´ 58
S . Macia J. Exp. Mar. Biol. Ecol. 246 2000 53 –67
appropriate replication for the algae factor. Because no algal effects were detected see Results, the 1 and 2 algae treatments were pooled into their respective urchin
densities, and a one-way ANOVA was then used to compare these to the unmanipulated controls.
3. Results
The long-term average population density of L . variegatus at the field site September
22
1995–September 1998 was 1.460.08 S.E. urchins per m Fig. 2. Average monthly
22
urchin density at the site ranged from 0.5 to 2.2 m . There was no significant difference
in shoot density before and after the experiment in any of the cage control treatments, indicating that there was no cage effect for this variable Table 1. This was true for both
winter and summer experiments. In the summer none of the urchin treatments showed significant differences in shoot density before and after the 6 weeks Table 1. The
22
20-urchins m treatment, however, was only marginally non-significant, with a p value
22 22
of 0.056. In the winter, the urchin exclusion 0 m 6algae and 10 m
2 algae treatments did not result in significant changes in shoot density, while shoot density in
22 22
22
the 10 m 1 algae, 20 m
2 algae, and 20 m 1 algae treatments did decrease
significantly Table 1. No cage effects were detected in either the winter or summer experiments for above-
Fig. 2. Long-term population density of the sea urchin Lytechinus variegatus at West Point, Biscayne Bay, Florida. Data points represent monthly means; error bars are standard error.
´ S
. Macia J. Exp. Mar. Biol. Ecol. 246 2000 53 –67 59
Table 1 Results of paired t-tests comparing shoot counts in permanently marked quadrats at the beginning and end of
a
the experiment Treatment
Initial Final
t p
Summer 1997
Control 21.565.0
22.864.6 1.00
Top 22.765.3
26.064.3 2 0.84
0.441 Partial 1 top
28.369.6 26.364.0
0.50 0.642
Partial 2 top 28.068.0
22.063.5 1.35
0.235
22
0 m 26.067.9
20.364.1 0.99
0.366
22
10 m 30.0612.3
28.367.0 0.53
0.618
22
20 m 25.868.7
17.864.0 2.48
0.056 Winter
1998 Control
22.765.5 18.365.4
1.11 0.320
Top 25.865.1
26.765.3 2 0.26
0.802 Partial 1 top
19.369.4 23.065.9
2 1.48 0.200
Partial 2 top 28.264.3
24.565.9 1.09
0.327
22
0 m 1 algae
24.863.3 20.364.8
1.59 0.174
22
0 m 2 algae
18.063.3 23.865.3
2 1.16 0.297
22
10 m 1 algae
29.564.1 19.862.5
3.61 0.015
22
10 m 2 algae
16.062.6 16.762.0
2 0.23 0.830
22
20 m 1 algae
30.066.0 10.862.9
4.02 0.010
22
20 m 2 algae
16.061.0 11.362.1
2.59 0.049
a 22
Initial and final shoot count data presented as number of shoots per 0.04 m 6S.E.. See text for
explanation of treatments. Assumptions of normality and homoscedasticity were met. All df 5 5.
or below-ground seagrass biomass Table 2. In the winter, higher urchin density significantly decreased aboveground T
. testudinum biomass, but there was no effect of algae on seagrass biomass, either above- or below-ground Table 3. Winter below-
ground biomass was not affected by urchin grazing. Because there was no effect of algae, the 1 and 2 algae treatments were combined into their respective urchin
22
densities and the three treatments 0, 10, and 20 urchins m were compared to the
unmanipulated controls with a one-way ANOVA. This comparison showed a significant difference in aboveground biomass among the four treatments df 5 41; F 5 6.42;
22
p 5 0.001. The 20 m treatment had significantly lower biomass than the control and 0
22
m treatments Fig. 3a; Tukey–Kramer test. In the summer, urchin grazing, regardless
Table 2
a
One-way ANOVA for cage effects for both above- and below-ground seagrass biomass Control
Top Partial 1 top
Partial 2 top F
p Aboveground, winter
382.9667.8 383.4696.2
371.46126.5 363.4639.2
0.40 0.752
Aboveground, summer 266.4629.7
190.2664.0 234.0661.9
171.8647.6 0.36
0.786 Belowground, winter
302.4645.3 312.7642.5
201.3643.2 339.9626.1
2.29 0.109
Belowground, summer 184.5633.0
120.8622.9 143.4620.7
207.1635.3 1.85
0.170
a 22
Values given are in g dry weight m 6S.E.. Treatments included in ANOVA were: unmanipulated
control plots, tops only, partial cages with tops, and partial cages without tops. Assumptions of both normality and homoscedasticity were met. All df 5 23.
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S . Macia J. Exp. Mar. Biol. Ecol. 246 2000 53 –67
Table 3
a
Two-way ANOVA for T . testudinum biomass in the winter experiment
Source df
MS F
p Aboveground biomass
Urchin density 2
50.67 8.21
0.001 Algae
1 10.35
1.67 0.205
Urchin 3 algae 2
0.52 0.08
0.919 Error
30 6.17
– –
Belowground biomass Urchin density
2 2.41
0.54 0.590
Algae 1
9.35 2.08
0.160 Urchin 3 algae
2 1.65
0.37 0.697
Error 30
4.50 –
–
a
Cage control treatments were not included in the analysis because no cage effects were detected see Table 2. Assumptions of both normality and homoscedasticity were met.
of density, had no significant effect on seagrass biomass Fig. 3b; aboveground: df 5 23; F 5 1.34; p 5 0.289; belowground: df 5 23; F 5 0.13; p 5 0.939.
A total of 34 macroinvertebrate species were identified from the site, but the vast
22
majority were present in very small numbers 1 per m . Overall macrofaunal abundance including echinoderms, molluscs, polychaetes and sponges was 144.5615.3
22
individuals m in the summer, and 117.7612.0 in the winter. The four most common
species were the epifaunal gastropod Lithopoma americanum Turbinidae, the sponge Haliclona permollis Haliclonidae, and the infaunal polychaetes Marphysa sanguinea
Eunicidae and Asychis elongata Maldanidae. The density of H . permollis, averaged
22 22
over all treatments, was 0.24 m 60.07 in the summer and 0.31 m
60.07 in the winter. Results of the paired t-tests indicated that for no treatment, in either winter or
summer, was there a change in the density of H . permollis all p . 0.05. Similarly, no
significant differences were found in numbers of L . americanum, A. elongata, or M.
sanguinea Table 4.
4. Discussion
