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Pygospio elegans size distribution - The P. elegans size-frequency distributions from the unmanipulated control and weed treatment plots after 6 weeks and 20 weeks are shown in Figures 4.6(i-ii) and the X 2 goodness of fit and Kolmogorov-Smirnov tests results are presented in Table 4.1. The results suggest that after 6 weeks of weed cover, although there was no significant difference between the size distributions of P. elegans in the weed treatment plots compared to the unmanipulated control plots, the significant decrease in abundance in the weed treatment (Figure 4.3(ii)) was mainly due to both the smallest individuals, 0.33mm 5th setiger width (38.8% of total variation), and those individuals with 5th setiger widths between 0.50 and 0.53mm (46.6%). However, caution must be taken because of the low abundances in the weed treatment plots. 30 — 25 — 20 — '5- 10 — 35 — 30 — 25 — a-, C' 20 — 'A ,L2 15 — 10 — 5 — 0 5 — 0 INE II • , 0 Ci NJ r•I 6 (i) 6 Weeks Fifth setiger width (mm) OD 20 Weeks I , 111 ,111,111, , II 0 en en en en 6 6 06 6 Fifth setiger width (mm) 0 en N 0 en 0 control, n=145 • weed, n=40 o control, n=275 • weed, n=75 0 t•-• 0 r— N.0 6 6 6 6 6 6 Figures 4.6(i-ii) : Size-frequency results of P. elegans from control and weed treatment plots after 6 and 20 weeks. Total number of individuals measured for each treatment is given in legend. 99 %.0 rs: r-
After 20 weeks there was a significant difference in the size distribution of P. elegans between the two plot types. The significant reduction in numbers shown in Figure 4.3(iii) was mainly due to the smallest individuals, 0.33mm 5th setiger width (Figure 4.6(ii)). 5th setiger width (mm) X 2 goodness of fit K-S 0.33 0.37-0.40 0.43-0.47 0.50-0.53 0.57 P value P value 6 weeks 38.8% 1.0% 13.4% 46.6% 0.1% 0.103 >0.05 20 weeks 37.9% 0.5% 3.7% 0.1% 57.8% 0.001 90% of the sediment surface within the weed treatments. The mean biomass of the E. prolifera estimated after 6 weeks (maintained density) and 20 weeks (during decay) was 139g DW/m 2 and 66g DW/m 2 respectively. This density is a realistic value when compared with the densities naturally occurring on intertidal mud- and sandflats (see Chapter 5). Table 4.2 shows the weed densities used in published controlled, algal-manipulation experiments. The mean weed biomass in this study after 6 weeks was generally low compared to those previously 100
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After 20 weeks there was a significant difference in the size distribution of P. elegans<br />
between the two plot types. The significant reduction in numbers shown in Figure<br />
4.3(iii) was mainly due to the smallest individuals, 0.33mm 5th setiger width (Figure<br />
4.6(ii)).<br />
5th setiger<br />
width (mm)<br />
X 2 goodness of fit K-S<br />
0.33 0.37-0.40 0.43-0.47 0.50-0.53 0.57 P<br />
value<br />
P<br />
value<br />
6 weeks 38.8% 1.0% 13.4% 46.6% 0.1% 0.103 >0.05<br />
20 weeks 37.9% 0.5% 3.7% 0.1% 57.8% 0.001 90% of the sediment surface within the weed treatments.<br />
The mean biomass of the E. prolifera estimated after 6 weeks (maintained density)<br />
and 20 weeks (during decay) was 139g DW/m 2 and 66g DW/m 2 respectively. This<br />
density is a realistic value when compared with the densities naturally occurring on<br />
intertidal mud- and sandflats (see Chapter 5). Table 4.2 shows the weed densities<br />
used in published controlled, algal-manipulation experiments. The mean weed<br />
biomass in this study after 6 weeks was generally low compared to those previously<br />
100
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