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stands, creating expansive meadows, and generally prefers mud or sand substrate for |
colonization. Their thick rhizomes can be found penetrating to depths of 20 cm below |
the surface (Phillips and Meñez 1988; Fonseca 1994; GMP 2004; Short et al. 2010d). It |
can be long-lived and is extremely resilient to storms and disturbances; however, it has |
very slow rhizome expansion rates which can slow the regrowth process of this species |
following a disturbance event (Whitfield et al. 2004). Individual shoots of the plant have |
been found to live for over 10 years, enduring seasonal temperature changes and |
powerful tropical storms (Phillips and Meñez 1988; Fonseca, 1994; GMP 2004). |
From the two methods of plant increase and dispersal used by T. testudinum, |
sexual reproduction was found to be secondary to rhizome elongation and clonal growth |
(Phillips 1960; Zieman 1972; Les 1988; Gallegos et al. 1993; Schlueter and Guttman |
1998). Thalassia testudinum plays an important role in sediment production and is very |
important in the prevention of coastal erosion (Zieman 1982; UNESCO 1998; Hemminga |
and Duarte 2000; Green and Short 2003; Larkum et al. 2006). It can be found in low |
density in oligotrophic areas (low nutrients) and is replaced by other species when there |
are continuous high nutrient inputs (Fourqurean and Rutten 2004). Because T. |
testudinum is a major habitat-forming species that cannot be replaced functionally by |
another species, it is suggested that its available habitat should be closely monitored (Van |
Tussenbroek et al. 2006; Short et al. 2010c). |
11 |
1.2.2 Syringodium filiforme |
Syringodium filiforme, manatee grass, is common, locally dominant seagrass and |
a major habitat forming species that occurs in the western tropical Atlantic from Florida |
(USA) to Venezuela, including the Gulf of Mexico and the Caribbean Sea, as well as |
Bermuda (Short et al. 2010c). Manatee grass is an important food source for many |
marine animals including manatees (Zieman 1982), and is named after the endangered |
marine mammals. It is distinguished by its cylindrical leaf blades which are |
approximately 1-3 mm wide and ranging from 10 to 40 cm in length (Phillips and Meñez |
1988; Littler et al. 1989; Fonseca 1994; GMP 2004). The blades are flexible and are able |
to withstand high current velocities (Littler et al. 1989), but are also very brittle and |
broken pieces are often found floating in large rafts along the coast after a storm event |
(Phillips 1960). The leaf length is affected by the depth of the water and the overall leaf |
length was found to be greater in deeper waters (Phillips 1960). This species creates a |
shallow but dense rhizome matrix and has a tall, dense leaf canopy, which makes this |
grass an ideal nursery for many other species (Sargent et al. 1995). The elongated blades |
from these plants can trap suspended particulates in the water column, increasing water |
clarity and improving the quality of the water by incorporating pollutants into their |
biomass and the surrounding sediments (Sargent et al. 1995). |
In South Florida S. filiforme is a major habit forming species and is often found |
growing together with T. testudinum and H. wrightii (Green and Short 2003; Short et al. |
2010c), but can also form large monospecific stands down to 18 m (Phillips and Meñez |
1988; Fonseca 1994; GMP 2004). Rhizome elongation and new branch production are |
primarily responsible for the dispersion of this species (Phillips 1960). The flowering of |
S. filiforme plants in Florida is actually very rare, so it is assumed that for the most part it |
does not use sexual reproduction in this region (Phillips 1960). Rhizomes have been |
reported to extend into the water column, presumably using this as a means of |
reproduction (Phillips and Meñez 1988; Fonseca 1994; GMP 2004). |
1.2.3 Halodule wrightii |
Halodule wrightii, commonly known as shoal grass, is a seagrass species with a |
disjoint global and predominantly tropical distribution, with the main part of its range in |
12 |
the Atlantic Ocean and others found in the eastern tropical Pacific and the Indian Ocean |
(Short et al. 2010b). It has fine blades with a bidentate tip (Phillips 2006). The blades |
grow to between 5 and 40 cm in length and their width can range from 1 to 3 mm |
(Phillips and Meñez 1988; Fonseca 1994; GMP 2004). Large continuous meadows are |
predominant on shallow shoals and flats, with the grass often exposed at times of low tide |
(Phillips and Meñez 1988; Fonseca 1994; GMP 2004). |
Halodule wrightii is a common species in Florida and usually found mixed with |
other seagrass species (like T. testudinum and S. filiforme) (Short et al. 2010b). It is fast |
growing and has a high turnover rate (Sargent et al. 1995; Short et al. 2010b). Shoal |
grass is highly tolerant to a range of environmental conditions and is considered a pioneer |
species that can replace less tolerant species under conditions of habitat deterioration, |
eutrophication, and increased turbidity (Short et al. 2010b). It is usually an early |
colonizing species but studies in Florida Bay show that with increased nutrient levels, it |
can become the dominant species locally as it is able to out-compete T. testudinum for |
light resources (Fourqurean et al. 1995). |
1.2.4 Halophila species |
Three species of Halophila have been recorded in South Florida waters; H. |
decipiens, H. engelmannii, and H. johnsonii. Halophila seagrasses are generally |
restricted to low light environments such as deeper water, under docks, as an understory |
plant and in shallow turbid waters (Fourqurean et al. 2002). These three species are able |
to tolerate a wide range of conditions such as low light intensities and higher levels of |
turbidity , which allows them to thrive in environments which are not suitable for other |
seagrass species (Fourqurean et al. 2002; GMP 2004; Short et al. 2010a). These plants |
are relatively small, just a few cm in height, with shallow root structures that are easily |
dislodged. Halophila are also known as pioneer species and are one of the first species to |
settle on disturbed sites and available substrate. |
1.2.5 Ruppia |
The final species of seagrass found in Florida is Ruppia maritima, or widegeon |
grass. Fourqurean et al. (2002) found that R. maritima was restricted to areas closer to |
13 |
freshwater sources. This species is found to have a high tolerance for low and variable |
salinity so they are more prevalent in areas with canal discharges (Lirman et al. 2008). |
1.3 Algae: Benthic and Epiphytic |
Associated with seagrass beds are characteristic benthic and epiphytic algae, |
which attach themselves to sediments, rocky outcroppings, and the seagrasses themselves |
(FMNH 2015). Algae are important to consider because they may contribute |
significantly to the structure and function of the seagrass community (Heijs 1987, Verheij |
and Erftemeijer 1993, Jupp et al. 1996, Sidik et al. 2001). Typical (common) benthic |
macroalgae observed in Florida include several species of Red algae (Rhodophyta), |
Green algae (Chlorophyta), and Brown algae (Phaeophyta). There are also several types |
of Calcareous-green algae (Calcareous-Rhodophyta) that produce calcium carbonate, |
which eventually becomes incorporated into the surrounding sediments (FMNH 2015). |
Seagrasses provide sufficient surface area on which hundreds of species of epiphytic |
algae could attach. The epiphytes cover the tips of the seagrass blades, rather than the |
bases, in order to receive more sunlight for photosynthesis, subsequently reducing |
seagrass growth as a result of shading. Eventually the epiphytes will become part of the |
detritus, along with the seagrass blades as they break off and decompose (FMNH 2015). |
Research on drift algae-epiphyte-seagrass interactions suggest that temporary, moderate |
cover of macroalgae may benefit seagrass by reducing epiphyte loads if the epiphyte |
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