282
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
Patch growth may also be affected by intrinsic factors and has been found to accelerate with patch size
and age (Duarte and Sand-Jensen, 1990a; Vidondo
et al., 1997). In a study of Cymodocea nodosa the
rate of lateral extension increased with patch size and
shoot number in an exponential manner whereas isolated single shoots survived for several years without
developing into patches (Vidondo et al., 1997). Such
positive effects of increasing patch size are probably
linked to reduction of water movement and increased
sediment stabilization as patches grow in size (Fonseca et al., 1983). Moreover, the gradual formation
of physiologically integrated shoot systems through
clonal growth enhances the potential translocation
of resources from older shoots on the rhizomes to
the apical shoots at the patch edge (Terrados et al.,
1997). Such a growth pattern has not, however, been
found for Zostera marina (Olesen and Sand-Jensen,
1994b) or for Z. novazelandica (Ramage and Schiel,
1999), presumably because of the slower horizontal
growth of these species resulting in densely packed
patches near edge and relatively high nutrient availability at the study sites.
Whereas patch extension is governed by the capacity for rhizome growth there are no constraints
on patch recession or mortality. Net growth of
patches can be substantially lower than expected
from the potential rhizome growth due to loss processes caused by physical and biological disturbance
agents. Hence, sediment reworking by burrowing animals can cause disruption of the patch edge (Philippart, 1994; Townsend and Fonseca, 1998) and the
erosion of patches at windward margins represents
significant disturbances to inhibit expansion of seagrass patches or to cause recession (Fonseca and
Bell, 1998). Restriction of patch expansion by the exposure to high flow velocity and the predominantly
growth of patches in the shelter, greatly influence the
shape and heterogeneity of patches (Fonseca et al.,
1983). Accordingly, patch edges are expected to be
highly dynamic as confirmed by the high rates of
shoot mortality and recruitment found at patch margin compared to inside the patches (Duarte and SandJensen, 1990b).
Disturbances above a certain magnitude are also
a common source of patch mortality and even large
meadows can disappear during extreme storm events
(e.g. Orth and Moore, 1983; den Hartog, 1987). The
mortality risk is size-dependent and patch losses
are often confined to the smaller patches below a
certain threshold size, presumably defined by the
species involved and the disturbance regime within
the study area (Duarte and Sand-Jensen, 1990a;
Olesen and Sand-Jensen, 1994b; Vivondo et al.,
1997; Ramage and Schiel, 1999). These negative
effects of size are probably linked to lack of mutual protection and firm anchorage leading to higher
susceptibility to physical disturbances and nutrient
stress in small patches. Consequently, patch formation from seeds is typically very inefficient due to
high seed and seedling mortality and often less than
10% of newly established seedlings survive past
their first year (Churchill, 1983; Duarte and SandJensen, 1990a; Harrison, 1993; Kaldy and Dunton,
1999), although higher survival probabilities have
been reported in some populations of Zostera marina
(24%, Olesen and Sand-Jensen, 1994b) and for Enhalus acoroides, and Thalassia hemprichii (19 and
22%, Olesen et al., in press). Moreover, the probability of newly established patches to reach a large
size is low, as small patches are subject to rapid
turnover, as indicated by positively skewed patch
size distribution that is frequently found in patchy
seagrass stands (e.g. Vidondo et al., 1997). The production of sexual and vegetative propagules remains
the term that serves to maintain the positive side
of patch dynamics = patch production – thereby
ensuring the recovery and formation of seagrass
meadows.
C. Resulting Patch Dynamics
The spatial and temporal dynamics of seagrass
patches is strongly influenced by the magnitude and
frequency of physical disturbances in a given area
and by the capacity of the species involved to persist and recover from disturbances. Some seagrass
populations experience continuous patch extinction
and replacement, which maintains the vegetation in
a permanent state of colonization and promotes the
development of a mosaic of patches of different age
and developmental stages (Duarte and Sand-Jensen,
1990a; Olesen and Sand-Jensen, 1994b; Vidondo
et al., 1997). When in balance, such populations
will maintain a dynamic equilibrium with a uniformity of patch distribution in time and space such
that an overall landscape equilibrium of patches applies. This has been demonstrated for Cymodocea
nodosa growing on highly mobile sediments where
the time interval between the passage of consecutive
sub-aquatic sand dunes allowed a close balance between loss of vegetation caused by erosion and burial
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
Patch growth may also be affected by intrinsic factors and has been found to accelerate with patch size
and age (Duarte and Sand-Jensen, 1990a; Vidondo
et al., 1997). In a study of Cymodocea nodosa the
rate of lateral extension increased with patch size and
shoot number in an exponential manner whereas isolated single shoots survived for several years without
developing into patches (Vidondo et al., 1997). Such
positive effects of increasing patch size are probably
linked to reduction of water movement and increased
sediment stabilization as patches grow in size (Fonseca et al., 1983). Moreover, the gradual formation
of physiologically integrated shoot systems through
clonal growth enhances the potential translocation
of resources from older shoots on the rhizomes to
the apical shoots at the patch edge (Terrados et al.,
1997). Such a growth pattern has not, however, been
found for Zostera marina (Olesen and Sand-Jensen,
1994b) or for Z. novazelandica (Ramage and Schiel,
1999), presumably because of the slower horizontal
growth of these species resulting in densely packed
patches near edge and relatively high nutrient availability at the study sites.
Whereas patch extension is governed by the capacity for rhizome growth there are no constraints
on patch recession or mortality. Net growth of
patches can be substantially lower than expected
from the potential rhizome growth due to loss processes caused by physical and biological disturbance
agents. Hence, sediment reworking by burrowing animals can cause disruption of the patch edge (Philippart, 1994; Townsend and Fonseca, 1998) and the
erosion of patches at windward margins represents
significant disturbances to inhibit expansion of seagrass patches or to cause recession (Fonseca and
Bell, 1998). Restriction of patch expansion by the exposure to high flow velocity and the predominantly
growth of patches in the shelter, greatly influence the
shape and heterogeneity of patches (Fonseca et al.,
1983). Accordingly, patch edges are expected to be
highly dynamic as confirmed by the high rates of
shoot mortality and recruitment found at patch margin compared to inside the patches (Duarte and SandJensen, 1990b).
Disturbances above a certain magnitude are also
a common source of patch mortality and even large
meadows can disappear during extreme storm events
(e.g. Orth and Moore, 1983; den Hartog, 1987). The
mortality risk is size-dependent and patch losses
are often confined to the smaller patches below a
certain threshold size, presumably defined by the
species involved and the disturbance regime within
the study area (Duarte and Sand-Jensen, 1990a;
Olesen and Sand-Jensen, 1994b; Vivondo et al.,
1997; Ramage and Schiel, 1999). These negative
effects of size are probably linked to lack of mutual protection and firm anchorage leading to higher
susceptibility to physical disturbances and nutrient
stress in small patches. Consequently, patch formation from seeds is typically very inefficient due to
high seed and seedling mortality and often less than
10% of newly established seedlings survive past
their first year (Churchill, 1983; Duarte and SandJensen, 1990a; Harrison, 1993; Kaldy and Dunton,
1999), although higher survival probabilities have
been reported in some populations of Zostera marina
(24%, Olesen and Sand-Jensen, 1994b) and for Enhalus acoroides, and Thalassia hemprichii (19 and
22%, Olesen et al., in press). Moreover, the probability of newly established patches to reach a large
size is low, as small patches are subject to rapid
turnover, as indicated by positively skewed patch
size distribution that is frequently found in patchy
seagrass stands (e.g. Vidondo et al., 1997). The production of sexual and vegetative propagules remains
the term that serves to maintain the positive side
of patch dynamics = patch production – thereby
ensuring the recovery and formation of seagrass
meadows.
C. Resulting Patch Dynamics
The spatial and temporal dynamics of seagrass
patches is strongly influenced by the magnitude and
frequency of physical disturbances in a given area
and by the capacity of the species involved to persist and recover from disturbances. Some seagrass
populations experience continuous patch extinction
and replacement, which maintains the vegetation in
a permanent state of colonization and promotes the
development of a mosaic of patches of different age
and developmental stages (Duarte and Sand-Jensen,
1990a; Olesen and Sand-Jensen, 1994b; Vidondo
et al., 1997). When in balance, such populations
will maintain a dynamic equilibrium with a uniformity of patch distribution in time and space such
that an overall landscape equilibrium of patches applies. This has been demonstrated for Cymodocea
nodosa growing on highly mobile sediments where
the time interval between the passage of consecutive
sub-aquatic sand dunes allowed a close balance between loss of vegetation caused by erosion and burial