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METHODS
Experimental design - Eighty P. elegans patches were individually numbered from
within the 250x400m study area (Chapter 1) so that patches could be selected
randomly for experimentation. Six P. elegans patches (hereafter referred to as 'patch'
plots) were randomly chosen and a 1m 2 area marked within each by a small cane at
each corner. The patches themselves were approximately 1-1.5m 2 in area (Chapter 2).
Similarly, 6 plots were chosen outside P. elegans patches (hereafter referred to as
'non-patch' plots) by random co-ordinates within the study area and a 1m 2 area
marked in the same way as for patch plots. Six replicates were chosen as this was
decided to be the most that could be logistically sampled with respect to the amount of
work involved and this was a sufficient number to allow non-parametric statistical
analyses to be carried out if necessary. None of these plots had macroalgal mat cover
either at the time of sediment implantation or at the time of sampling.
Sediment was obtained by inserting 12 corers, 10cm diameter, into the sand within the
study area from non-patch sediments to a depth of 12cm. The sediment was kept in
the corers and frozen at -20°C for 3 days, thawed for 1 day and then refrozen.
Sediment defaunation by freezing at this temperature has been successfully achieved
in many studies (e.g., Bonsdorff and Osterman, 1984; Savidge and Taghon, 1988;
Bonsdorff, 1989; Frid, 1989; Thrush et al., 1992). This freeze-thawing was repeated 6
times for each core. After treatment, no animals were found alive (n=6) and all the
invertebrate taxa killed by this process were so severely damaged that they could
easily be distinguished from invertebrates alive at the time of subsequent sampling
(except nematodes). Any P. elegans tubes protruding from the treated sediment
surfaces were removed. The sediment cores were kept intact throughout the freeze-
thawing process, maintaining the original sediment structure was thought to reduce
erosion when later replaced in the sand.
The 12 defaunated sediments were implanted on the 18th April 1997, ensuring that the
tops were flush with the surrounding sediment surface. One defaunated sediment core
was inserted into each of the 6 patch plots and 6 non-patch plots (see Figure 6.1). A
1m2 quadrat was located on the predefined 1m 2 areas (see above), divided into 100
sections (10x10cm) with string and the defaunated sediments inserted after the
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