Components of Seagrass Meadows from Apical Meristems to Meadows

Seagrasses are clonal plants, whereby the plant growth occurs through the reiteration of a basic set of modules, connected by rhizome material to develop the clone (Marba and Duarte, 1998; Hemminga and Duarte, 2000). This basic module consists of a shoot, bearing a leaf bundle in all species except some Halophila species that have a leaf pair at each shoot (den Hartog, 1970), and a set of adventitious roots and a rhizome piece connecting them to neighboring shoots (Fig. 1). The reiteration of these modules is achieved through cell division at the apical rhizome meristem, which provides, therefore, the basis for seagrass clonal growth (Tomlin-son, 1974). In addition, to produce new modules, the apical rhizome meristem may divide, producing a branch also containing an apical rhizome meristem, which extends the clone in a different direction (Fig. 1). Hence, an adequate representation of clonal growth patterns requires characterization of the size of the clonal modules and their organs, the spacing in between consecutive modules along the rhizome, the rhizome elongation rate and its branching rate, and angle (Fig. 1; Marba and Duarte, 1998). There has been, therefore, considerable effort to quantify these properties across the seagrass flora (Tables 1 and 2).

Table 1. Mean and range of components of clonal growth of seagrass species. Based on data compiled by Marba and Duarte (1998).

Trait

Mean

Min

Max

Rhizome elongation

79

2

3.56

(cm year-1)

Horizontal rhizome branching

5.8

0.06

25.97

rate (% of internodes)

Horizontal rhizome branching

47

19

angle (degrees)

The components of clonal growth all range greatly across the seagrass flora (Table 1, range of variation of clonal properties across the seagrass flora), including significant plasticity within species (Perez et al., 1994; Marba and Duarte, 1998). However, much of this variability can be explained through allometric relationships between these components and module size, as represented by either shoot weight or rhizome diameter (Duarte, 1991; Marba and Duarte, 1998; Hemminga and Duarte, 2000). Hence, small seagrasses show faster clonal growth rates than large species (Table 2), which tend to

Table 2. Average rhizome elongation rates of seagrass species. Based on data compiled by Marba and Duarte

Rhizome elongation

Species (cm year-1)

Amphibolis antarctica 20

Amphibolis griffithii 4

Cymodocea nodosa 40

Cymodocea rotundata 210

Cymodocea serrulata 153

Enhalus acoroides 3

Halophila decipiens 215

Halophila hawaiiana 89

Halophila ovalis 356

Heterozostera tasmanica 103

Halodule uninervis 101

Haludule wrightii 223

Posidonia angustifolia 12

Posidonia australis 9

Posidonia oceanica 2

Posidonia sinuosa 4

Phyllospadix scouleri 17

Phyllospadix torreyi 26

Syringodium filiforme 123

Syringodium isoetifolium 109

Thalassia hemprichii 54

Thalassia testudinum 69

Thalassodendron ciliatum 16

Thalassodendron pachyrhizum 3

Zostera marina 26

Zostera noltii 68

grow slowly (Duarte, 1991; Marbá andDuarte, 1998; Hemminga and Duarte, 2000). On the basis of the existence of such allometric relationships, the seagrass flora has been described as composed of scale models of a generic design (Marba and Duarte, 1998). Whereas this statement holds if examining individual properties, the simultaneous variation in average clonal properties across species renders clonal patterns complex, thereby resulting in contrasting growth strategies across species.

The simplest models of clonal growth could not elucidate these differences for they portrayed clonal growth as a simple radial growth process, with circular-shaped clones extending at a constant radial growth rate equivalent to the average rhizome elongation rate of the modeled species (Duarte, 1995; Kendrick et al., 1999). However, comparison of the resulting prediction of colonization rates with observed dynamics provided evidence that clonal growth does not proceed at a constant rate, but that it accelerates over time (Kendrick et al., 1999). More elaborate models of clonal growth used all components of clonal growth, as represented by their average value and observed within-species variability, to examine the development of clonal networks (Marba and Duarte, 1998; Sintes et al., 2005. Models using clonal growth rules to simulate clonal growth provided evidence that, as suggested by field observations (Vidondo et al., 1997; Kendrick et al., 1999), this is a strongly non-linear process (Marba and Duarte, 1998; Sintes et al., 2005). The radial growth of seagrass clones accelerates from very low values at the early stages of growth to high rates (Marba and Duarte, 1998; Sintes et al., 2005), equaling the extension rates of runners (i.e. rhizomes extending outside seagrass patches), by the time they reach highly compact structures (Fig. 2). The efficiency of space occupation, as described by the increase in patch size achieved for a given rhizome production, declines sharply with increasing clonal size (Sintes et al., 2005). The applicability of these models, developed using Cymodocea nodosa as the model species, to other species is yet to be assessed.

Whereas fast-growing seagrasses have been assumed to display a guerrilla strategy compared to the more compact, 'phalanx' growth strategy assumed for larger, slow-growing species, analysis of model results indicate that these expectations do not hold (Marba and Duarte, 1998). The broad branching angles of the fast-growing, small seagrass species (e.g. Zostera noltii) lead to a compact growth, following a

Fig. 2. The shape of modelled Cymodocea nodosa clones of different ages. From Sintes et al. (2005)—with permission.

spiral pattern around the origin of the clone, whereas the narrow branching angles of large-slow-growing seagrasses project them at relatively larger distances for a given investment in rhizome material, generating a guerrilla-like pattern but over a long period of time (Fig. 3).

Present depictions of clonal growth patterns cannot, however, be used to infer the resulting structure of the meadows, for these models examine the growth of individual clones and do not consider possible interferences from neighboring clones. Moreover, there is evidence that there is a limit to the maximum density of seagrass stands (e.g. Duarte and Kalff, 1987; Marba and Duarte, 2003), so that the presence of neighboring clones is expected to reduce the growth of adjacent clones. Indeed, models of seagrass clonal development can only reproduce the internal density of seagrass clones if an exclusion

-

Halophila ovalis

ywft - ^

Thalassodendron ciliatum

-

Posidonia oceanica // "

-

/ ^ -

time = 0.10 yr

produced

alive

rhizome (m)

5.7

5.4

number of

336

262

shoots

time = 6 yr

produced

alive

rhizome (m)

5.5

3.5

number of

201

59

shoots

time = 55 yr

produced

alive

rhizome (m)

5.3

1.7

number of

191

38

shoots

distance to X origin (m)

Fig. 3. The simulated spread of clones of different seagrass species predicted on the basis of their basic growth rules: horizontal rhizome elongation rate, and branching rules (probability and angle). The graphs depict the clonal topography after producing ca. 5 m of rhizome for three contrasting seagrass species (Halophila ovalis, Thalassodendron ciliatum, and Posidonia oceanica). The time required to develop the networks, and the rhizome length, and number of shoots produced and surviving since initiation of clonal spread are indicated. Dashed lines show the spatial distribution of the rhizomes and shoots produced, and continuous ones the distribution of surviving rhizomes and shoots. Reproduced from Marba and Duarte (1998)—with permission.

area, or per capita space, which is unlikely to be occupied by another shoot, is defined around each shoot (Sintes et al., 2005), thereby supporting empirical evidence for architectural-determined seagrass density (Marba and Duarte, 2003). The role of density-dependence in regulating clonal growth and space occupation in seagrasses is, however, insufficiently developed at present. Hence, whereas the expected dynamics of colonizing clones are adequately represented by existing knowledge and rate estimates, the dynamics of clones within established meadows is not sufficiently understood as yet to allow reliable models of meadow development and dynamics to be formulated. Moreover, the role of environmental factors, prominently hydrodynamics in shaping the landscape produced (cf. Bell et al., Chapter 26), is

Amphibolis antarctiva Amphibolis griffithii Cymodocea nodosa Cymodocea rotundata Cymodocea serrulata Enhalus acoroides Halodule uninvervis Halodule wrightii Halophila ovalis Heterostera tasmanica Posidonia angustifolia Posidonia australis Posidonia oceanica Syringodium filiforme Syringodium isoetifolium Thlassia hemprichii Thalassia testudinum Thalassodendron ciliatum Thalassodendron pachyrhizum Zostera marina

Amphibolis antarctiva Amphibolis griffithii Cymodocea nodosa Cymodocea rotundata Cymodocea serrulata Enhalus acoroides Halodule uninvervis Halodule wrightii Heterostera tasmanica Posidonia angustifolia Posidonia australis Posidonia oceanica Posidonia sinuosa Syringodium filiforme Syringodium isoetifolium Thlassia hemprichii Thalassia testudinum Thalassodendron ciliatum Thalassodendron pachyrhizum Zostera marina

0 1 2 3 4 5 Specific mortality rate (year-1)

0 1 2 3 4 5 Specific recruitment rate (year-1)

Fig. 4. Reported shoot mortality and recruitment rates for seagrass species. Solid circles represent average values, and bars extend across reported ranges. Data from tables in Hemminga and Duarte (2000).

also not captured as yet by models of how clonal growth develops into meadows.

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