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due to the persisting dry conditions from La Niña. Salinity generally remained in the
optimum range for seagrass growth with minimal variation among seagrass coverdensity, even during years with larger salinity variations.
A significant increase in water depth was seen between the 2007 and 2008
samples. This could be due to the fact that 2008 marked the end of the drought
conditions in South Florida that had persisted since 2006. The early part of 2008 was
wetter than expected and the summer was also wetter than normal, with heavy rainfall in
mid-August from tropical storm Fay passing through the area (NOAA 2008). 2009
experienced a very dry winter, very dry spring, then a very wet early summer, followed
by average rainfall until heavy rainfall in the last half of December (NOAA 2009).
Rainfall varied across the year; however, Miami records show the least amount of rainfall
during the study period occurred in 2009 (only 52.1 inches; -9.8 in departure from
average, Figure 15A). Due to La Niña conditions that developed in the summer of 2010,
dry conditions persisted from October 2010-June 2011, and this period is documented
with driest conditions for the region in 80 years (Molleda 2010).
The year 2011 had dryer than normal conditions, due to the wet season falling 20
days short of normal (NOAA 2011), and brought drought condition back to most of
South Florida by the end of the year (NOAA 2010). 2011 also marked the 6th
consecutive year with no hurricanes directly impacting South Florida (NOAA 2011).
The most significant changes seen in seagrass cover-density and water temperature were
between the 2009, 2010 and 2011 sampling years in POM. In this study, 2009 and 2011
had the warmest water temperatures on record, while in contrast, 2010 had the coldest.
These differences can be explained by the annual air temperatures recorded in Miami
during that time. December 2009 ended with a period of record setting cold temperatures
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that stretched into the early months of 2010 (NOAA 2010). 2010 experienced extremes,
with some of the coldest and hottest temperatures across South Florida (NOAA 2011),
and concluded with the coldest December on record (NOAA 2010). The cold
temperatures documented in the winter of 2009-2010 and in December of 2010 were
mainly caused by a strongly negative North Atlantic Oscillation (NAO) and Arctic
Oscillation (AO) (NOAA 2010). These strongly negative atmospheric oscillations
essentially “flip” the weather patterns across North America, forcing the jet stream to
plunge Arctic air masses from northern Canada into the southeastern U.S., including
Florida (NOAA 2010). There is an average of 51 days of 90+ °F and 2 days of sub-40 °F
temperatures annually in South Florida (NOAA 2010). In 2009 Miami-Dade experienced
121 days with temperatures at or above 90 °F, the highest number of days since the
record began in 1937 (NOAA 2009), 2010 recorded 103 days at 90+ °F and 6 mornings
with temperatures sub-40 °F, (NOAA 2010), and 2011 documented the 2nd most number
of days, 118, with a record 44 consecutive days at 90+ °F (NOAA 2011). The extreme
cold winter air temperatures from the AO in 2010 are likely responsible for the cooler
surface and bottom water temperatures measured throughout the year within the POM
basin. In 2011, on the opposite end of the spectrum, an unusually warm spring, warmer
than normal summer (with record high temps), and one of the warmest Decembers on
record (NOAA 2011) raised the water temperatures within the POM to their highest
measurements across the entire study period (Figure 12A).
The two major meadow building seagrasses, Syringodium and Thalassia,
experienced a decline in cover-density over the 2010 collection, but then rebounded by
2011 (Figure 8). This may be due to natural variability, but the influence of the colder
temperatures (Figure 12A and Figure 15B) in 2010 may have negatively impacted both
Syringodium and Thalassia in the short run, both measuring their lowest cover-density
during that collection year. The greatest cover-density measurements for Thalassia were
just a year prior, 2009. Thalassia was showing an increasing trend over the sample
period until 2010. Halodule showed minimal change over the collection years (Figure
8), most likely due to its high tolerance range of environmental conditions such as habitat
deterioration, eutrophication, and increased turbidity (Short et al. 2010a). Red algae and
calcareous algae cover-density experienced some of their greatest variation between 2009
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and 2011, with 2010 displaying the highest measurements over the entire collection
(Figure 11). The colder temperatures in 2010 may have allowed some algae species to
experience greater productivity over the seagrass, but overall there was no significant
negative impact on seagrass. The lowest occurrence of seagrass among sample sites (30)
was recorded at the beginning of the study in 2005 and the highest occurrence in 2011
when the study was completed (Figure 5).
This study found that turbidity is generally lower across the sample area (Figure
12E) than previous reports had indicted (Ecosummary Biscayne Bay 2002; Caccia and
Boyer 2005). However, Caccia and Boyer (2005) predicted that the turbidity in the North
Bay will continue to degrade over time. Before this study began in the spring of 2005,
three hurricanes affected the South Florida region in 2004, one in August and two in
September (NOAA 2004). 2005 then experienced two large hurricanes that directly
impacted South Florida at the end of August (Katrina) and October (Wilma) (NOAA
2005). There was also construction activity present in 2005. During the months of June
through August, 2005, the project that began in 1990 to deepen the POM shipping
channel from 35 to 42 feet was completed using new confined blasting techniques that
minimized impacts to the ecosystem (USACE 2007). The project was able to
successfully blast through limestone bedrock and deepen the port. Turbidity and
sediment depth measurements for 2005 were significantly higher than other years. The
amount of storm activity prior to, and during the first year of sampling, in addition to
blasting in the channels most likely attributed to the disturbance and resuspension of
sediment within the basin. After the 2005 collection year, there were no other hurricanes
impacting the South Florida region during the study period. The lack of storms in
proceeding years has most likely aided in the recovery of turbidity. By the end of the
study period turbidity levels measured below 2 NTU, which is much lower than the levels
measured in 2005 (Figure 12E).
After major modifications in the region, loss of the stabilizing vegetation and the
continuing resuspension and erosion of unconsolidated sediment are the principal causes
of chronic turbidity in areas of the bay (Wanless et al. 1984). The Florida Fish and
Wildlife Conservation Commission reported areas of significant seagrass decline between
1950 and 2000; including a 43 percent loss of seagrass in the northern section of
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Biscayne Bay near Miami (FFWCC 2002). Studies done by Blair et al. (2011) found that
seagrass cover was extensive (159,363 acres) in the Biscayne Bay region and was found
to be increasing in area in all subregions of the bay from 1992 to 2005 except in North
Biscayne Bay, where it lost 660 acres, or 11%. Results from this study show that from
2005 to 2011 the seagrass habitat within the POM in North Biscayne Bay has remained
fairly stable and even shows an overall increased in occurrence by the end of the last
collection year (Figure 5). The lack of major storm activity, improved water
management practices, and regulated construction activity in the Miami area may have
supported fairly stable salinity measurements and contributed to the general decline in
turbidity and overall improved water clarity in the POM basin. This may have aided in
the quick recovery of the seagrass from the 2010 cold event. There are many
contributing factors as to the distribution and cover of benthic vegetation in the POM