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lack, hydrogen sulphide-smelling sediments, suggesting that oxygen exchange
between the sediments and the overlying water column was severely reduced.
Raffaelli et al. (1999) suggested that the differences observed between the studies by
Everett (1994) and Hull (1987) was due to M. balthica in Hull's study being settling
spat while they were adults in Everett's study. The low ambient abundances of the
two bivalve species in the present study were probably partly responsible for the lack
of a significant weed effect, although increased food and more reducing sediments
could have been two opposing factors.
Macroalgal mats and P. elegans patch formation.
The results of this experiment support the conclusion of Thrush (1986a) who proposed
that patchily distributed accumulations of macroalgae result in spatial variations in the
faunal structure of the benthos. However, the results showed that the E. prolifera
implanted during this experiment on Drum Sands did not, at least in the short term,
result in increased numbers of P. elegans and, therefore, did not support the
suggestion that macroalgal mat formation has a role in P. elegans patch establishment.
Instead, the faunal communities under weed mats shortly after implantation, to just
after weed disappearance 20 weeks later, were dominated by C. capitata, with
increased numbers of oligochaetes and gammarids. Unfortunately, monitoring P.
elegans densities in weed treatment plots in the longer term was rendered impossible
due to mussel colonisation onto the plastic netting in the weed treatment and net plots.
Therefore, although E. prolifera had a negative effect on P. elegans, both on the post-
larval population and on larval recruitment, the potentiality of P. elegans later
increasing in numbers remains unresolved. An increase in P. elegans population
density would only have been possible during late November/early December when
larval recruitment occurred (Chapter 3), a time when weed-affected areas would have
been numerically dominated by C. capitata.
Controlled, manipulation experiments potentially allow a less equivocal way of
determining the effects of weed mats on intertidal sandflats. However, since the biotic
and abiotic responses are dependent upon factors such as weed type, biomass and
timing of establishment, experiments are unlikely to unequivocally link macroalgal
mat establishment with invertebrate patch formation in a particular environment
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