410
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
Considering the habitat types where the two
species of Thalassia occur, their beds could potentially extend over huge areas (hundreds of thousands
of square km). However, no serious attempts at mapping their total surface area have been undertaken.
Even without detailed knowledge of their exact distribution and abundance, both species are known to
play an enormous role in the ecological equilibrium
of many tropical coasts. Thalassia-dominated-beds
can either be found in marine lagoons or coastal estuaries, which are often fringed by mangroves or
coral reefs, and it is very likely that narrow interactions exist between seagrass, coral reef, and
mangrove systems. These interactions include modification of hydrodynamic environment (the reefs
provide low wave-energy environments necessary
for the establishment of seagrasses and mangroves,
and the seagrasses reduce water motion on a smaller
scale which allows stabilization of sediments), sediment production (by the reef organisms and calcareous algae associated with the seagrasses), export
of organic material (mainly by seagrasses and mangroves toward the reef), and migratory movements of
fauna between the three systems (Ogden and Gladfelter, 1983; Bosence, 1989; Hemminga et al., 1994;
Holmer et al., 1999; Nagelkerken et al., 2000). Thalassia has the potential to attain high biomass and
production rates, and Duarte and Chiscano (1999)
who compiled data from many sites, reported an average aboveground biomass of 519 and 87 g dry
mass m
−2 for T. testudinum and T. hemprichii, respectively. The differences in mean aboveground
biomass between the two species are most likely
not due to differences in the plant body but caused
by differences in the environmental settings and cooccurring seagrass and macroalgae species in the
Atlantic and Indo-Pacific (see also Section VIII).
These two species have the best developed root and
rhizome system of all seagrass species in the western
Atlantic, which is better developed than most other
grasses in the Indo-Pacific. Belowground biomass
(which includes all tissues except for the leaves) accounts for ∼80–92% of the total biomass (Brouns,
1985; van Tussenbroek, 1996; Kaldy and Dunton,
2000). Duarte and Chiscano (1999) reported mean
aboveground production rates of 5.0 and 3.7 g dry
mass m
−2 day
−1 for T. testudinum and T. hemprichii,
respectively; belowground tissues (excluding roots)
only contribute between 5 and 27% (generally varying between 10 and 20%) of the total production for
this genus (Brouns, 1985; Gallegos et al., 1993; Vermaat et al., 1995; van Tussenbroek, 1996), but Kaldy
and Dunton (2000) reported an exceptionally high
contribution of 35% by rhizomes in the total plant
production for T. testudinum in Laguna Madre. Although Thalassia is often the dominant primary producer in tropical coastal seagrass communities, other
macrophytes (seagrasses and macroalgae), benthic
and epiphytic diatoms and phytoplankton also contribute, to a greater or lesser degree, to the total
community production, and Thalassia-dominated
meadows are amongst the most highly productive marine systems on Earth (Westlake, 1963;
Erftemeijer, 1993; Duarte and Chiscano, 1999).
II. Basic Environmental Requirements
Thalassia is a genus of the tropics and subtropics:
T. testudinum occurs between 9
◦ S and 32
◦ N, and the
distribution range T. hemprichii falls between 28
◦ S
and 32
◦ N (Spalding et al., 2003). Thus, its global distribution is largely determined by incoming irradiation and related temperature. Regional distributions
are considerably more complex, with temperature,
salinity, light attenuation, sediment composition,
and sediment depth playing major roles.
Seagrasses from tropical environments generally
have reduced temperature tolerances compared with
temperate-zone counterparts. Their range of temperature tolerance is only half that of organisms from
temperate waters (Moore, 1963), which is due to loss
of resistance to cold environments, whereas their
upper thermal limit is generally not much greater
than that of organisms from warm-temperate regions (Zieman, 1975a). Thalassia can neither tolerate prolonged exposure to high temperatures nor
long-term desiccation on intertidal flats (Brouns,
1985; Stapel et al., 1997). Phillips (1960) found
that T. testudinum preferred temperatures of 20–
30
◦ C in South Florida, and the optimum temperature range for growth for this species is between
23 and 31
◦ C (Barber and Behrens, 1985). This
species tolerates short-term exposure to high temperatures (33–35
◦ C) but growth rapidly falls off
when these temperatures are sustained (Zieman,
1975a). In Puerto Rico, Glynn (1968) found that
temperatures of 35–40
◦ C killed the leaves of T. testudinum on a tidal flat, but that the rhizomes of the
plants were apparently unaffected. On shallow vegetated banks, temperatures rise rapidly during spring
low tides, and the high temperatures, coupled with
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
Considering the habitat types where the two
species of Thalassia occur, their beds could potentially extend over huge areas (hundreds of thousands
of square km). However, no serious attempts at mapping their total surface area have been undertaken.
Even without detailed knowledge of their exact distribution and abundance, both species are known to
play an enormous role in the ecological equilibrium
of many tropical coasts. Thalassia-dominated-beds
can either be found in marine lagoons or coastal estuaries, which are often fringed by mangroves or
coral reefs, and it is very likely that narrow interactions exist between seagrass, coral reef, and
mangrove systems. These interactions include modification of hydrodynamic environment (the reefs
provide low wave-energy environments necessary
for the establishment of seagrasses and mangroves,
and the seagrasses reduce water motion on a smaller
scale which allows stabilization of sediments), sediment production (by the reef organisms and calcareous algae associated with the seagrasses), export
of organic material (mainly by seagrasses and mangroves toward the reef), and migratory movements of
fauna between the three systems (Ogden and Gladfelter, 1983; Bosence, 1989; Hemminga et al., 1994;
Holmer et al., 1999; Nagelkerken et al., 2000). Thalassia has the potential to attain high biomass and
production rates, and Duarte and Chiscano (1999)
who compiled data from many sites, reported an average aboveground biomass of 519 and 87 g dry
mass m
−2 for T. testudinum and T. hemprichii, respectively. The differences in mean aboveground
biomass between the two species are most likely
not due to differences in the plant body but caused
by differences in the environmental settings and cooccurring seagrass and macroalgae species in the
Atlantic and Indo-Pacific (see also Section VIII).
These two species have the best developed root and
rhizome system of all seagrass species in the western
Atlantic, which is better developed than most other
grasses in the Indo-Pacific. Belowground biomass
(which includes all tissues except for the leaves) accounts for ∼80–92% of the total biomass (Brouns,
1985; van Tussenbroek, 1996; Kaldy and Dunton,
2000). Duarte and Chiscano (1999) reported mean
aboveground production rates of 5.0 and 3.7 g dry
mass m
−2 day
−1 for T. testudinum and T. hemprichii,
respectively; belowground tissues (excluding roots)
only contribute between 5 and 27% (generally varying between 10 and 20%) of the total production for
this genus (Brouns, 1985; Gallegos et al., 1993; Vermaat et al., 1995; van Tussenbroek, 1996), but Kaldy
and Dunton (2000) reported an exceptionally high
contribution of 35% by rhizomes in the total plant
production for T. testudinum in Laguna Madre. Although Thalassia is often the dominant primary producer in tropical coastal seagrass communities, other
macrophytes (seagrasses and macroalgae), benthic
and epiphytic diatoms and phytoplankton also contribute, to a greater or lesser degree, to the total
community production, and Thalassia-dominated
meadows are amongst the most highly productive marine systems on Earth (Westlake, 1963;
Erftemeijer, 1993; Duarte and Chiscano, 1999).
II. Basic Environmental Requirements
Thalassia is a genus of the tropics and subtropics:
T. testudinum occurs between 9
◦ S and 32
◦ N, and the
distribution range T. hemprichii falls between 28
◦ S
and 32
◦ N (Spalding et al., 2003). Thus, its global distribution is largely determined by incoming irradiation and related temperature. Regional distributions
are considerably more complex, with temperature,
salinity, light attenuation, sediment composition,
and sediment depth playing major roles.
Seagrasses from tropical environments generally
have reduced temperature tolerances compared with
temperate-zone counterparts. Their range of temperature tolerance is only half that of organisms from
temperate waters (Moore, 1963), which is due to loss
of resistance to cold environments, whereas their
upper thermal limit is generally not much greater
than that of organisms from warm-temperate regions (Zieman, 1975a). Thalassia can neither tolerate prolonged exposure to high temperatures nor
long-term desiccation on intertidal flats (Brouns,
1985; Stapel et al., 1997). Phillips (1960) found
that T. testudinum preferred temperatures of 20–
30
◦ C in South Florida, and the optimum temperature range for growth for this species is between
23 and 31
◦ C (Barber and Behrens, 1985). This
species tolerates short-term exposure to high temperatures (33–35
◦ C) but growth rapidly falls off
when these temperatures are sustained (Zieman,
1975a). In Puerto Rico, Glynn (1968) found that
temperatures of 35–40
◦ C killed the leaves of T. testudinum on a tidal flat, but that the rhizomes of the
plants were apparently unaffected. On shallow vegetated banks, temperatures rise rapidly during spring
low tides, and the high temperatures, coupled with