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studies may have been completely different environments and, therefore, not
comparable.
Controlled, experimental manipulations are much more objective and less equivocal
ways of investigating the effects of weed mats on the sediments and their fauna.
Unfortunately, of the few experiments previously carried out, Everett (1991)
suggested that the interpretation of the results is often difficult due to the lack of
experimental controls (Olafsson, 1988) or lack of replication (Reise, 1983a). Such
experiments have been carried out either by implanting weed into the sediments of
areas where weed mats are not present (e.g., Thrush 1986a; Hull, 1987) or by
removing algae from the sediments within algal-affected areas (e.g., Reise, 1985;
Everett, 1991). These two approaches have been shown to produce similar results
(Raffaelli et al., 1999). The algal-removal method, however, makes two assumptions.
Firstly, it assumes that the physical removal of weed has no effect on the sediment and
its fauna, and secondly, that the effect of the weed upon hydrodynamics is very
localised, i.e., algal-free plots are not affected by the surrounding weed. The latter
may be an unreasonable assumption for weed species which form long strands such as
Enteromorpha spp.. However, an experimental approach overcomes the inherent
problem of surveys (i.e., the large-scale separation of weed-affected and weed-free
areas) and allows variables known to affect the sediment and its invertebate fauna,
e.g., weed species, biomass, weed-mat size and the timing of weed establishment, to
be controlled. Furthermore, treatments can be fully interspersed (Krebs, 1989).
During the summer of 1996, part of the present study area within Drum Sands (Figure
4.1) was covered with weed mats. The weed (mainly Vaucheria subsimplex, with
some Enteromorpha prolifera) occurred in patches, covering approximately 30-40%
of the total sediments and 100% within patches. Unfortunately, Drum Sands is not
regularly monitored and therefore it is not known whether this was an annual
occurrence.
In order to investigate the faunal effects of these weed mats within this area of Drum
Sands, a controlled weed-implantation experiment was conducted during the summer
of 1997. It has been shown (see Chapter 3) that the life-history and the reproductive
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