Numbers of dispersers

Figure 2.14 shows the mean (n=5) numbers of Nereis caught at each station and the total hours of immersion, for each of the eight week periods. The greatest overall abundances were at Mud 1, Mud 2 and Salicornia Front. The numbers were related to elevation with the lowest abundances at the higher stations within the saltmarsh vegetation. Table 2.5 lists the total hours of immersion for each station in each period.

The first, third, fifth and seventh weeks, encompassed spring tides. During week four the lowest neap tides occurred and the Spartina and Atriplex stations were not immersed and the two sets of Salicornia traps were immersed for only 36 minutes during this week. There was a temporal variation in the numbers of Nereis caught in the traps and the total hours of tidal immersion.

Table 2.5 Sampling schedule and variation in total hours of tidal immersion for each station (Sal = Salicornia). Shaded rows indicate period in which spring tides occurred at Two Tree Island.

Stations & height above chart datum (m)

Mud 1 Mud 2 Sal Front Sal Back Spartina Atriplex 4.26 m 4.49 m 4.85 m 4.85 m 5.16 m 5.25 m

Week Sample date Total Hours Immersed

1 27^th June 55.82 47.18 30.37 30.37 8.63 3.37

49 Figure 2.14 Mean abundance (mean number per trap + SE, n=5) of Nereis recorded weekly (bar) at each of the six stations, plotted against total hours immersed (dashed line) for each sampling period.

50 Throughout there were significantly greater numbers of Nereis caught at Salicornia Front than Salicornia Back (ANOVA, F= 22.29, p = < 0.0001), (Figure 2.15) even though they are at the same tidal level. There was a significant change in numbers caught with time for these two stations (ANOVA, F = 10.37, p =< 0.0001), but differences in numbers of dispersers between the two stations remained , as there was no significant interaction term between mean abundances and sampling date (ANOVA, F = 1.51, p = 0.181).

Figure 2.15 Temporal variation in mean abundance of Nereis (mean per trap ± SE, n=5) at Salicornia Front and Salicornia Back stations.

The relationship between total hours of immersion and mean abundance for each station is presented in Figure 2.16. There was a positive correlation between the mean number of Nereis caught and the time that each trap was immersed, and for only Salicornia Front (Pearson = 0.72, p = 0.044, n = 8) and Spartina (Pearson = 0.800, p = 0.018, n = 8) was this relationship significant. When all stations were plotted against total hours of immersion, there was a significant relationship between these two factors (Figure 2.17, Pearson = 0.73 p = < 0.001, n = 48).

51 Figure 2.16 The relationship between total hours immersed and mean abundance of Nereis recorded for each of the eight sampling periods, for each station (* denotes a significant correlation, p = < 0.05) (x- and y- axes are the same scale for consistency between stations).

*

*

52 Figure 2.17 The relationship between total hours immersed and mean abundance of Nereis recorded for all stations for each of the eight sampling periods, n = 48 (* denotes a significant correlation, p = < 0.05) .

To standardise the numbers caught with immersion time the mean numbers of worms caught per hour of immersion were calculated for each station (Figure 2.18). The largest number was at Salicornia Front in week 4, but there was a large variation reported for this week (0.67 ± 0.67 S.E), and this was due to only one of the five traps having caught two individuals of Nereis during this neap tide period. There was no obvious pattern in the first four weeks of the experiment with relation to spring/neap tidal cycles. However, for the remaining four weeks the data indicate a pattern of higher dispersal rates during a spring tide period, and lower rates during a neap.

*

53 Figure 2.18 The mean numbers of Nereis (mean per trap ± SE, n=5) caught per hour of tidal immersion for each of the stations. S = spring tide period and N = neap tide period.

Figure 2.19 shows the numbers of worms caught per hour of immersion at each station, with the weekly trap data pooled for spring or neap tide periods. A higher number of captures occurred at the two lower stations, Mud1 and Mud 2 and for the front of the Salicornia zone, than at the back of the Salicornia zone and the Spartina and Atriplex stations. There was a significant difference in numbers between the stations (ANOVA F= 5.47, p = < 0.001), but no significant difference between spring and neap tide periods (ANOVA, F = 0.38, p = 0.538). The data for the eight sampling periods were pooled and the results of a post-hoc ANOVA test (Fisher‟s LSD), are shown in Table 2.6. The Fisher‟s LSD test revealed which of the six stations are significantly different to each other. There was a significantly higher rate of dispersal occurring above the muddy substrata (Mud1 Mud 2) and Salicornia front than the three more densely vegetated stations (Salicornia back, Spartina and Atriplex).

54 Figure 2.19 The mean numbers of Nereis (mean per trap ± SE) caught per hour of tidal immersion for each of the stations, plotted with height above chart datum for each station. The Spring tide data are pooled from weeks 1, 3, 5 and 7, and the Neap tide data pooled from weeks 2, 4, 6 and 8. SalF = Salicornia Front, SalB = Salicornia Back, Spar

= Spartina and Atri = Atriplex stations.

Table 2.6 Results from the non-parametric Fisher‟s LSD pair-wise test for the mean numbers of dispersers (no.hr^-1 of tidal immersion) for the six sampling stations, with data pooled from all eight sampling weeks. The stations are arranged in ascending order of means, and the lines between stations indicates a non-significant difference (p = >

0.05). Atri = Atriplex, Spart = Spartina, SalB = Salicornia Back, SalF = Salicornia Front.

STATIONS: n Atri Spart SalB Mud1 Mud2 SalF

Mean dispersal (no.hr^-1) 40 0.009 0.012 0.023 0.135 0.177 0.206

55 Core samples were not taken to assess densities of Nereis from the sediments of the six stations during this experiment. However, an earlier (October 2006) vertical transect at Two Tree Island showed Nereis to have their highest densities in the mud adjacent to the saltmarsh (Figure 2.20). Within the S. europaea agg. area Nereis densities were low, and in dense vegetation Nereis are never found (Paramor & Hughes, 2005).

Comparing the densities from Figure 2.20 against the tidal heights for the stations Mud 1 (4.26 m above c.d.) and Mud 2 (4.49 m above c.d.) it can be expected that there would have been high densities of Nereis within the sediments at those sites. For Figure 2.20 there was a peak in mean densities at 4.75 m tidal elevation, at the top of the mudflat.

Figure 2.20 Mean densities (mean per core ± SE, n = 5) of Nereis with elevation (height above chart datum) at Two Tree Island, (October 2006).

56 2.6.2 Sizes of dispersing Nereis

A total of 617 worms were caught. Figure 2.21 shows the L3 frequencies of all worms, caught at each station pooled for all sampling dates. Total body lengths for damaged worms were predicted from L3 using Equation 1, as there was a significant relationship between these two measurements for intact specimens (F= 5940, p = < 0.0001, R² = 91.2 %, n = 561 worms). No worms with L3 lengths longer than 1.30 mm (total body length = 24.84 mm) were caught and almost all had L3 lengths less than 0.50 mm (total body length = 4.71 mm).

Equation 1: Log TL (mm) = 1.197 + (1.739 * Log 10 L3 (mm))

The relationship between lengths L3 (mm) and number of chaetigerous segments (Equation 2, F= 5420, p = < 0.0001, R² = 90.6 %, n = 561 worms) was used to predict the number of segments for the damaged specimens No worms with more than 80 chaetigerous segments were caught and most had fewer , than five (Figure 2.22).

Equation 2: Log10 Number of segments = 1.811 + (1.440 * Log10 L3 (mm))

57 Figure 2.21 Length L3 (mm) frequencies for Nereis from the six stations (frequencies pooled from the eight samples).

58 Figure 2.22 Numbers of chaetigerous segments for Nereis, from the six stations (frequencies pooled from the eight samples).

59 Dales (1950), found a good linear correlation between the total length and number of chaetigerous segments for larvae up to the age of about 10 weeks (20 chaetigerous