Chapter 18 The Biology of Thalassia
411
desiccation, kill vast quantities of leaves that are later
sloughed off. This defoliation process occurs sporadically throughout the year and seems to pose no longterm problem for the plants. Brouns (1985) reported
similar negative effects of desiccation at extreme
low tides for monospecific T. hemprichii meadows
in Papua New Guinea. In the subtropical waters of
Texas and Florida, the most severe mortality of T.
testudinum is usually caused by severe cold rather
than heat, even though these seagrasses from the extreme northern distribution range showed a greater
tolerance to chilling temperatures (at 2
◦ C, during 4–
12 h), than their southern counterparts (McMillan,
1979).
The optimum salinity range for T. testudinum has
been reported to be from 24 to 35 psu (Phillips, 1960;
McMillan and Moseley, 1967; Zieman, 1975a), but
Thalassia tolerated a broad salinity range for very
brief exposures, ranging from 3.5 to 5.0 psu (den
Hartog, 1957 in Sculthorpe 1967) to 60 psu (McMillan and Moseley, 1967), although such exposures
commonly result in leaf loss. Following the passage of hurricane Donna in South Florida in 1960,
Thomas et al. (1961) considered the damage to T.
testudinum by fresh water runoff to have been more
severe than the physical effects of the high winds and
water surge. Heavy freshwater releases reported in
South Florida (Thomas et al., 1961) and Venezuela
(P´ erez and Galindo, 2000) caused mortality of macro
algae as well as vertebrate and invertebrate fauna
associated with seagrass meadows, but although
T. testudinum was heavily defoliated, the buried
rhizomes remained intact, and leaf recovery was
rapid.
T. testudinum requires a minimum sediment depth
of ∼25–50 cm to achieve lush growth (Zieman,
1975b), although meadow formation occurs at lesser
sediment depths (Scoffin, 1970). Sediment composition and the proportion of finer-grained particles vary as a function of leaf density of T. testudinum. In the Bahamas, bare sediments had only 1–
3% fine-grained material, while dense T. testudinum
had more than 15% of fine-grained sands (Scoffin,
1970). Similarly, dense T. testudinum meadows raise
the elevation within the meadows due to sedimentation (Ginsburg and Lowenstam, 1958; Zieman,
1975b; Durako and Moffler, 1985c), and increase the
concentration of sedimentary organic matter (Wood
et al., 1969) resulting in higher protein levels in rhizomes of T. testudinum from the middle of the beds
(Durako and Moffler, 1985c).
T. testudinum may be found as deep as 10 m in
clear water (Phillips and Me˜ nez, 1988), but is confined to maximum depths of 1–2 m in turbid habitats (Phillips, 1960; Zieman, 1982). For seagrasses
in general, lower depth limits are set by ambient
light levels (Dennison and Alberte, 1985), as the
photosynthates produced in the leaves in light, are
transported to subterranean tissues, where they are
invested in new shoot growth (an investment on top
of carbohydrate storage and respiration). Lee and
Dunton (1996) reported that T. testudinum in Laguna
Madre (Texas, USA), required an annual quantum
flux in excess of 1628 mol m
−2 in order to maintain
a positive carbon balance, which corresponded with
14% of the surface irradiance.
III. Historical Development in Research
Few areas of marine or environmental science have
seen an explosion of research and literature to match
that produced by studies of seagrasses in the past
three to four decades. Early work with Thalassia focused principally on taxonomy, anatomy, distribution, and limited autecology (Phillips, 1960;
Tomlinson and Vargo, 1966; Tomlinson, 1969a,b,
1972, 1974; den Hartog, 1970). Most early studies
were limited to sites where the scientists could wade
at low tide, and deeper samples were restricted to random and disruptive ship-borne dredges and cores.
This changed dramatically and rapidly in the mid1960s with the emergence of reliable and simpler
SCUBA, which enabled the scientist to dive throughout the entire depth range of Thalassia and other
seagrasses, and to spend considerable time in situ.
In a study on the marking method of estimating production of T. testudinum, Zieman (1974) cited 16
references. Less than 10 years later, a synthesis of
seagrass studies in South Florida (Zieman, 1982),
where T. testudinum is the dominant species, contained 529 references, of which ∼40% concerned
(directly or indirectly) T. testudinum communities.
Of these, 33 were from prior to 1960, 119 were from
1960 to 1969, and 377 were post 1970, showing
the rapid increase in studies of seagrass ecosystems
during this time. While field instrumentation underwent tremendous changes and development during
this period, particularly the size reduction and reduced power demands brought about by integrated
solid-state technology, it is impossible to overestimate the importance of allowing the scientist to
411
desiccation, kill vast quantities of leaves that are later
sloughed off. This defoliation process occurs sporadically throughout the year and seems to pose no longterm problem for the plants. Brouns (1985) reported
similar negative effects of desiccation at extreme
low tides for monospecific T. hemprichii meadows
in Papua New Guinea. In the subtropical waters of
Texas and Florida, the most severe mortality of T.
testudinum is usually caused by severe cold rather
than heat, even though these seagrasses from the extreme northern distribution range showed a greater
tolerance to chilling temperatures (at 2
◦ C, during 4–
12 h), than their southern counterparts (McMillan,
1979).
The optimum salinity range for T. testudinum has
been reported to be from 24 to 35 psu (Phillips, 1960;
McMillan and Moseley, 1967; Zieman, 1975a), but
Thalassia tolerated a broad salinity range for very
brief exposures, ranging from 3.5 to 5.0 psu (den
Hartog, 1957 in Sculthorpe 1967) to 60 psu (McMillan and Moseley, 1967), although such exposures
commonly result in leaf loss. Following the passage of hurricane Donna in South Florida in 1960,
Thomas et al. (1961) considered the damage to T.
testudinum by fresh water runoff to have been more
severe than the physical effects of the high winds and
water surge. Heavy freshwater releases reported in
South Florida (Thomas et al., 1961) and Venezuela
(P´ erez and Galindo, 2000) caused mortality of macro
algae as well as vertebrate and invertebrate fauna
associated with seagrass meadows, but although
T. testudinum was heavily defoliated, the buried
rhizomes remained intact, and leaf recovery was
rapid.
T. testudinum requires a minimum sediment depth
of ∼25–50 cm to achieve lush growth (Zieman,
1975b), although meadow formation occurs at lesser
sediment depths (Scoffin, 1970). Sediment composition and the proportion of finer-grained particles vary as a function of leaf density of T. testudinum. In the Bahamas, bare sediments had only 1–
3% fine-grained material, while dense T. testudinum
had more than 15% of fine-grained sands (Scoffin,
1970). Similarly, dense T. testudinum meadows raise
the elevation within the meadows due to sedimentation (Ginsburg and Lowenstam, 1958; Zieman,
1975b; Durako and Moffler, 1985c), and increase the
concentration of sedimentary organic matter (Wood
et al., 1969) resulting in higher protein levels in rhizomes of T. testudinum from the middle of the beds
(Durako and Moffler, 1985c).
T. testudinum may be found as deep as 10 m in
clear water (Phillips and Me˜ nez, 1988), but is confined to maximum depths of 1–2 m in turbid habitats (Phillips, 1960; Zieman, 1982). For seagrasses
in general, lower depth limits are set by ambient
light levels (Dennison and Alberte, 1985), as the
photosynthates produced in the leaves in light, are
transported to subterranean tissues, where they are
invested in new shoot growth (an investment on top
of carbohydrate storage and respiration). Lee and
Dunton (1996) reported that T. testudinum in Laguna
Madre (Texas, USA), required an annual quantum
flux in excess of 1628 mol m
−2 in order to maintain
a positive carbon balance, which corresponded with
14% of the surface irradiance.
III. Historical Development in Research
Few areas of marine or environmental science have
seen an explosion of research and literature to match
that produced by studies of seagrasses in the past
three to four decades. Early work with Thalassia focused principally on taxonomy, anatomy, distribution, and limited autecology (Phillips, 1960;
Tomlinson and Vargo, 1966; Tomlinson, 1969a,b,
1972, 1974; den Hartog, 1970). Most early studies
were limited to sites where the scientists could wade
at low tide, and deeper samples were restricted to random and disruptive ship-borne dredges and cores.
This changed dramatically and rapidly in the mid1960s with the emergence of reliable and simpler
SCUBA, which enabled the scientist to dive throughout the entire depth range of Thalassia and other
seagrasses, and to spend considerable time in situ.
In a study on the marking method of estimating production of T. testudinum, Zieman (1974) cited 16
references. Less than 10 years later, a synthesis of
seagrass studies in South Florida (Zieman, 1982),
where T. testudinum is the dominant species, contained 529 references, of which ∼40% concerned
(directly or indirectly) T. testudinum communities.
Of these, 33 were from prior to 1960, 119 were from
1960 to 1969, and 377 were post 1970, showing
the rapid increase in studies of seagrass ecosystems
during this time. While field instrumentation underwent tremendous changes and development during
this period, particularly the size reduction and reduced power demands brought about by integrated
solid-state technology, it is impossible to overestimate the importance of allowing the scientist to