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artefacts associated with the method of weed attachment and it was assumed that the
wire hoops did not confound the interpretation of the results. There seemed very little
evidence from the results of the present experiment at Drum Sands to suggest that the
plastic mesh had an effect upon either the physical or physico-chemical conditions of
the sediments or upon the fauna.
The effects of weed cover on species abundances are sometimes both dramatic and
complex and can play an important role in structuring benthic assemblages. Hull
(1988) suggested that the observed changes result from the interaction of many
factors. These include a reduced current velocity enhancing larval settlement, shelter
from predation (although some epibenthic predators may be attracted to weed mats), a
reduction in oxygen exchange between the sediment surface with the overlying water,
accumulation of silt, anoxia and the production of toxic H2S. Consequently, the
effects of weed cover on the fauna (Bonsdorff, 1992) and the mechanisms by which
these effects are brought about (Raffaelli et al., 1991) are poorly understood. Many of
the effects observed during controlled algal-manipulation experiments are similar to
those resulting from organic enrichment (Pearson and Rosenberg, 1978), notably the
large increases in Capitella capitata and Malacoceros fuliginosus (Hull, 1987, 1988;
Cha, in prep.). Hull (1987) suggested that areas such as the Ythan Estuary, Scotland,
where the community comprises many opportunistic species, are likely to cope with
the anoxic environment which algal mats create and, therefore, the observed effects of
macroalgae are likely to be small. This is in contrast to Drum Sands where reduced
sediments were well below the sediment surface and opportunistic species such as C.
cap itata were present in very low numbers suggesting that the fauna there were likely
to show a more dramatic response to the disturbance imposed by weed cover.
A review of the literature revealed that C. capitata tends to be the only benthic
infaunal species to show a consistent response to weed cover in controlled
manipulation experiments and in general, polychaetes exhibit a mixed response to
macroalgal cover (Woodin, 1977). Warren (1976) found that C. capitata colonised by
larval settlement giving increased densities during July and October. The timing of C.
capitata larval availability therefore coincided with the period of E. prolifera cover in
weed treatment plots in this experiment and presumably enabled this species to
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