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statistical difference from net plot values using One-way ANOVA with Tukey
multiple comparison test at 5% level of significance.
Redox potentials - Redox potential has been shown to be well correlated with
sediment sulphide, oxygen, and organic matter concentrations (Pearson and Stanley,
1979). The redox potential values at the start of the experiment were very similar for
all three plot types and thus the initial values for the control plots only are shown in
Figure 4.5(i). The redox potentials significantly decreased at all sediment depths
measured in the weed treatments after 6 weeks, Figure 4.5(i), (One-way ANOVA with
Tukey test). Although the mean redox potential for the weed treatment was never
negative (except for 8cm depth with a mean of -2.3mV) the sediments at this time
were black just below the sediment surface. In contrast, the sediments did not appear
black until a depth of 7-8 cm within the net and control plots.
After 12 weeks, the effect of the weed on sediment redox potential (Figure 4.5(ii)) was
similar to that after 6 weeks. The sediments within the weed treatment plots had
significantly lower potentials for each depth relative to the net plots. The values for
the net plots were lower, but never significantly lower, than the unmanipulated
controls at every depth. This suggests that there was a slight decrease in the oxygen
exchange between the sediments and the overlying water, perhaps due to the partial
covering of the sediments by the plastic mesh. The net plot and unmanipulated
control values after 12 weeks were both noticeably lower than those after 6 weeks
suggesting that as the summer progressed the sediments at Drum Sands naturally
became more reducing. This was also apparent between the initial and the 6 week
control values, Figure 4.5(i).
After 20 weeks, when the majority of the weed had disappeared, the sediments of the
weed treatment plots were no longer significantly more reducing than those of the net
or control plots (Figure 4.5(iii)). This is similar to the other sediment variables
measured in that the effects of the weed were short-lived once the weed started to
disappear.
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