416
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
et al., 1999) and internal resorption of nutrients from
senescent leaves (Nienhuis et al., 1989; Hemminga
et al., 1991). The average leaf lifespan for Thalassia
is ∼50 days, which is rather short compared to other
(sub) tropical seagrass species, like Enhalus and Syringodium (∼100 and 90 days, respectively; see review by Hemminga et al., 1999). Stapel and Hemminga (1997), furthermore, found for T. hemprichii
that the maximum potential resorption of nitrogen
and phosphorus from senescent leaves was equivalent to 18–28% of the plants’ N- and 31% of the
plants’ P-demand, respectively. Usually, only 56–
76% of these maximum values could be realized,
because of loss of leaf tissue due to leaf fragmentation and premature detachment. Stapel et al. (2001)
reported evidence for efficient external re-use of
nutrients via the detrital pathway. The uptake of
nutrients by the leaves probably plays a very important role in this process by efficiently recapturing
the nutrients that are released in the water column
from decomposing Thalassia leaves, and thereby
contributing to nutrient conservation for the Thalassia meadow as a whole. This external re-use of
nutrients is closely related to nutrient acquisition via
the trapping of seston particles in the meadow and
nutrient uptake by epiphytic and epibenthic primary
producers. These organic nutrient pools are eventually regenerated in the sediments and the liberated
nutrients then become available for root uptake and,
after diffusion, for leaf uptake. The capturing and
retention of these particulate organic nutrient pools
depends on Thalassia bed size. Increasing Thalassia
bed size may therefore also coincide with increasing
nutrient retention, which, especially in nutrient-poor
environments, may increase chances of survival. The
highly dynamic conditions of the marine coastal environment, causing premature loss of leaves and leaf
fragments, may have favored the development of an
efficient nutrient conservation strategy outside the
living plant over internal conservation strategies.
Research into the nutrient dynamics of T.
hemprichii and T. testudinum meadows are, like their
geographic distribution, strikingly isolated from
each other. For a better understanding of the nutrient dynamics of Thalassia, comparative research
should be carried out, including studies along the
East African coast. Important aspects that are less
well understood in the nutrient dynamics of Thalassia and that should receive considerable attention
are the role of the microbial community in the regeneration of nutrients from Thalassia tissue, the role
of other primary producers, especially microalgae
(diatoms), in fixing nutrients in the system, and the
role of microelements, especially iron. The sediment
compartment with its complex biogeochemical interactions remains, so far, under investigated, and
only recently has received serious attention (Holmer
et al., 1999; Terrados et al., 1999b; Enr´ ıquez et al.,
2001; Holmer et al., 2001). Also, considerably more
research is needed to link the sedimentary and nutrient regeneration processes to nutrient absorption
by the living seagrass.
VI. Plant Development
A. Vegetative (Clonal) Growth
As with all seagrasses, the vegetative habit of Thalassia is clonal and consists of a horizontally creeping rhizome (long shoot) with branches (vertical
rhizomes or short-shoots), which bear the foliageleaves (Figs. 1A and B). But in contrast with the other
seagrasses, in Thalassia, the distinction between the
leaf-bearing vertical rhizomes and the horizontal
rhizome is absolute. The following description is
mainly based on the studies of Tomlinson on T. testudinum. The morphological traits of the vegetative
body of T. hemprichii seem to be similar to those of T.
testudinum (Tomlinson, 1974), and no detailed analysis of vegetative development has been realized for
the first species to allow comparison. The meristem
at the apex of the horizontal rhizome (Figs. 1C and D)
forms scale-leaves (or scales), roots and squamules
(or intravaginal scales: very small scale-like structures in between the leaves), whereas the meristem at
the apex of the vertical rhizome forms foliage-leaves
(or leaves), roots, squamules, and flowers (Figs. 1E
and F). The root primordia always emerge unilaterally, on the lower side of the horizontal rhizome and
either on the left or right site of the vertical rhizomes
(Tomlinson and Vargo, 1966; Fig. 1I). Branching
is monopodial (Tomlinson and Vargo, 1966), and
the apical meristems of the horizontal and vertical
rhizomes produce leaf-opposed lateral branches of
the opposite kind; that is, horizontal rhizome apical meristems produce vertical rhizomes, and vice
versa (Tomlinson and Bailey, 1972; Figs. 1A–C and
E). However, in exceptional cases, two foliar groups
emerge from the vertical rhizomes (‘twin shoots’;
Durako, 1995; van Tussenbroek, unpublished data),
and Durako (1995) found that up to 40% of the
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
et al., 1999) and internal resorption of nutrients from
senescent leaves (Nienhuis et al., 1989; Hemminga
et al., 1991). The average leaf lifespan for Thalassia
is ∼50 days, which is rather short compared to other
(sub) tropical seagrass species, like Enhalus and Syringodium (∼100 and 90 days, respectively; see review by Hemminga et al., 1999). Stapel and Hemminga (1997), furthermore, found for T. hemprichii
that the maximum potential resorption of nitrogen
and phosphorus from senescent leaves was equivalent to 18–28% of the plants’ N- and 31% of the
plants’ P-demand, respectively. Usually, only 56–
76% of these maximum values could be realized,
because of loss of leaf tissue due to leaf fragmentation and premature detachment. Stapel et al. (2001)
reported evidence for efficient external re-use of
nutrients via the detrital pathway. The uptake of
nutrients by the leaves probably plays a very important role in this process by efficiently recapturing
the nutrients that are released in the water column
from decomposing Thalassia leaves, and thereby
contributing to nutrient conservation for the Thalassia meadow as a whole. This external re-use of
nutrients is closely related to nutrient acquisition via
the trapping of seston particles in the meadow and
nutrient uptake by epiphytic and epibenthic primary
producers. These organic nutrient pools are eventually regenerated in the sediments and the liberated
nutrients then become available for root uptake and,
after diffusion, for leaf uptake. The capturing and
retention of these particulate organic nutrient pools
depends on Thalassia bed size. Increasing Thalassia
bed size may therefore also coincide with increasing
nutrient retention, which, especially in nutrient-poor
environments, may increase chances of survival. The
highly dynamic conditions of the marine coastal environment, causing premature loss of leaves and leaf
fragments, may have favored the development of an
efficient nutrient conservation strategy outside the
living plant over internal conservation strategies.
Research into the nutrient dynamics of T.
hemprichii and T. testudinum meadows are, like their
geographic distribution, strikingly isolated from
each other. For a better understanding of the nutrient dynamics of Thalassia, comparative research
should be carried out, including studies along the
East African coast. Important aspects that are less
well understood in the nutrient dynamics of Thalassia and that should receive considerable attention
are the role of the microbial community in the regeneration of nutrients from Thalassia tissue, the role
of other primary producers, especially microalgae
(diatoms), in fixing nutrients in the system, and the
role of microelements, especially iron. The sediment
compartment with its complex biogeochemical interactions remains, so far, under investigated, and
only recently has received serious attention (Holmer
et al., 1999; Terrados et al., 1999b; Enr´ ıquez et al.,
2001; Holmer et al., 2001). Also, considerably more
research is needed to link the sedimentary and nutrient regeneration processes to nutrient absorption
by the living seagrass.
VI. Plant Development
A. Vegetative (Clonal) Growth
As with all seagrasses, the vegetative habit of Thalassia is clonal and consists of a horizontally creeping rhizome (long shoot) with branches (vertical
rhizomes or short-shoots), which bear the foliageleaves (Figs. 1A and B). But in contrast with the other
seagrasses, in Thalassia, the distinction between the
leaf-bearing vertical rhizomes and the horizontal
rhizome is absolute. The following description is
mainly based on the studies of Tomlinson on T. testudinum. The morphological traits of the vegetative
body of T. hemprichii seem to be similar to those of T.
testudinum (Tomlinson, 1974), and no detailed analysis of vegetative development has been realized for
the first species to allow comparison. The meristem
at the apex of the horizontal rhizome (Figs. 1C and D)
forms scale-leaves (or scales), roots and squamules
(or intravaginal scales: very small scale-like structures in between the leaves), whereas the meristem at
the apex of the vertical rhizome forms foliage-leaves
(or leaves), roots, squamules, and flowers (Figs. 1E
and F). The root primordia always emerge unilaterally, on the lower side of the horizontal rhizome and
either on the left or right site of the vertical rhizomes
(Tomlinson and Vargo, 1966; Fig. 1I). Branching
is monopodial (Tomlinson and Vargo, 1966), and
the apical meristems of the horizontal and vertical
rhizomes produce leaf-opposed lateral branches of
the opposite kind; that is, horizontal rhizome apical meristems produce vertical rhizomes, and vice
versa (Tomlinson and Bailey, 1972; Figs. 1A–C and
E). However, in exceptional cases, two foliar groups
emerge from the vertical rhizomes (‘twin shoots’;
Durako, 1995; van Tussenbroek, unpublished data),
and Durako (1995) found that up to 40% of the