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. Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
by the alga through the spread originating from the three experimental units; in fact, after spring 1998 the three algal patches belonging to each area were mostly
indistinguishable because of edge collapse and thus we decided to measure the total surface coverage per area. As above, data relative to the last sampling time August
1998 were analysed by a two-way ANOVA with ‘position’ two levels and ‘density’ three levels as factors.
For all analyses ‘position’ and ‘density’ were treated as fixed and orthogonal, while ‘area’, ‘time’ and ‘plot’ random, where ‘area’ is nested in the ‘position
3 density’ interaction, ‘plot’ is nested in ‘area’ and ‘time’ is orthogonal. Cochran’s test was used
prior to the analyses to remove heterogeneity of variances when necessary and SNK test to compare levels of significant factors.
3. Results
Vertical blade length of Caulerpa racemosa at Posidonia oceanica edge was greatly affected by the season; minimum size was found in winter while maximum was
observed in summer Fig. 1. Variable size was also evident at a small spatial scale, as there were significant differences among time
3 plot combinations Table 1. Vertical blade length underwent great fluctuations through time also depending on shoot density
and position significant ‘position 3 density 3 time’ interaction, Table 1: smaller blades
were found at the edge of higher P . oceanica shoot density 50 and 100 with respect
to the less dense treatment areas, especially at the east position Table 1A SNK test. Despite the very large size of blades found along 10 shoot density areas at the east
Table 1 C
. racemosa: analysis of variance on algal blade length cm
a
Source of variation df
MS F
P Position
5 Po 1
8.63 Density
5 De 2
8.54 Time
5 T 2
40.56 Area Po
3 De 5 A 12
2.75 Plot APo
3 De 5 Pl 36
1.04 Po
3 De 2
6.13 Po
3 T 2
4.59 De
3 T 4
0.85 T
3 APo 3 De 24
0.39 0.58
0.9317 T
3 Pl APo 3 De 72
0.68 3.21
0.0000 Po
3 De 3 T 4
1.40 3.53
0.0209 Error
162 0.21
Cochran’s test C
5 0.0638 P
. 0.05
a
Po 5 position, De 5 density, T 5 time, A 5 area and Pl 5 plot.
Significant.
G . Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
233 Table 1A
SNK test of the interaction term Po 3 De 3 T Table 1: i shoot density of 10, 50 and 100; ii position,
W 5 west and E 5 east; iii time, Oct 5 October 1997, Aug 5 August 1998 and May 5 May 1998, SE 5
0.148, df 5 24
i Shoot density W Oct
100 5 50 5 10
W May 100
5 50 5 10 W Aug
100 , 50 5 10
E Oct 100
5 50 , 10 E May
100 5 50 , 10
E Aug 100
5 50 , 10 ii Position
Oct 10 E
. W Oct 50
E 5 W
Oct 100 E
5 W May 10
E 5 W
May 50 E
, W May 100
E , W
Aug 10 E
, W Aug 50
E , W
Aug 100 E
5 W iii Time
W 10 AUG
. MAY 5 OCT W 50
AUG . MAY 5 OCT
W 100 AUG
. MAY 5 OCT E 10
AUG . MAY 5 OCT
E 50 AUG
. OCT . MAY E 100
AUG . OCT . MAY
position, taller blades were found at the west position with respect to the east either in August or May, 10 vs. 50 and 50 vs. 100 densities, respectively. Blade length was
significantly greater in August relative to October and May of all position–density combinations, except at the east edge of the seagrass where the short-term effect
October 97 of higher densities 50 and 100 was intermediate than the longer-term one May 98.
Growth of stolons of Caulerpa racemosa into the meadow from transplants also underwent great fluctuations with high penetration distance found during Autumn 1997,
followed by a loss of penetration in winter–spring 1998 Fig. 2. However, the direction of growth appears to be into the meadow with the greatest distance of growth recorded
after 1 year of experiment summer 1998, when stolons had elongated on average from 25 to 50 cm. Density greatly affected this response variable: significant higher
penetration into the seagrass meadow was observed where shoot density was lowest 10 Table 2, SNK test. Although not significant, position of C
. racemosa at the east or west margin of the seagrass meadow had a moderate effect on the short-term, as
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. Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
Fig. 2. Temporal variation of mean 6SE Caulerpa racemosa stolon penetration or growth cm into the
Posidonia oceanica meadow to treatments of 10, 50 and 100 of shoot density at the east and west edge of the seagrass meadow n
5 9.
penetration was higher 53 at the west position where shoot density was 50 Fig. 2. However, stolon elongation of the alga was highly dependent on the experimental area.
Percent cover of Caulerpa racemosa in the experimental units was significantly
Table 2 C
. racemosa: analysis of variance on penetration distance of stolons into the meadow cm Source of variation
df MS
F P
Position 5 Po
1 1075.57
1.67 0.2205
Density 5 De
2 2634.02
4.09 0.0442
AreaPo 3 De 5 A
12 643.96
5.79 0.0000
Po 3 De
2 82.57
0.13 0.8808
Error 36
111.29 Cochran’s test
C 5 0.276
P . 0.05
Significant: SNK test of the term De: SE 5 5.98, df 5 12. 10 . 50 5 100.
G . Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
235 Table 3
C . racemosa: analysis of variance on algal percent cover in each experimental unit
Source of variation df
MS F
P Position
5 Po 1
146.61 0.46
0.5080 Density
5 De 2
462.84 1.47
0.2685 AreaPo
3 De 5 A 12
314.91 8.58
0.0000 Po
3 De 2
609.02 1.93
0.1870 Error
18 Cochran’s test
C 5 0.318
P . 0.05
affected by the characteristics of the area and not by other sources of variation Table 3. Although sampling time was not evaluated by the analysis, temporal fluctuations were
observed for this variable and a decrease of cover was evident during winter Fig. 3. However, after 1 year of growth algal cover had increased to more than 80 of the
sampling unit.
Fig. 3. Temporal variation of mean 6SE percent cover of the experimental unit substrate by Caulerpa
racemosa at the edge of defoliated Posidonia oceanica 10, 50 and 100 of shoot density at the east and west edge of the seagrass meadow n
5 6 .
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. Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
Fig. 4. Temporal variation of mean 6SE total coverage of the experimental area substrate by Caulerpa
racemosa at the edge of defoliated Posidonia oceanica 10, 50 and 100 of shoot density at the east and west edge of the seagrass meadow n
5 3.
The estimation of total area of substrate affected by Caulerpa racemosa in each experimental area has shown a substantial growth after 9 months of study from spring to
2
summer 1998, since coverage reached more than 6 m per experimental area Fig. 4. The effect of neither position nor density was highlighted by the analysis Table 4.
Table 4
2
C . racemosa: analysis of variance on algal total cover per experimental area m
Source of variation df
MS F
P Position
5 Po 1
7.220 2.248
0.1596 Density
5 De 2
9.757 3.038
0.0855 Po
3 De 2
1.145 0.35
0.7073 Error
12 3.211
Cochran’s test C
5 0.56 P
. 0.05
G . Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
237
4. Discussion
