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cover negatively impacts the seagrass (Irlandi et al. 2004). While a moderate amount of
nutrient input may increase seagrass growth, dramatic or prolonged increases in available
nutrients can be harmful, creating excessive epiphyte growth and algae blooms that result
in reduced light availability to the seagrasses (FMNH 2015).
Algae blooms are controlled by physical, chemical, and biological factors (Brush
and Nixon 2010). Bloom events are considered an indicator of degraded conditions in an
area, and these events can highlight how sensitive a system is to both human and natural
disturbances (RECOVER 2014). According to Qiuying and Dongyan (2014), seagrass
decline caused by algae blooms is becoming a more common phenomenon in the
temperate and tropical regions across the world, through direct and indirect impacts.
Competition for living space and resources are the most direct impacts associated with
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algae blooms, while the result of the bloom (e.g., light reduction, hypoxia, and
decomposition) can lead to significant indirect impacts on the seagrass beds (Qiuying and
Dongyan 2014). Short-term disturbances from algae blooms are tolerable because the
seagrass beds can usually recover, but long-term events can lead to a significant decrease
in seagrass biomass (Qiuying and Dongyan 2014).
1.4 Study Site: Port of Miami, North Biscayne Bay, FL
1.4.1 Physical Environment
The Port of Miami (POM) is the southernmost major port on the Atlantic Coast of
the USA (USACE 2004). It is an island facility (NOAA 2012) situated on 520-acres of
land mass that was created through beneficial reuse from the combination of the three
manmade spoil disposal islands (Dodge, Lummus and Sam's Islands) within the Northern
portion of Biscayne Bay in South Florida (CDMP 2011). The North Bay extends from
the Broward/Miami-Dade County line, south to the Rickenbacker Causeway (Corcoran et
al. 1984; Hale 1993) and only represents about 10% of the entire bay area (Ecosummary
Biscayne Bay 2002). The POM is located in the southernmost portion of this region (see
Figure 2). The POM basin is bounded to the north by the Fisherman’s Channel adjacent
to the busy commercial shipping harbor of Port of Miami, to the west by the most densely
populated area of the state, the City of Miami, including the Miami River, to the east by
Miami Beach and Fisher Island, and to the south by the Rickenbacker Causeway (CDMP
2011; Caccia and Boyer 2005; Ecosummary Biscayne Bay 2002). Channels and turning
basins adjacent to the port provide ship access to the cargo-handling and cruise passenger
facilities. The vessels enter and exit through the Government Cut Channel, which is
federally maintained. The channel branches at the Fisher Island Turning Basin to run
along the north (Main Ship Channel) and south (Fisherman’s Channel) sides of the port
(CDMP 2011) (Figure 3). Land surrounding the Port of Miami is essentially fully
developed, except for Virginia Key, and the diverse terrestrial and marine habitats in the
area include beaches, mangroves, seagrass beds, hardbottom and reef communities,
rock/rubble bottom, and unvegetated bottom (USACE 2004).
Water depth in the basin is generally shallow, except for the channel leading into
the port which measured around 500 ft wide and 42 ft deep before new dredging began in
2013. The bay bottom is characterized in most areas by a thin layer of sand and mud
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sediment less than six inches (15.2 cm) in depth (USACE 2004), but near Miami Beach,
sediment thickness is increased up to 40 in (101.6 cm) (USACE 2004). The tidal range in
the bay is relatively small, approximately one meter, with salinities ranging from 30 to 40
‰ (McNulty et al. 1962) and annual water temperature fluctuations ranging from 18 to
31°C (McNulty et al. 1962; Maciá 2000). Salinity within the bay is influenced by
precipitation, freshwater inputs from land, canal, and groundwater sources, and tidal
influx of oceanic water (Alleman et al. 1995; Wang et al. 2003; Lirman and Cropper
2003; Lirman et al. 2008). Because it is a shallow water lagoon, the bay experiences
sudden changes in salinities throughout the year from both natural and anthropogenic
factors (Serafy et al. 2003; Lirman et al. 2008).
The natural weather patterns in South Florida determine the amount of rainfall
over the year. There are two distinct seasons, the dry and wet season represented by
spring and fall collections, respectively. The dry season, December through May, has
milder air and water temperatures and significantly lower precipitation typically resulting
from frontal passages (Sutula et al. 2003; Tabb et al. 1962). The wet season lasts from
May through November, during which time there is an increased amount of rainfall from
frequent tropical storms and thunderstorms (Sutula et al. 2003). The wet season is also
characterized by the hurricane season; the passage of hurricanes can dramatically
increase precipitation in some years. Hurricanes and tropical storms can change the
sediment dynamics, salinity, water quality, nutrient fluxes, vegetative cover and biotic
community structure (Davis et al. 2004; Tilmant et al. 1994). Droughts are also a natural
part of the climate variability in South Florida, but the duration, extent, severity, and
reoccurrence intervals can impact the coastal ecosystems (Gilbert et al. 2012; Petes et al.
2012). They can alter species composition, distribution, abundance and health due to
changes in salinity, water quality, and freshwater influx (Gilbert et al. 2012). Water
management practices have greatly altered the impact of seasonal rainfall on the wetlands
and estuaries of South Florida, but the regional patterns still contribute to the balance
between fresh and salt water in the transition zones (Jiang et al. 2011, McIvor et al. 1994,
Shomer and Drew 1982).
The port experiences a certain level of chronic turbidity and sedimentation due to
erosion, daily outflow from the Miami River, and daily ship and tug activity in addition
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to the natural sources of turbidity from runoff, and wind or tide-driven shifting of shallow
sediments (USACE 2004). The POM basin, within North Biscayne Bay, holds a
significant environmental and economic importance to South Florida. This region is
continually impacted by human activities and the habitat has been mornitored by many
different agencies including the South Florida Fish and Invertebrate Assessment Network
(FIAN). North Biscayne Bay has the distinction of being an aquatic preserve and as such,
no impact can occur without state permit under the condition that any damage is
mitigated by planting an equivalent amount of seagrass at the same site or a nearby
already damaged site (Thorhaug 1980). In 1980, The Biscayne Bay Aquatic Preserve
was established, under Ch. 18-18, F.A.C (USACE 2004) for the purpose of preserving
and enhancing the natural waterways of Biscayne Bay, so that future generations can
enjoy its biological and aesthetic values (Kardys et. al 2012). Within the aquatic preserve
is the Bill Sadowski Critical Wildlife Area (BSCWA), located just south of the Port of
Miami and next to Virginia Key (Figure 3). BSCWA was established in 1990 by the
Florida Game and Fresh Water Fish Commission (now called the Florida Fish and
Wildlife Conservation Commission) to protect important habitats including the shallow
submerged seagrass and hardbottom habitats, intertidal mudflats, and coastal mangrove
wetlands in the Biscayne Bay area of Virginia Key (USACE 2004). The protected area
covers about 700 acres and is closed to boating year-round, except for authorized
channels that are excluded from the aquatic preserve due to their status as Federal
navigation channels (USACE 2004). The boundaries are marked on-site with buoys
(USACE 2004) (see Figure 2).
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Figure 2. Study area located in North Biscayne Bay on the southeast coast of Florida.