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composition and distribution of biota that occur in the coastal ecosystem will shift
(Scavia et al. 2002).
1.1.4 Anthropogenic Impacts to Seagrasses
Anthropogenic seagrass losses have been attributed to many direct and indirect
causes, with most such losses resulting from human activities that increase inputs of
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nutrients and sediment into the coastal zone (Short and Wyllie-Echeverria 1996), thereby
reducing water clarity. Although dredging and filling activities are strictly regulated,
localized turbidity impacts can occur in connection with dredging, coastal construction,
or vessel traffic (Hefty et al. 2001). The resuspension of sediments and the introduction
of nutrients from runoff and pollutants from damaged structures (e.g. landfills, water
treatment plants and ports/marinas) can affect the water quality (Tilmant et al. 1994;
Davis et al. 2004). Physical damage can also occur from vessel groundings and scarring
of the bottom with propellers. Direct and indirect impacts to the seagrasses in Florida
have been attributed mainly to increased urbanization and coastal development, which in
turn have brought about sewage pollution, eutrophication, sedimentation and destructive
motor vessel activity (Littler et al. 1989; Sargent et al. 1995; Hall et al. 1999; Carruthers
et al. pers. comm. 2007; Short et al. 2010b; Short et al. 2010c; BBAP 2012). The major
consequence of all these activities is reduced water clarity and quality as well as physical
destruction of habitat. Decreases in seagrass populations reported in Florida Bay were
mostly attributed to environmental change (e.g. changes in water clarity, light attenuation
and salinity) and anthropogenic-induced damage to the habitat with the introduction of
pollutants, coastal development and motor vessel damage (Littler et al. 1989; Sargent et
al. 1995; Hall et al. 1999).
1.1.5 Ecological Role and Economic Importance of Seagrasses
Seagrasses are keystone components of coastal ecosystems throughout the world
where they contribute to productivity, carbon budget, and sediment stability, as well as
provide essential habitat to a large number of associated organisms (Zieman 1972; Davis
and Dodrill 1989; Holmquist et al. 1989; Thayer et al. 1997; Walker et al. 2001;
Fourqurean et al. 2002; Lirman and Cropper 2003; Lirman et al. 2008). With the many
physical, chemical and biological services they offer, seagrasses are essential to the
marine environment. The structural components of seagrass leaves, rhizomes, and roots
modify currents and waves, trapping and storing both sediments and nutrients, and
effectively filtering nutrient inputs to the coastal ocean (Hemminga and Duarte 2000).
They protect the coast from erosion by trapping and stabilizing the marine sediments,
raising the sea floor at rates of around 0.04 inch per year (Duarte et al. 2007). These
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habitats act as nutrient sinks and play a significant role in global carbon and nutrient
cycling (Hemminga and Duarte 2000). Blue carbon is the carbon captured by living
organisms in the oceans and represents more than 55% of the green carbon (Nellemann
et. al 2009). This carbon is stored in the form of sediments from mangroves, salt marshes
and seagrasses (Nellemann et. al 2009). Seagrasses cover less than 0.5% of the entire
seafloor and are responsible for capturing and storing up to some 70% of the carbon
permanently stored in the marine realm (Nellemann et. al 2009). Current studies suggest
that mangroves and coastal wetlands trap carbon at an annual rate two to four times
greater than mature tropical forests and store three to five times more carbon per
equivalent area than tropical forests mainly due to the fact that the blue carbon is stored
in the soil, not in above-ground plant materials (Murray et al. 2011). The carbon from
these habitats can remain stored for millennia, rather than decades or centuries as with
terrestrial plants (Nellemann et. al 2009).
The seagrass habitats help support the thriving, multimillion-dollar recreational
fishery industry (Dawes et al. 2004). Nearly all of the commercially and recreationally
valuable estuarine marine animals depend on seagrasses for parts or all of their life cycles
(Kikuchi and Peres 1977; Thayer et al. 1978; Kikuchi 1980; Ogden 1980; Thayer and
Ustach 1981; Phillips 1984). In 1997, the economic value of global seagrass habitats was
estimated at $19,004 per hectare per year, generating an annual value of $3.8 trillion.
However, this value did not include fisheries, climate regulation, habitat, recreational and
cultural values, or erosion control (Costanza et al. 1997; Unsworth and Cullen-Unsworth
2010). Seagrasses are a highly significant part of the Florida coastal economy and
provide millions of acres of habitat, supporting both commercially and recreationally
important fisheries species and bring in millions of dollars annually from out-of-state and
resident recreational boaters and fishermen (Bell 1993; Milon and Thunberg 1993;
Virnstein and Morris 1996; Virnstein 1999; Wingrove 1999; Thomas and Stratis 2001).
In Florida, more than 70% of recreational and commercially important fish, shellfish and
crustacean species spend part of their lives in seagrass beds (FFWCC 2003). In 2000,
FDEP estimated that each acre of seagrass in Florida had an economic value of
approximately $20,500 per year, creating a statewide economic benefit of 55.4 billion
dollars annually (Hill 2002). In 2010, an estimated $5 million of commercial harvest
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came from crab, shrimp, lobster, and fish species that were supported by seagrass
communities in Miami‐Dade County (Sweeney 2011). Protecting the species targeted by
commercial fisheries and the habitats on which these species depend helps to ensure both
a productive ecosystem and economy (BBAP 2012).
1.2 South Florida Seagrass Species
Seven species of seagrasses are commonly found in southern Florida waters:
Thalassia testudinum (turtle grass), Syringodium filiforme (manatee grass), Halodule
wrightii (shoal grass), Halophila decipiens (paddle grass), Halophila engelmannii (star
grass), Halophila johnsonii (Johnson’s seagrass), and Ruppia maritima (widgeon grass)
(Figure 1) (Sweeney 2011; Fourqurean et al. 2002; Hemminga and Duarte 2000; Sargent
et al. 1995; Eiseman and McMillan 1980). Thalassia testudinum, S. filiforme, and H.
wrightii each have a tropical to subtropical distribution and are the three most abundant
species of seagrasses found in Florida’s near-shore waters (Zieman and Zeiman 1989).
Sixty percent of Biscayne Bay bottom substrate is thought to be covered by T.
testudinum, S. filiforme, and H. wrightii, leaving the rest of the substrate bare sand or
hard bottom (Browder et al. 2005; Lirman and Cropper, 2003). Given the occurrence of
these three seagrasses in South Florida, they are the primary focus of this study; however,
other species will be discussed.
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Figure 1. Seagrass species occurring in Florida (from Sargent et al. 1995, based on
drawings by Mark D. Moffler).
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1.2.1 Thalassia testudinum
The seagrass Thalassia testudinum, commonly known as turtle grass, is the most
abundant and thought to be one of the most important habitat-forming seagrass species in
Florida because it creates extensive dense grass beds (Short et al. 2010d). It gets its name
from the endangered Green Sea Turtle which depends on the grass for its diet (GMP
2004; Short et al. 2010d). It has the largest and most complex rhizome and root systems
and is distinguished by its deep-green broad ribbon like leaves (Whitfield et al. 2004;
GMP 2004). The blades are normally over 1 cm wide and range from 10 to 75 cm long
(Phillips and Meñez 1988; Fonseca 1994; GMP 2004). It grows in dense, extensive