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RESULTS
Species abundances - The mean numbers per core of each species on each sampling
occasion, together with standard errors of the mean, are shown in Figures 4.3(i-iii).
Only 5 taxa were sufficiently abundant at the start of the experiment for statistical
analyses, these were the polychaetes P. elegans and C. cap itata, the bivalve molluscs
C. edule and M. balthica and oligochaetes. Prior to setting up the experiment, there
were no significant differences in numbers for any taxa between any plot type (One-
way ANOVA). After 6 weeks, 6 taxa were statistically analysed since Gammarus spp.
had increased in density within the weed treatments. Four of these showed a
significant weed treatment effect, i.e., a Tukey multiple comparison test revealed
significant differences between weed treatment and net plots. C. capitata,
oligochaetes and Gammarus spp. all showed significantly increased densities in the
weed treatment plots compared to the net plots while the abundance of P. elegans
significantly decreased. The two bivalve species, C. edule and M. balthica, showed
no treatment effect. Tukey multiple comparison tests suggested that for no species
were the net plots significantly different from the unmanipulated controls, suggesting
there were no artefacts associated with the method of weed attachment after 6 weeks.
The data for all these taxa were log(x+1)-transformed before statistical testing.
After 20 weeks, 8 taxa were analysed since the polychaetes Anaitides mucosa and
Spio martinensis were sufficiently abundant for statistical analysis in at least one of
the plot types. Of these, 5 taxa showed significant density changes in the weed
treatment. The abundance data for M. balthica, S. martinensis, A. mucosa, Gammarus
spp. and oligochaetes were log(x+1)-transformed before statistical analysis. C.
capitata, oligochaetes and Gammarus spp. again showed significant increases in
abundance with the weed treatments while P. elegans was lower in abundance in the
weed treatment plots. S. martinensis, in addition to a significant weed effect, had a
significant net plot effect, i.e., densities in the net treatments were significantly lower
than in the unmanipulated controls while there was no significant difference between
the weed and net plots. Therefore, although S. martinensis densities were
significantly lower in the weed treatments than the controls, it appears that this was
caused by the netting rather than the weed itself. This suggests that there was a slight
artefact in the method of weed attachment in the longer term, although only one
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