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sediment sampling, together with redox potential measurements, were repeated. At
the end of the experiment, only 6% of each plot had been sampled and the precise core
positions had been noted so that nowhere was repeatedly sampled.
The weed biomass was determined twice during the experiment, after 6 weeks
(maintained density) and 20 weeks (during decay). This was done by inserting a
6x6cm corer randomly positioned into each weed treatment plot. The weed lying on
the top of the sediment surface was collected and preserved in 10% saline
formaldehyde solution. The weed was later washed in water to remove any fauna or
sand grains then dried at 80°C for 24 hours and weighed (Everett, 1991). At low tide,
the sediment was consistently more than 90% covered by the algae until it started
disappearing in early October. The net plots noticeably collected some sediment and
the nets became partially covered. Since the sediment is mobile at Drum Sands, it is
likely that this could have been natural sediment movement and not due to the
presence of the plastic mesh. Very small amounts of unanchored weed were present
within the net plots after two weeks which were almost certainly drift fragments
which had been deposited by the receding tide. These remained only loosely attached
to the plastic mesh rather than establishing themselves within the sediments.
The 5th setiger widths of the P. elegans were measured directly using the same
technique as in Chapter 3. Size measurements were carried out on the individuals
from the weed treatment and the control plots only since there were no significant
differences in their numbers between the weed treatment and net plots (see Results).
Therefore, it was concluded that there were no artefacts associated with the plastic
mesh and any differences in the size-frequency distributions between the weed
treatments and the control plots were due to the weed alone. All individuals within
these plots which were complete to at least their 10th setiger were measured.
Data analyses - Only those species with a mean abundance of more than one
individual per core for any one treatment were included for analyses. The data were
checked for normality using the Anderson-Darling test and for homogeneity of
variance using the Bartlett test. Data not meeting these criteria were transformed
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