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May-June 1997 (see Chapter 3) did not occur with any success in the weed treatment
plots of this experiment.
It is not possible to suggest the mechanism responsible for the lack of P. elegans
recruitment in the weed treatment plots in this study. Although reduced water velocity
in weed-affected areas may facilitate planktonic larval settlement, larvae may be
'filtered out' by the weed (Olafsson, 1988) preventing them reaching the sediment
surface. However, many studies (see Raffaelli et al., 1999) have suggested that
reduced P. elegans recruitment to weed-affected areas results from decreased juvenile
survival in the more reduced sediments and/or smothering due to siltation.
Ragnarsson (1996) found that weed significantly reduced P. elegans colonisation of
azoic sediments, however, the present study on Drum Sands is the first experimental
study which has reported that macroalgal cover has a negative effect on polychaete
recruitment to ambient sediments.
The densities of the two bivalves, C. edule and M. balthica, in this study were not
significantly affected by weed cover. Everett (1994) found that M. balthica numbers
decreased in weed plots compared to algal-free plots and suggested that tellinid
bivalves are expected to suffer from the physical barrier formed by algae between the
sediment surface and the overlying water column. The results obtained by the algal-
removal experiment of Cha (in prep.) supported this. In contrast, Hull (1987) reported
greater numbers of M. balthica under his experimentally-implanted algal-covered
plots compared to unmanipulated controls. Everett (1994) suggested that the
contrasting results obtained by his experiment and the one by Hull (1987) could have
been due to two factors. Firstly, Hull's method of weed attachment resulted in an
unnatural algal-sediment interface. However, this cannot be the reason for the lack of
a negative result in the present study because sediment accumulation resulted in the
algae being held in place below the sediment surface, similar to natural weed mats.
Secondly, Everett (1994) suggested that the Enteromorpha spp. Hull (1987) used, in
contrast to the Ulva spp. in Everett's experiment, being filamentous rather than
laminar, may not have formed a barrier between the infauna and the water column to
the same extent. However, in Hull's study and the present one on Drum Sands, a
significant decrease in the redox profiles occurred under the weed mats leading to
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