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spans (Hemminga and Duarte 2000). Seagrasses continually produce new leaves, roots
and rhizomes, leaving the old plant material to enter the detrital food web (Björk et al.
2008). This steady-state condition can be maintained over long periods, leading to longlived seagrass meadows (Hemminga and Duarte 2000).
1.1.2 Environmental Constraints for Seagrasses
Seagrasses are able to tolerate a wide range of climates (GMP 2004) and are
present in all U.S. coastal states, with the exception of Georgia and South Carolina,
where a combination of freshwater inflows, high turbidity, and large tidal amplitude
restricts their occurrence (Thayer et al. 1997). Many environmental variables influence
the growth and survival of each seagrass species. The consensus among studies revealed
that light, depth, sediment characteristics salinity, temperature, and nutrient
concentrations were among the most important variables that produced a response in a
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measured seagrass indicator (Kirkman 1996, Dennison et al.1993; Livingston et al. 1998;
Koch 2001, and Fourqurean et al. 2003; Wilson and Dunton 2012). The various
combinations of these parameters will permit, encourage or eliminate seagrass from a
specific location (McKenzie 2008).
Light is a critical factor controlling seagrass productivity and spatial distribution
because the amount of light that reaches the submerged plants dictates the daily growth
and seasonal productivity (Björk et al. 2008). Light requirements are greater for
seagrasses (Duarte 1995) because of their extensive below-ground roots and rhizomes
that they must support (Duarte 199l). Limited light can stress the plants and result in a
reduction of below-ground biomass, reduced carbohydrate content of rhizomes, loss of
tissue nutrients and other growth issues (Coles and McKenzie 2004). Water depth and
turbidity (the amount of suspended particles in the water) are major factors influencing
the amount of light available to seagrasses. Turbidity measures water clarity and
expresses the degree to which light is scattered and absorbed by molecules and particles
suspended in the water (Radke et al. 2003). Increased turbidity and lowered light
transmission can adversely impact an ecosystem, limiting the distribution and depth at
which species can grow (Abal and Dennison 1996). Suspended sediment that causes
turbidity can precipitate and accumulate and smother communities. The depth
distribution of seagrasses is largely determined by light penetration, restricting their range
to depths less than 70 m (Short et al. 2007), although some species have been reported at
a maximum of 90 m (Hemminga and Duarte 2000). In areas near large river discharges
or in areas of development, seagrass depth limits are reduced by significant light
attenuation (Björk et al. 2008). Distribution may even be limited in some shallow water
habitats due to cloudiness and low irradiances levels (Björk et al. 2008). In more turbid
waters, 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). Seagrass species are mostly confined to sandy or muddy
sediment where their roots can penetrate (Hemminga and Duarte 2000) and their
placement and development has also been found to correlate with sediment depth
(Zieman et al 1989; Hall et al. 1999). Colonizing seagrasses (e.g., Halodule) are better
suited to mobile sediments than some of the larger species (McKenzie 2008).
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Each species of seagrass can tolerate different ranges of salinity. In general
seagrasses can survive a range of salinities from 5-60 ‰ (parts per thousand, ppt
equivalent to practical salinity units, PSU) (McMillan and Moseley 1967, Walker 1989),
with some species having the ability to tolerate both lower and higher salinities ranging
from 0 -140 ‰ (Björk et al. 2008). Water temperature can also influence the health and
growth rate of seagrasses (McKenzie 2008). Temperature tolerances vary widely for
individual species, but seagrasses have been found to withstand temperatures as low as -6
°C (Eelgrass in Alaska is known to tolerate encasement in ice during winter) and brief
readings as high as 40.5 °C (Phillips and Meñez 1988). Temperature also controls the pH
and the dissolved carbon dioxide (CO2) in the water column, which can influence
seagrass growth.
Seagrasses require nutrients such as inorganic carbon (C), nitrogen (N) and
phosphorus (P) for growth (McKenzie 2008). Coastal waterways obtain nutrients from
terrestrial, atmospheric and oceanic sources, but human activities often enrich the waters
too much (McClelland and Valiela 1998). High nitrate and ammonium concentrations in
the water column may also limit seagrass growth (Burkholder et al. 1992; Van Katwijk et
al. 1997). A compilation of data on the nutrient environment of seagrass meadows
world-wide shows that in seagrass beds the average water column has an ammonium
concentration of 3.1 uM, a nitrate concentration of 2.7 uM, and an average phosphate
concentration of 0.35 uM (Hemminga 1998). High levels of chlorophyll-a in a system
often indicate turbid, poor water clarity with low levels suggesting higher water clarity
and good conditions for seagrass (McPherson and Miller 1994; Tomasko et al. 2001).
Too high nutrient levels in the water column can cause excessive algal growth and
organic matter loading to the bottom waters, which in turn can cause an increase in
bacterial decomposition of the organic matter and consume oxygen, depleting the water
column of dissolved oxygen (DO) (Clement et al. 2001) and potentially creating hypoxic
and anoxic conditions (Hypoxia = DO < 2 mg/L, and anoxia = DO < 0.1 mg/L) (Vitousek
et al. 1997). These conditions can cause negative impacts to aquatic life ranging from
mortality to chronic impairment of growth and reproduction (USEPA 2001). In estuaries
and coastal waters, low DO is one of the most widely reported consequences of nitrogen
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and phosphorus pollution and one of the best predictors of a range of biotic impairments
(Bricker et al. 2003).
1.1.3 Natural Impacts to Seagrasses
Natural changes in weather patterns and storm events cause mixing in the water
column and can increase the amount of nutrient input into an ecosystem. Seasonal
changes in the environment can alter the amount of light penetration and other habitat
conditions that influence the growth of seagrass. Seasonal changes in temperature affect
the capacity of water to hold DO, with colder waters having the ability to hold more DO
than warm waters (US EPA 2006). Solar heating can cause layering, with the
dramatically warmer surface layer becoming isolated from the colder bottom layer (US
EPA 2006). Stratification in salinity often results from colder denser salt water intruding
at depth, while warmer less dense fresh water sits at the surface. However, storms, tides,
and wind can cause mixing and eliminate the layering caused by salinity and temperature
differences (US EPA 2006). Gradual changes that come with seasonality allow
organisms to acclimate, whereas rapid shifts may cause shock and adversely affect their
distribution and abundance (US EPA 2006).
Over the next century, the predicted changes in global climate will alter many of
the factors that shape the coastal ecosystem of South Florida (RECOVER 2014). Climate
change could cause sea level rise, increases in temperature, changes in precipitation
patterns, and changes in the intensity and/or frequency of extreme events (ICLEI 2010).
This could lead to a rapid loss and substantial changes to the benthic communities along
the coasts (Wanless et al. 1994). Studies done by the Organization for Economic
Cooperation and Development (OECD) identified Miami–Dade as the county with the
highest amount of vulnerable assets exposed to potential coastal flooding, with costs
projected at around $3.5 trillion (Nicholls et al. 2007). As the climate changes, the