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been reported growing in abundance in salinities from 12.0 - 38.5 ‰ (Phillips 1960).
Generally, H. wrightii tends to be more abundant in shallow inshore areas because it can
tolerate frequent tidal exposure and low salinities (Dawes et al. 2004). Salinities of 45–
60 ‰ can cause S. filiforme and T. testudinum to stop growing, but H. wrightii can
continue to grow even at 72‰ (McMillan and Moseley 1967). Laboratory studies done
by Lirman and Cropper (2003) found that out of the three main species found in Florida,
S. filiforme was most susceptible to changes in salinity with the highest mean blade
extension rates recorded at 25 ‰ and dropping dramatically at both higher and lower
salinity. Thalassia testudinum exhibited peak leaf elongation rates at 40 ‰. Seagrasses
can tolerate fluctuations in salinity, but prolonged exposure to extreme conditions can
become detrimental to their survival. Within the POM basin, 87.9% of the sample sites
had salinity ranging between 30-40 ‰. The surface salinity within the POM basin had a
much greater range than bottom salinity most likely due to the influence of freshwater
canal discharge and precipitation. Salinity conditions are ideal for seagrass growth in the
POM basin as long as there are no sudden or prolonged changes to the delivery of
freshwater to the basin.
Water depth influences the distribution of seagrass species. The optimum depth
for growth in S. filiforme ranges from around 1-3 m (Duarte et al. 2007) and for T.
testudinum the ideal depth is between 1-20 m (Littler et al. 1989). Halodule wrightii is a
hardy species that can tolerate exposure at low tides (Phillips and Menez 1988; Fonseca
1994; Haynes 2000; Short et al. 2010a) and prefers depths ranging between 0-2 m
(Haynes 2000). All three of these seagrass species can be observed at much greater
depths in clearer waters (Phillips and Menez 1988; Fonseca 1994; GMP 2004; Short et al.
2010a; Short et al. 2010b), most likely as a function of light penetration (Phillips 1960).
Halodule wrightii has also been observed to exhibit a second abundance peak in some
areas along the deep-water edge of T. testudinum and S. filiforme meadows (Iverson and
Bittaker 1986; Zieman and Zeiman 1989). The water depth in the POM basin averaged
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around 2 m (ranging between 1-3 m across the basin, see Appendix 3), which is within
the ideal range for all three major seagrass species.
The thick sandy-mud sediments in the POM basin average over 1 m in depth (see
Appendix 3) and are ideal for seagrass root and rhizome growth. Halodule wrightii are
often found in sediment less than 5 cm, but may be found in sediments as deep as 25 cm
(Phillips and Menez 1988; Fonseca 1994). Syringodium filiforme rhizomes are usually
found in sediment 1-10 cm deep and T. testudinum rhizomes often grow down to 20 cm
below the surface (Phillips and Menez 1988; Fonseca 1994). The sediment depth within
POM averaged over twice as deep as most benthic vegetation requires for stable growth.
This is due to the bottom and shorelines being altered drastically by dredging in the
1900’s, to provide for the development of the surrounding lands and for navigation
channels (Hefty et al. 2001). The bay was dredged and filled with spoil from
construction in order to create land, causing unstable shorelines (Caccia and Boyer 2007).
Sediment that is too deep or loose can be resuspended easily and reduce water clarity.
The seawalls created to stabilize the developed land reflect wave energy, which
contributes to the resuspension of bottom sediments (Hefty et al. 2001). Since the
seagrass and benthic algae communities rely on light reaching the bottom, water clarity is
of crucial significance (Hefty et al. 2001).
Previous studies done by Caccia and Boyer (2005) found that the North Bay had
the highest turbidity, almost twice that observed in other areas, because it is influenced by
the runoff of five canals, stormwater runoff, the Port of Miami, Munisport landfill and the
urban landscape (Caccia and Boyer 2005). The overall turbidity in Biscayne Bay
averages around 2 NTU (Caccia and Boyer 2005). Levels observed in the POM basin in
the data used in this study averaged just slightly over 2 NTU. When the waters are less
clear, seagrasses may produce elongated leaves so that they can reach the light closer to
the surface and may form less dense canopies in order to avoid self-shading (Short et al.
1995; Collier et al. 2008). However, the seagrass canopy height does not seem to
correlate with seagrass density within the POM basin, in fact, Syringodium canopy height
is taller when seagrass density is greater (Figure 8 and Figure 9).
Available Results from SFWMD during the collection period (2006-2011) show
that there has been minimal variation in the measured environmental variables (Table
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11). Also, all variables were within a suitable range for seagrass production. The
Biscayne Bay region is an oligotrophic estuary that requires minimal phosphorus and
nitrogen inputs (Carey et al 2011). The nutrient environment of seagrass meadows
world-wide has an average phosphate concentration of 0.35 uM (0.033 mg/L)
(Hemminga 1998). The SFWMD reported average TP concentrations (2008-2011) at
0.004 ± 0.017 mg/L and NOx concentrations (2008-2011) at 0.004 ± 0.017 mg/L (Table
11). Hemminga (1998) documented that seagrasses worldwide require DO
measurements averaging above 2 mg/L and within the POM basin, DO measured an
average of 6.26 ± 0.98 mg/L (Table 11). The average pH (2006-2011) within the POM
basin, 8.03 ± 0.14 units (Table 11), is basic in nature and at an ideal level for the coastal
region. The average CHL-A across all of Biscayne Bay is reported as 1.0 ± 1.5 mg/m3
(Johns and Kelble 2013). SFWMD average CHL-A concentrations (2007-2011)
remained on the low level in the POM basin, 0.73 ± 0.56 mg/m3
(Table 11). High levels
of chlorophyll-a in a system often indicate poor water quality and low levels often
suggest good conditions (McPherson and Miller 1994; Tomasko et al. 2001). SFWMD
OC concentrations (2008-2011) were generally low, averaging 2.06 ± 2.60 mg/L (Table
11). Within the POM basin available nutrient concentrations remained fairly low and as
such did not have any significant adverse impact on the seagrass community.
4.4 Environmental, Physical, Weather and Anthropogenic Changes Related to Seagrass
Variations
Annual rainfall and storm activity in the Miami area, coupled with air
temperatures, can influence the environmental and physical measurements (especially
salinity, water temperature, and turbidity) within the POM basin and in turn impact the
seagrass. Some slight changes in seagrass cover-density were documented between
certain years in possible relation to weather or construction events. Salinity fluctuations
between sample years in the POM are most likely due to storm activity, or lack thereof,
and influences from ocean exchange and river outputs. Salinity measurements in 2006
were the lowest on record in the POM basin (Figure 12B), possibly due to the increased
rain in the region resulting from the high storm activity in 2005, which included two
hurricanes that directly impacted South Florida (NOAA 2006). The 2006 collection year
marked the beginning of a drought period in South Florida (NOAA 2006) and 2007 saw
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drought conditions persisting across most of South Florida; however, the Miami area
measured rainfall just a few inches above normal for the year (NOAA 2007) (Figure
15A). The data shows the largest increase in salinity between 2006 and 2007 (Figure
12B). This could be due in part to the overall shallower water depths recorded during
2007, tide conditions, and the drought conditions across South Florida. Less freshwater
flow from canals and rivers inland could have influenced the salinity within the basin as
well. Salinity was seen to increase between the 2010 and 2011 sample years, possibly