428
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
elongation rates of T. testudinum are comparatively
low. Gallegos et al. (1994) reported rhizome elongation rates of 51.6 and 80.9 cm year
−1 per apex for the
seagrasses S. filiforme and H. wrightii respectively,
which are considerably higher than the rates of 19–
35 cm year
−1 per apex reported for T. testudinum
(see section 4.1). Fourqurean et al. (1992) showed
that H. wrightii had a four-fold higher demand for
phosphorus (which is the limiting nutrient in carbonate sediments) than T. testudinum. A consequence of
this high nutrient demand of H. wrightii is that after
the continuous supply of external nutrients, retrograde succession occurs, resulting in a community
with dominance of the early colonizer H. wrightii
(Fourqurean et al., 1995: see section 6.2). Williams
(1990) in her study of secondary succession in the
experimentally denuded plots in a reef lagoon at
the US Virgin Islands, found that when the community advanced toward its climax state, co-existence
rather than replacement of species occurred. She
suggested that co-existence of more species in a T.
testudinum community in the Caribbean could be
attributed to niche partitioning (mainly caused by
different rooting depths and differential nutrient demand of the species) or to increasing resource supply
(nutrient accumulation) as succession progresses, in
accordance with the resource-ratio model of Tilman
(1985). But Clarke and Kirkman (1989) and Duarte
et al. (1997a), suggested that (intermediate) perturbations were of primary importance in maintaining
multispecific seagrass meadows in the Indo-Pacific.
In the Indo-Pacific, T. hemprichii often occurs intermixed with various seagrass species (Table 4, see
section 6.1), and colonization sequences of cleared
areas are closely related to the rate of rhizome expansion of each species. Rollon et al. (1998) experimentally denuded plots in a mixed seagrass meadow in
the Philippines and observed that Halophila ovalis
was the most conspicuous early colonizer, reaching
high density within half a year after disturbance.
But invasion of larger species such as Cymodocea
rotundata, Syringodium isoetifolium, and Halodule
univervis reduced the H. ovalis densities to predisturbance levels. The naturally-dominant species
T. hemprichii and Enhalus acoroides were the last
to colonize, the first reaching pre-disturbance levels
after 2 years, whereas the latter required a little less
than 10 years for full recovery. Some early species
colonized from seeds, but in general, rhizome expansion of plants in nearby meadows was most important. The sequence of succession reported by Rollon
et al. (1998) was in accordance with the rhizome expansion rates reported by Vermaat et al. (1995) for
these species in a mixed seagrass meadow, where
they found highest expansion rates for the early colonizers such as H. ovalis (141.0 cm year
−1 apex
−1 ),
lowest rates for the climax species T. hemprichii
and Enhalus acoroides (at 20.6 and 5.3 cm year
−1
apex
−1 , respectively) and intermediate rates for the
species S. isoetifolium (134.7 cm year
−1 apex
−1 ),
C. serrulata (78.3 cm year
−1 apex
−1 ), C. rotundata
(33.9 cm year
−1 apex
−1 ), and H. uninervis (28.4 cm
year
−1 apex
−1 ), which followed H. ovalis in succession but preceded the climax species. The shoots of
the later colonizers attained a greater age and were
larger than those of the early arrivers (Vermaat et al.,
1995). The sequence of secondary succession after gap formation through experimental burial in a
mixed Philippine seagrass meadow was similar to
that reported by Rollon et al. (1998), with the exception that the climax species E. acoroides was resistant to the burial treatment (Duarte et al., 1997a).
Although the structure of T. testudinum and T.
hemprichii communities may vary considerably,
mainly because T. hemprichii meadows contain
more seagrass species, general patterns in structural
changes and processes during succession are similar. With the progression toward a climax community, there is an increase in the belowground biomass
of the community as well as the leaf portion exposed in the water column, and more nutrients are
sequestered by the seagrasses (Fig. 2). The increase
in leaf area provides an increase in surface area for
colonization by epiphytic algae and fauna, with the
surface area of the climax community being many
times that of either the pioneer seagrasses or the initial algal colonizers. In addition to providing a substrate, the larger leaf area also increases leaf-baffling
and sediment-trapping effects. Thus, as the canopy
component increases, so does newly deposited material in the sediment. The climax species T. testudinum and T. hemprichii (with also E. acoroides
in the Indo-Pacific), have the highest leaf area, the
highest total biomass, and by far the greatest amount
of material in the sediments of any species dominant
in the earlier stages of succession (Fig. 2).
D. Faunal Assemblages
Thalassia meadows contain a wide range of benthic,
demersal, and pelagic organisms that are either permanent residents of the system or transients. The
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
elongation rates of T. testudinum are comparatively
low. Gallegos et al. (1994) reported rhizome elongation rates of 51.6 and 80.9 cm year
−1 per apex for the
seagrasses S. filiforme and H. wrightii respectively,
which are considerably higher than the rates of 19–
35 cm year
−1 per apex reported for T. testudinum
(see section 4.1). Fourqurean et al. (1992) showed
that H. wrightii had a four-fold higher demand for
phosphorus (which is the limiting nutrient in carbonate sediments) than T. testudinum. A consequence of
this high nutrient demand of H. wrightii is that after
the continuous supply of external nutrients, retrograde succession occurs, resulting in a community
with dominance of the early colonizer H. wrightii
(Fourqurean et al., 1995: see section 6.2). Williams
(1990) in her study of secondary succession in the
experimentally denuded plots in a reef lagoon at
the US Virgin Islands, found that when the community advanced toward its climax state, co-existence
rather than replacement of species occurred. She
suggested that co-existence of more species in a T.
testudinum community in the Caribbean could be
attributed to niche partitioning (mainly caused by
different rooting depths and differential nutrient demand of the species) or to increasing resource supply
(nutrient accumulation) as succession progresses, in
accordance with the resource-ratio model of Tilman
(1985). But Clarke and Kirkman (1989) and Duarte
et al. (1997a), suggested that (intermediate) perturbations were of primary importance in maintaining
multispecific seagrass meadows in the Indo-Pacific.
In the Indo-Pacific, T. hemprichii often occurs intermixed with various seagrass species (Table 4, see
section 6.1), and colonization sequences of cleared
areas are closely related to the rate of rhizome expansion of each species. Rollon et al. (1998) experimentally denuded plots in a mixed seagrass meadow in
the Philippines and observed that Halophila ovalis
was the most conspicuous early colonizer, reaching
high density within half a year after disturbance.
But invasion of larger species such as Cymodocea
rotundata, Syringodium isoetifolium, and Halodule
univervis reduced the H. ovalis densities to predisturbance levels. The naturally-dominant species
T. hemprichii and Enhalus acoroides were the last
to colonize, the first reaching pre-disturbance levels
after 2 years, whereas the latter required a little less
than 10 years for full recovery. Some early species
colonized from seeds, but in general, rhizome expansion of plants in nearby meadows was most important. The sequence of succession reported by Rollon
et al. (1998) was in accordance with the rhizome expansion rates reported by Vermaat et al. (1995) for
these species in a mixed seagrass meadow, where
they found highest expansion rates for the early colonizers such as H. ovalis (141.0 cm year
−1 apex
−1 ),
lowest rates for the climax species T. hemprichii
and Enhalus acoroides (at 20.6 and 5.3 cm year
−1
apex
−1 , respectively) and intermediate rates for the
species S. isoetifolium (134.7 cm year
−1 apex
−1 ),
C. serrulata (78.3 cm year
−1 apex
−1 ), C. rotundata
(33.9 cm year
−1 apex
−1 ), and H. uninervis (28.4 cm
year
−1 apex
−1 ), which followed H. ovalis in succession but preceded the climax species. The shoots of
the later colonizers attained a greater age and were
larger than those of the early arrivers (Vermaat et al.,
1995). The sequence of secondary succession after gap formation through experimental burial in a
mixed Philippine seagrass meadow was similar to
that reported by Rollon et al. (1998), with the exception that the climax species E. acoroides was resistant to the burial treatment (Duarte et al., 1997a).
Although the structure of T. testudinum and T.
hemprichii communities may vary considerably,
mainly because T. hemprichii meadows contain
more seagrass species, general patterns in structural
changes and processes during succession are similar. With the progression toward a climax community, there is an increase in the belowground biomass
of the community as well as the leaf portion exposed in the water column, and more nutrients are
sequestered by the seagrasses (Fig. 2). The increase
in leaf area provides an increase in surface area for
colonization by epiphytic algae and fauna, with the
surface area of the climax community being many
times that of either the pioneer seagrasses or the initial algal colonizers. In addition to providing a substrate, the larger leaf area also increases leaf-baffling
and sediment-trapping effects. Thus, as the canopy
component increases, so does newly deposited material in the sediment. The climax species T. testudinum and T. hemprichii (with also E. acoroides
in the Indo-Pacific), have the highest leaf area, the
highest total biomass, and by far the greatest amount
of material in the sediments of any species dominant
in the earlier stages of succession (Fig. 2).
D. Faunal Assemblages
Thalassia meadows contain a wide range of benthic,
demersal, and pelagic organisms that are either permanent residents of the system or transients. The