426
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
to be the climax species and competitively superior
(see section 6.3). Fourqurean et al. (1995) explained
the dominance of H. wrightii in their study by the
higher nutrient demand of H. wrightii than that of
T. testudinum; and during succession T. testudinum
replaces H. wrightii due to its capacity to draw nutrients down to concentrations below the requirements
of the latter seagrass. Davis and Fourqurean (2001)
demonstrated that there is competition for nutrients
between co-occurring T. testudinum and the rhizophytic calcareous alga Halimeda incrassata in the
Florida Keys. The presence of seagrass decreased
the size of the algal thalli by 20% but the algae only
decreased T. testudinum shoot size by 10%. Tissue
nitrogen of T. testudinum decreased in the presence
of H. incrassata, suggesting that nitrogen competition was the mechanism of interaction. In Thailand, Nakaoka and Izumi (2000), studied interference at patch level for the seagrasses T. hemprichii
and Halophila ovalis. They found that H. ovalis had
higher biomass and lower production in the center
than at the edges of the patches joining unvegetated
areas, whereas these differences between center and
edge were not detected when the H. ovalis patches
joined T. hemprichii.
Rose and Dawes (1999) studied intra-specific interference in shoots of T. testudinum by comparing
their leaf mass and growth in low and high-density
monospecific meadows. They reported that in highdensity monospecific meadows growth and biomass
of the shoots were reduced (compared with lower
density meadows), suggesting that the foliar shoots
competed for available resources. Van Tussenbroek
et al. (2000) reported inhibition of foliar development at high-rhizome density in T. testudinum, and
found that the spacing of foliar shoots was regulated
just below the critical density at which negative effects on foliar development would occur, which is
quite common for clonal plants (Hutchings, 1979).
The shoots in which development was inhibited became dormant. Subsequent foliar development of
these dormant shoots after experimental fertilization suggests that competition for belowground nutrients played a significant role in the inhibition of
the development of foliage leaves. These findings are
in apparent contrast with the findings of Rose and
Dawes (1999), who found that reduction of light by
dense leaf canopies was a likely mechanism of intraspecific competition at high densities. However, it
has to be borne in mind that these studies were carried out in contrasting areas, the first an oligotrophic
reef lagoon, and the second a mangrove-lined
estuary.
2. Drifting and Epiphytic Algae
Drifting (free-living, detached, unattached or looselying) algae can form extensive mats on top of, or
occasionally in between the seagrasses. These mats
may remain stationary on the seagrass meadows for
days, weeks, or even months, and Bell and Hall
(1995) found a clear relationship between the occurrence of drifting masses in T. testudinum meadows
and sediment grain size, which in its turn is determined by hydrodynamic regimes. Alternatively, the
drift algae in or close to T. testudinum vegetation may
form balls in response to the constant rolling caused
by water movement (Littler and Littler, 2000). These
balls are assemblages with one to four dominant
species and are hollow, the cavity housing a wide
variety of invertebrates (Ballantine et al., 1994).
Epiphytes may contribute up to 24% of the total
aboveground biomass of Thalassia meadows (Heijs,
1984), and include a diatom and a macro-algal component. Both drifting algal mats and epiphytes inhibit growth of Thalassia (Tomasko and Lapointe,
1991; Holmqvist, 1997), and possible mechanisms
of these interactions are shading and smothering of
the host plant, and competition for nutrients and
gases (van Montfrans et al., 1984; Lapointe et al.,
1994; Duarte, 1995). There are indications that the
associated fauna of the seagrass meadows increases
in number and diversity at higher degree of structural complexity of the seagrass meadows through
the presence of epiphytes and drifting algal masses
(Gore et al., 1981; Stoner and Lewis, 1985), but
long-term cover with high density mats of drifting algae in a T. testudinum community resulted in
a decrease in associated fauna (Holmqvist, 1997).
The abundance of epiphytes and drift algae is often
mediated by nutrients and grazers (Lapointe et al.,
1994; Heck et al., 2000; Peterson and Heck, 2001),
which is also dealt with elsewhere in this chapter
(sections 6.4 and 7).
C. Vegetation Succession
The sequence of succession in the seagrass beds generally involves early species that stabilize the sediments and increase the sediment nutrient content,
and by doing so allow establishment of later species
(Clarke and Kirkman, 1989; Williams, 1990). This
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
to be the climax species and competitively superior
(see section 6.3). Fourqurean et al. (1995) explained
the dominance of H. wrightii in their study by the
higher nutrient demand of H. wrightii than that of
T. testudinum; and during succession T. testudinum
replaces H. wrightii due to its capacity to draw nutrients down to concentrations below the requirements
of the latter seagrass. Davis and Fourqurean (2001)
demonstrated that there is competition for nutrients
between co-occurring T. testudinum and the rhizophytic calcareous alga Halimeda incrassata in the
Florida Keys. The presence of seagrass decreased
the size of the algal thalli by 20% but the algae only
decreased T. testudinum shoot size by 10%. Tissue
nitrogen of T. testudinum decreased in the presence
of H. incrassata, suggesting that nitrogen competition was the mechanism of interaction. In Thailand, Nakaoka and Izumi (2000), studied interference at patch level for the seagrasses T. hemprichii
and Halophila ovalis. They found that H. ovalis had
higher biomass and lower production in the center
than at the edges of the patches joining unvegetated
areas, whereas these differences between center and
edge were not detected when the H. ovalis patches
joined T. hemprichii.
Rose and Dawes (1999) studied intra-specific interference in shoots of T. testudinum by comparing
their leaf mass and growth in low and high-density
monospecific meadows. They reported that in highdensity monospecific meadows growth and biomass
of the shoots were reduced (compared with lower
density meadows), suggesting that the foliar shoots
competed for available resources. Van Tussenbroek
et al. (2000) reported inhibition of foliar development at high-rhizome density in T. testudinum, and
found that the spacing of foliar shoots was regulated
just below the critical density at which negative effects on foliar development would occur, which is
quite common for clonal plants (Hutchings, 1979).
The shoots in which development was inhibited became dormant. Subsequent foliar development of
these dormant shoots after experimental fertilization suggests that competition for belowground nutrients played a significant role in the inhibition of
the development of foliage leaves. These findings are
in apparent contrast with the findings of Rose and
Dawes (1999), who found that reduction of light by
dense leaf canopies was a likely mechanism of intraspecific competition at high densities. However, it
has to be borne in mind that these studies were carried out in contrasting areas, the first an oligotrophic
reef lagoon, and the second a mangrove-lined
estuary.
2. Drifting and Epiphytic Algae
Drifting (free-living, detached, unattached or looselying) algae can form extensive mats on top of, or
occasionally in between the seagrasses. These mats
may remain stationary on the seagrass meadows for
days, weeks, or even months, and Bell and Hall
(1995) found a clear relationship between the occurrence of drifting masses in T. testudinum meadows
and sediment grain size, which in its turn is determined by hydrodynamic regimes. Alternatively, the
drift algae in or close to T. testudinum vegetation may
form balls in response to the constant rolling caused
by water movement (Littler and Littler, 2000). These
balls are assemblages with one to four dominant
species and are hollow, the cavity housing a wide
variety of invertebrates (Ballantine et al., 1994).
Epiphytes may contribute up to 24% of the total
aboveground biomass of Thalassia meadows (Heijs,
1984), and include a diatom and a macro-algal component. Both drifting algal mats and epiphytes inhibit growth of Thalassia (Tomasko and Lapointe,
1991; Holmqvist, 1997), and possible mechanisms
of these interactions are shading and smothering of
the host plant, and competition for nutrients and
gases (van Montfrans et al., 1984; Lapointe et al.,
1994; Duarte, 1995). There are indications that the
associated fauna of the seagrass meadows increases
in number and diversity at higher degree of structural complexity of the seagrass meadows through
the presence of epiphytes and drifting algal masses
(Gore et al., 1981; Stoner and Lewis, 1985), but
long-term cover with high density mats of drifting algae in a T. testudinum community resulted in
a decrease in associated fauna (Holmqvist, 1997).
The abundance of epiphytes and drift algae is often
mediated by nutrients and grazers (Lapointe et al.,
1994; Heck et al., 2000; Peterson and Heck, 2001),
which is also dealt with elsewhere in this chapter
(sections 6.4 and 7).
C. Vegetation Succession
The sequence of succession in the seagrass beds generally involves early species that stabilize the sediments and increase the sediment nutrient content,
and by doing so allow establishment of later species
(Clarke and Kirkman, 1989; Williams, 1990). This