative effect of weed cover on this species abundance.
A possible cause of P.elegans decline in the experiment on Drum Sands could have
been sediment accumulation rather than the effect of siltation. During the first few
days of the experiment, 5-6cm of sediment had accumulated within the weed
treatment plots and it is perhaps unlikely that P. elegans individuals were able to cope
with such accretion and were therefore buried. This is supported by the size-
frequency distributions of P. elegans: although sediment accumulation affected all
size classes, the smaller ones were more severely affected, as would have been
expected from sediment smothering. Sediment accumulation by mats of
Enteromorpha has been reported elsewhere (e.g., Frostick and McCave, 1979) and is
probably a common phenomenon on more exposed sandflats.
In addition to the detrimental effect on adults, this study indicated that the decrease in
abundance of P. elegans in the long term was likely to have been due to a lack of
larval recruitment. Bonsdorff (1992) pointed out that the success of the settling stages
of benthic infauna are of crucial importance in determining the structure of the
community. It was shown in Chapter 3 that while P. elegans is capable of
reproducing by both benthic and planktonic larvae, only planktonic larvae were
produced by the population at Drum Sands. Size-frequency distributions clearly
showed that the decrease in adult density was mostly due to the decrease in the smaller
size classes. After 20 weeks, P. elegans size-frequency distributions in weed
treatment plots were significantly different from those in the control plots, mostly due
to the dramatic reduction of those individuals smaller than 0.33mm wide (5th setiger).
Therefore, the P. elegans larval recruitment which occurred at Drum Sands during
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