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productivity and cover-density, with maximums usually occurring during warmer
summer months (Dineen 2001). The warmer fall sampling season is expected to have
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taller canopy heights as well as greater cover-densities of the 3 major seagrasses. In the
POM basin, generally, the fall (wet season) collections measured greater cover-density of
Halodule and Syringodium. Unexpectedly, Thalassia measurements were generally
higher in the spring (dry season) collections (Table 5). Seagrass canopy heights are
generally higher in the fall, after the growth that follows spring seagrass reproduction
(Appendix 2). The seasonal changes in water quality (natural or anthropogenic) and algae
presence are important to this study because of their likely impact on seagrass growth and
distribution; however, the fluctuations in water quality and algae between the spring and
fall seasons may not have been extreme enough to cause a substantial seasonal impact on
the seagrass cover-densities or canopy heights within the POM basin.
4.2 Environmental and Physical Measurements
Environmental conditions within the POM basin were typical of South Florida
inshore waters; water temperatures ranged between 25-30 °C and salinity between 30-40
‰. Water clarity was generally clear with turbidity around 2 NTU, which is a safe level
for seagrass habitat growth. Water depth measurements were generally between 200-300
cm and the substrate was comprised of a mixed mud and sand sediment that mostly
measured 100-200 cm deep (see Appendix 3).
Within the POM basin, the sites located within or near the channels and cuts
generally recorded the deepest water depths and shallowest sediment depths (Figure 3).
The protected Bill Sadowski Critical Wildlife Area near Virginia Key (USACE 2004)
displayed much shallower water depths and measured the deepest sediment depths. Due
to previous dredge-and-fill activities to permit boat traffic through the basin, sediment is
more easily disturbed in the channels where less vegetation is present to stabilize loose
sediment. The shallow protected areas have more stable sediment that is more suitable
for supporting grass and algae. Surface and bottom salinity measurements within the
basin are highest near the channels and cuts that lead out to the Atlantic Ocean, while the
lowest salinities are located in the coastal sites near the Miami River, where freshwater
and nutrients enter the system from inland (Figure 3, Appendix 6).
Studies by Caccia and Boyer (2005) showed higher salinity in the dry season in
all areas of Biscayne Bay, except in the Central bay area, which did not show seasonal
variability because it is a well-mixed zone, exchanging waters with the Atlantic. The
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water flow in the North Bay region is restricted due to the construction of dredged islands
and causeways (Bialczak et al. 2001), and also has greater freshwater influence from the
canals and the Miami River, and in turn is influenced more heavily during the wet season
from increased river flow (Caccia and Boyer 2005). Compared to other regions of
Biscayne Bay, salinities are generally lower and water clarity is diminished in the North
Bay region due to relatively high freshwater discharge combined with a low flushing rate
(Browder et al. 2005). Larsen (1995) also found that during the dry season less
freshwater reached the bay because of increased upstream storage and lower groundwater
levels. This large range in annual salinity can impact the benthic seagrass community
(Montague and Ley 1993) by affecting growth, survival, reproduction, and other critical
physiological processes of the plants (Browder et al. 2005).
The water temperature, salinity and water depth data collected from the POM
basin exhibited the typical seasonal pattern observed in South Florida. Within the basin,
the wet season displays significantly lower salinities, higher water temperatures, and
deeper water depths than the dry season (2005-2011). Surface salinity decreases during
the wet season due to the influx of freshwater from precipitation and subsequent Miami
River flow, while the opposite occurs during the dry season (Duever et al. 1994).
Warmer air temperatures and larger quantities of summer rain raise water temperatures
and decrease the salinity levels by the end of the wet season. During the dry season
months (spring/winter), air temperatures are cooler and in turn the salinity levels are
usually higher.
As expected, sediment depth and turbidity did not show substantial seasonal
patterns over the sample period; turbidity measurements were slightly lower and
sediment depths were slightly higher during the fall (see Figures 13E and 13D).
Research by Caccia and Boyer (2005) found turbidity in Biscayne Bay is generally lower
in the wet season (fall), most likely due to less wind influence and less mixing of bay
waters. The POM basin is surrounded by land and manmade structures which can
influence the impacts from wind and potentially reduce mixing. Sedimentation is usually
greater in the fall as well, which could be due to more detritus entering the system after
spring seagrass reproduction (Phillips 1960; Moffler and Durako 1982; Dineen 2001).
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4.3 Relationships between Seagrass and the Environmental and Physical Measurements
The current study investigated the extent to which seagrass cover-density would
change seasonally and annually over the collection years (2005-2011) and whether the
measured environmental and physical variables explained significant variation among
seagrass presence and cover-density. It was found that sedimentation and water depth are
the only major physical variables influencing the cover-density of seagrass within the
POM basin. The single best predictor of seagrass cover-density (Table 13, Models 2-4
A-D) was water depth. The models also showed that seagrasses are more likely to be
present in areas with shallower water depths characterized by low turbidity where light
levels are sufficient for photosynthesis (Table 12, Models 1A-D). The conditions in the
POM basin, excluding deep channels, offer a suitable environment for growth of benthic
vegetation. Most of the basin contains shallow enough waters with thick sediment for
seagrass rhizomes to grow well. It was expected that temperature and salinity would
have a greater effect on seagrass distribution, but due to the low variability in
measurements across the entire collection, no significant relationships were expressed
between these variables.
Tropical seagrasses can tolerate a wide range of temperatures, but temperatures
above 43 °C can cause mortality (Biebl and McRoy 1971; Campbell et al. 2006; DiazAlmela et al. 2007; Ehlers et al. 2008) and temperatures lower than 20 °C can inhibit
photosynthesis and eventually lead to death (Thomas et al. 1961; Mazzotti et al. 2007).
Temperature primarily controls flowering in S. filiforme, with the optimal range to induce
flowering between 22-24 °C (McMillian 1980). Leaf kill can occur in S. filiforme when
temperatures fall below 20 °C (Phillips 1960). Optimum water temperature for T.
testudinum and H. wrightii growth ranges between 20-30 °C (Phillips 1960; Fourqurean
et al. 2002; Whitfield et al. 2004; Short et al. 2010c). Within the POM basin, water
surface and bottom temperatures did not exceed 32.5 °C and did not drop below 20.8 °C.
Salinity for optimal growth of various seagrass species has been found to occur
between 10 to 30 ‰ (Phillips and Meñez 1988). In general, seagrass growth declines at
salinities in excess of 45 ‰ (Quammen and Onuf 1993) and if exposed to extreme
salinity levels, seagrass tissues suffer from osmotic stress which can lead to a loss of
functionality, and eventually the tissue becomes necrotic and dies (Biebl and McRoy
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1971). Observations made by Phillips (1960) found that the optimum salinity range in
Florida for growth of S. filiforme is around 20 - 25 ‰. The optimum range for T.
testudinum is approximately 25- 38.5 ‰ and H. wrightii, with the greatest range, has