Chapter 11 Dynamics of Seagrasses
275
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
Specific mortality rate (year -1 )
0
3 4
5
2
1
11.6
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
Specific recruitment rate (year -1 )
3
2
0
1
4
5
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.
III. Shoot Dynamics
A. Shoot Recruitment: Vegetative and Sexual
Shoot recruitment is the addition of new individuals to the population occurring by the vegetative
production of new shoots through clonal growth or
by the recruitment of new genets through production and germination of seeds or fragments. Uprooted shoot modules may also act as recruitment
units (Ewanchuk and Williams, 1996; Reusch, 2001;
Campbell, 2003) although the successful establishment and survival of such vegetative fragments
inside established vegetation has yet to be documented. Vegetative shoot recruitment proceeds at
highly variable rates and is largely a species characteristic although individual species also show plastic response of clonal growth to ambient conditions.
Hence, vegetative shoot recruitment does not proceed at constant rates in time and space and experimental studies have demonstrated reduced rates of
shoot recruitment in nutrient and light limited stands
(Gordon et al., 1994; P´ erez et al., 1994; Agawin
et al., 1996; Ru´ ız and Romero, 2001). In dense stands
light also tends to impose an upper limit to shoot
recruitment such that rates may be constrained by
the density of neighbouring shoots, thereby avoiding
overcrowding of the populations (Duarte and Kalff,
1987; Olesen and Sand-Jensen, 1994a). Variability
in clonal growth also has a seasonal pattern, particularly in temperate regions, with shoot formation rates
proceeding slowly during winter when growth is restricted by adverse growth conditions and rapidly in
early summer concomitant with increasing temperature and light (Bigley and Harrison, 1986; Marb` a
et al., 1996a). Accordingly, shoot formation rates are
influenced by resource availability imposing a limit
to overall rates of seagrass growth but the substantial
plasticity observed may also be an important component of their capacity to acclimate to growth under
a range of environmental conditions.
The high variability across species in rates of vegetative shoot formation scales to size such that the
time interval between the production of consecutive
shoots on the horizontal rhizome is much longer
(months) in large seagrass species than in small
species (days) (Duarte, 1991; Marb` a and Duarte,
1998; Marb` a and Walker, 1999; Hemminga and
Duarte, 2000). Hence, the average specific vegetative recruitment rates of new shoots into seagrass
populations proceed at rates spanning more than
10-fold from the large seagrass species Enhalus
acoroides (0.26 year
−1 ) to the small species Halodule wrightii (4.81 year
−1 ; Fig. 4). The variability
275
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
Specific mortality rate (year -1 )
0
3 4
5
2
1
11.6
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
Specific recruitment rate (year -1 )
3
2
0
1
4
5
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.
III. Shoot Dynamics
A. Shoot Recruitment: Vegetative and Sexual
Shoot recruitment is the addition of new individuals to the population occurring by the vegetative
production of new shoots through clonal growth or
by the recruitment of new genets through production and germination of seeds or fragments. Uprooted shoot modules may also act as recruitment
units (Ewanchuk and Williams, 1996; Reusch, 2001;
Campbell, 2003) although the successful establishment and survival of such vegetative fragments
inside established vegetation has yet to be documented. Vegetative shoot recruitment proceeds at
highly variable rates and is largely a species characteristic although individual species also show plastic response of clonal growth to ambient conditions.
Hence, vegetative shoot recruitment does not proceed at constant rates in time and space and experimental studies have demonstrated reduced rates of
shoot recruitment in nutrient and light limited stands
(Gordon et al., 1994; P´ erez et al., 1994; Agawin
et al., 1996; Ru´ ız and Romero, 2001). In dense stands
light also tends to impose an upper limit to shoot
recruitment such that rates may be constrained by
the density of neighbouring shoots, thereby avoiding
overcrowding of the populations (Duarte and Kalff,
1987; Olesen and Sand-Jensen, 1994a). Variability
in clonal growth also has a seasonal pattern, particularly in temperate regions, with shoot formation rates
proceeding slowly during winter when growth is restricted by adverse growth conditions and rapidly in
early summer concomitant with increasing temperature and light (Bigley and Harrison, 1986; Marb` a
et al., 1996a). Accordingly, shoot formation rates are
influenced by resource availability imposing a limit
to overall rates of seagrass growth but the substantial
plasticity observed may also be an important component of their capacity to acclimate to growth under
a range of environmental conditions.
The high variability across species in rates of vegetative shoot formation scales to size such that the
time interval between the production of consecutive
shoots on the horizontal rhizome is much longer
(months) in large seagrass species than in small
species (days) (Duarte, 1991; Marb` a and Duarte,
1998; Marb` a and Walker, 1999; Hemminga and
Duarte, 2000). Hence, the average specific vegetative recruitment rates of new shoots into seagrass
populations proceed at rates spanning more than
10-fold from the large seagrass species Enhalus
acoroides (0.26 year
−1 ) to the small species Halodule wrightii (4.81 year
−1 ; Fig. 4). The variability