K .L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209
197
2
per holdfast area m ; averages and 95 confidence intervals for these ratios were calculated Cochran, 1977.
3. Results
3.1. Substratum and exposure effects on juveniles Relative to the percent cover of substrata types open for recruitment Fig. 3, the
number of juveniles found on each type were different Table 1 and juveniles were most likely to be attached to articulated coralline algae. Only juveniles on articulated
corallines AC in the exposed site and on articulated AC and crustose coralline algae CC in the protected site had sufficient sample size n
.2 and thus were the only sample groups used in analyses. Thus, juvenile attachment data were used to compare
the difference between juvenile attachment on AC in exposed and protected sites, and to compare the attachment between AC and CC within the protected site.
For all substratum types and wave-exposure sites, juveniles primarily broke at the holdfast substratum boundary Fig. 4. All holdfasts completely broke off the sub-
stratum, leaving no holdfast remnants; for this reason, holdfast break area is considered
Fig. 3. Percent cover of substrata available for recruitment. Substratum types are abbreviated as AC, articulated coralline algae; CC, crustose coralline algae; RC, non-calcareous encrusting red algae; BR, bare
rock. Data are from exposed solid bars and protected open bars sites, n 510, and error bars represent 95
confidence intervals. Recruitment also occurs on Hedophyllum sessile holdfasts, and these data are not shown.
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.L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209 Table 1
Differential juvenile recruitment by Hedophyllum sessile on different substrata types in May 1995, calculated
a
by Eq. 1 Substratum type
Differential recruitment Exposed site
Protected site Articulated coralline algae
3.29 2.06
Crustose coralline algae 0.18
1.72 Non-calcareous encrusting red algae
Bare rock 0.37
0.38 Total sample size
n 536
n 558
a
Differential recruitment is the percent of juveniles found on each substratum type normalized by the percent cover area of that substratum available for recruitment. Substratum bias for recruitment is
demonstrated by differential recruitment values exceeding one.
equal to the holdfast area which was originally attached. Analysis of holdfast removal data Fig. 5 showed that juveniles on articulated coralline algae AC in the exposed
site required significantly more force to remove than those on AC in the protected site
Fig. 4. Break locations for juveniles on different substrata and at two sites. Breaks abbreviated as HO, holdfast; ST, stipe; S BL, stipe blade junction; SU, substratum. Sites labeled as EXP, exposed site; PRO,
protected site. Samples from the exposed site on articulated coralline algae EXP,AC are represented by solid bars. Samples from the protected site are represented by open bars for articulated coralline algae PRO, AC
and by open, hatched bars for crustose coralline algae PRO,CC.
K .L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209
199
Fig. 5. Forces d required to break juvenile holdfasts and the holdfast break areas n on different substrata and at two sites. See Fig. 4 for site and substratum abbreviations. Error bars are standard error. Asterisks above
symbols represent groups that are significantly different one-way ANOVA from each other: P 0.001,
and 0.001 ,P0.01. Groups significantly different were: holdfast break area in protected site PRO,AC.
PRO,CC at 0.001 ,P 0.01; removal force from articulated coralline algae EXP,AC.PRO,AC at P 0.001;
and removal force in the protected site PRO,AC .PRO,CC at 0.001,P0.01.
P ,0.001, n523. Within the protected site, juveniles on AC required more force to
remove than those on crustose coralline algae CC; P 50.003, n523 and had less
juvenile holdfast area attached on CC than AC P 50.008, n523. Juveniles on CC,
therefore, had weaker holdfast strength than juveniles on AC in the protected site Table 2.
3.2. Patterns of adult attachment Adult holdfast mechanics were quantified at pre-storm and post-storm dates. For both
sampling dates and compared to the number of complete holdfast breaks, partial holdfast breaks occurred rarely in adult Hedophyllum sessile Fig. 6. As with the juveniles, adult
H . sessile breaks primarily at the holdfast base Fig. 7. However, upon removal, either
small remnants of holdfast were left holdfast-only break or some of the coralline algae beneath the holdfast holdfast substratum break was removed as well. Since both of
these break-types resulted in complete thallus dislodgment, both of these types of breaks were considered holdfast breaks and pooled for statistical analyses.
For complete holdfast breaks in each site, shoreward and seaward pulls were not
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.L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209 Table 2
22 a
Hedophyllum sessile holdfast attachment strengths MN m 6S.E. of sporophyte juveniles and adults
22
Sample group Holdfast attachment strength MN m
6S.E. n
Juveniles Exposed, AC
0.023 60.002
32 Protected, AC
0.012 60.002
31 Protected, CC
0.005 60.002
23 Adults
Exposed, pre-storm 0.070
60.011 17
Exposed, post-storm 0.078
60.012 17
Protected, pre-storm 0.073
60.015 17
Protected, post-storm 0.045
60.005 17
a
AC represents juveniles on articulated coralline algae and CC represents juveniles on crustose coralline algae; n is the sample size within each sample group.
different in removal force P 50.279, n511 or holdfast break area P50.797, n511.
Shoreward and seaward pulls were pooled and used to compare force to break and break area between sites and seasons.
Fig. 6. Proportion of complete holdfast breaks in adult Hedophyllum sessile at two sites in pre-storm and post-storm sampling dates. Complete holdfast break was when the whole holdfast separated from the
substratum. A partial holdfast break was when only part of the holdfast detached, leaving a portion of the holdfast on the substratum. Sites are labeled as EXP, wave-exposed site; PRO, wave-protected site. Solid bars
are complete breaks and open bars are partial breaks.
K .L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209
201
Fig. 7. Percent occurrence of different break locations for adults at two sites in pre- and post-storm sampling dates. Data include only complete holdfast breaks from Fig. 6 and do not include partial holdfast breaks.
Breaks are abbreviated as HO, holdfast only total holdfast separates from the substratum; H S, holdfast and substratum total holdfast detaches with the underlying substratum; TTL HO, holdfast only
1holdfast substratum; BL, blade holdfast does not detach but the blade breaks. Samples from the exposed site are
represented by solid bars and those from the protected site are represented by open bars.
The force pooled mean 5100.69 N66.79 S.E. required to remove H. sessile adults
was consistent over sites and seasons one-way ANOVA, P 50.355, n513; Fig. 8.
2
Holdfast break area pooled mean 50.0019 m 60.0001 S.E. was also consistent over
sites and seasons, with the exception that the protected, post-storm group was significantly larger than the exposed, pre-storm group one-way ANOVA, P
50.025, n
513. Thus, the protected, post-storm group had a weaker holdfast attachment strength than other experimental groups Table 2. There was no direct relationship between
holdfast break area and the force to remove in the pre-storm sampling, but a significant positive relationship existed in the post-storm sampling at both sites exposed post-
storm: R 50.612, P50.009, n517; protected post-storm: R50.563, P50.019, n517.
Larger holdfasts supported more thallus area Fig. 9, and holdfasts at both sites support significantly less blade area in the post-storm sampling than in the pre-storm sampling
date Table 3. When a holdfast was not completely removed and only part of the holdfast became
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.L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209
Fig. 8. Forces d required to break adult holdfasts and the holdfast break areas n at two sites in pre- and post-storm sampling dates. Sites are labeled as EXP, wave-exposed site; PRO, wave-protected site. Error bars
are standard error. Asterisks above symbols represent the sampling dates and sites for holdfast break area, pre-storm EXP
,post-storm PRO that are significantly different, 0.01,P0.05 one-way ANOVA.
detached, there was preliminary evidence that the remaining holdfast on the rock was attached less firmly. However, sample sizes were too small for statistical analyses;
therefore, it can not be concluded that partial removal affects the integrity of the remaining holdfast.
Even though analysis of attachment properties using one-way ANOVA showed no differences in average attachment forces among sites and seasons Fig. 8, there is a
seasonal effect on the distribution of attachment forces within the sampled populations. Despite no change in holdfast size, within each site, there is a shift to more resistant
holdfasts as a result of increased seasonal wave-exposure Fig. 10. For simplicity, 95 confidence intervals Table 4 are not shown around the estimated distributions in Fig.
10; however, above the normalized force of 1.5, there is a seasonal effect at each site P
,0.05 towards more resistant holdfasts where the post-storm population had a lower probability of removal than the pre-storm population.
4. Discussion
