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of sites (see Table 3). Ruppia maritima was not documented in any samples.
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Mixed seagrass beds were identified at 57% of the sites consisting of
Syringodium-Thalassia-Halodule (20%), Syringodium-Thalassia (18%), SyringodiumHalodule (11%), and Thalassia-Halodule (8%). Homogenous beds for Syringodium
occurred at 17% of the sites; whereas Thalassia (5%) and Halodule (11%) were rarely
monospecific and were often mixed with the Syringodium (Table 4).
Syringodium measured the tallest average canopy height, 38.6 ± 16.3 cm,
followed by Thalassia, 27.6 ± 11.3 cm, and Halodule, 16.6 ± 8.2 cm. The Halophila
species were the shortest, measuring an average of 3.6 ± 2.1 cm. The average seagrass
canopy height was calculated to be 29.96 ± 16.97 cm over the sample period.
Algae species were present at 90.5% of the 420 sites and 64.6% of the 2159
quadrats sampled over the collection period. Red and calcareous-green algae were the
most abundant types of algae, present at 61.4% and 60% of the 420 monitoring sites,
respectively. Green algae were the third most abundant type, followed by brown algae as
the least abundant type (Table 3).
Table 3. FIAN Percentage of each vegetation genera present at sample sites and quadrats
over the study period. Dominance ranks are listed for all vegetation present. Total Sites
n = 420, total quadrats n = 2159.
Vegetation
Groups
(n) sites
present
% sites
present
(n) quads
present
% quads
present Dominance
Rank
Halodule 210 50% 570 26.4% 5
Halophila 32 7.6% 67 3.1% 8
Syringodium 279 66.4% 1049 48.6% 1
Thalassia 215 52.4% 628 29.1% 4
Seagrass Total 385 91.7% 1662 77% -
Red algae 258 61.4% 758 35.1% 2
Green algae 157 37.3% 312 14.5% 6
Brown algae 45 10.7% 93 4.3% 7
Calcareous algae 256 61% 713 33% 3
Algae Total 380 90.5% 1394 64.6% -
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Table 4. FIAN Frequency of sites within the POM basin with mixed or monospecific
seagrass beds and no grass present. (S = Syringodium, T = Thalassia, H = Halodule)
Grass Mix
# Sites
Present Frequency
S+H 46 11.0%
S+T 76 18.1%
T+H 32 7.6%
S+T+H 85 20.2%
S 71 16.9%
T 22 5.2%
H 46 11.0%
No grass 42 10.0%
From 2005 to 2011 the occurrence of seagrass among the 30 sample sites
increased by 10%, the occurrence of algae increased by 8%, and the total occurrence of
vegetation increased by 2%. Results of a general linear model showed seagrass
occurrence did not significantly vary over years or between seasons (p > 0.05). However,
algae occurrence showed a significant annual (F6 = 2.465, p = 0.024) (Figure 5) and
seasonal (F1 = 2.465, p = 0.018) variation (Appendix 4). A greater percentage of algae
were present in the sample sites during the spring (dry) season (93.81%), than in the fall
(wet) season (87.14%). Across sample years, the greatest variation in algae occurrence
was observed between the 2006 and 2007 collections; the lowest occurrence of algae
within sample sites was documented in 2006, followed by the highest occurrence in the
2007sampling year (see Figure 5).
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Figure 5. Mean annual seagrass and algae frequency of occurrence among the 30 sample
sites measured in the Port of Miami basin over the 2005-2011 sampling period with 95%
CI error bars.
Results from Mann-Whitney U tests showed seagrass cover-densities did not vary
significantly between seasons for any species, p > 0.05 (Table 5). Generally,
Syringodium (Figure 6A) and Halodule (Figure 6C) cover-densities were higher during
the fall season, and Thalassia (Figure 6B) cover-density was higher during the spring
season. Significant seasonality was observed in seagrass canopy height measurements
for Thalassia (Z = 6462.5, p = 0.002) and Halodule (Z = 6877, p < 0.001), but not for
Syringodium or the average seagrass canopy height, p > 0.05. Overall, taller canopy
heights for all seagrass species and taller average seagrass canopy height (Table 5) were
observed during the fall season (Figure 7).
Cover-density differed between all algae groups, except brown algae (p > 0.05).
The spring sampling season measured greater red algae cover-density, while the fall
measured higher green and calcareous algae cover-densities (Table 5).
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Table 5. Mann-Whitney U test of significance for seasonal differences of canopy height,
average canopy height (cm) and cover-density (#/m2
) among the four seagrass and four
algae groups between seasons in POM.
Vegetation Canopy Height and Cover-Density Measurements between Seasons
(dry/wet) Mann-Whitney U Test
Measure Season N range mean +/- SD U(Z) p
Canopy Height
Average Seagrass spring 192 1.0 - 93.33 28.62 ± 16.14
fall 191 1.0 -101.2 31.31 ± 17.7 19987.5 0.127
Halodule spring 97 4.0 - 27.0 13.31 ± 5.1
fall 99 4.5 - 44.0 19.81 ± 9.34 6877 0.000
Halophila sp. spring 9 1.0 - 7.0 2.98 ± 2.21
fall 16 1.0 - 7.0 3.9 ± 2.07 92.5 0.251
Syringodium spring 138 7.0 - 93.33 37.43 ± 15.08
fall 137 8.0 - 101.2 39.68 ± 17.42 10288 0.205
Thalassia spring 97 3.0 - 45.0 24.62 ± 8.86
fall 106 5.0 - 61.17 30.33 ± 12.57 6462.5 0.002
Cover-Density