284
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
landscape dominated by a superior competitor. The
blowouts in seagrass meadows are very similar to the
wind-induced migrating wind-throws responsible
for the ‘wave-regenerated’ evergreen high-latitude
forests, in which gaps in the forest generated move
slowly upwind at a rate of 1–3 m year
−1 as old
trees succumb to wind fall and younger trees recruit into the space cleared by the wind falls (e.g.
Cooper, 1913; Sprugel, 1976). As gap formation
and closure are not synchronized in the meadow,
a mosaic of different stages of gap dynamics may
be encountered in a meadow, maintaining a mosaic
of species diversity in the meadow (Duarte et al.,
2000). There are, of course, exceptions to this simplified successional pattern, as pioneer species may
sometimes develop strategies, such as the formation
of a three-dimensional canopy, preventing their exclusion (Fourqurean et al., 1995).
The closure of gaps is primarily dependent on
clonal processes, through the extension of rhizomes
of the plants at the periphery of the patches onto the
gap, as demonstrated by multiple examinations of
gap dynamics, including experimental approaches
(e.g. Williams, 1987; Rasheed, 1999), as well as observations of recovery of gaps following disturbance,
such as those produced by propellers (e.g. Andorfer
and Dawes, 2002; Kenworthy et al., 2002).
VI. Dynamics of Seagrass Meadows
at Different Time Scales
A. Disturbance
As seagrass meadows provide a variety of ecosystem services, there is much focus on the range and
time scales of their variability. At a given site, this
variability reflects the frequency and magnitude of
disturbances relative to the capacity of the species
to resist and recover. Disturbances can be natural or human-induced and are defined here as factors preventing seagrasses from reaching their maximum potential abundance. Natural disturbances
most commonly responsible for seagrass loss include extreme climatic events (such as hurricanes)
and biological interaction such as diseases, grazing,
and bioturbation, while the most common humaninduced disturbances are eutrophication, leading to
reduced water clarity and quality, and dredging,
filling, and certain fishing practices causing direct
physical damage (see review by Short and WyllieEcheverria, 1996). Changes in light conditions, temperature, and water level, due to climate changes, are
also likely to affect the world’s seagrass meadows
both directly and indirectly and cause large-scale
variations, but this aspect is not treated separately
here (for further discussion see Walker et al., Chapter 23, and Ralph et al., Chapter 24 and Kenworthy
et al., Chapter 25). Tolerance toward disturbances as
well as growth and recolonization potentials differ
among species and various seagrass species therefore show different temporal and spatial dynamics.
While individual seagrass shoots have a life
span of weeks or decades depending on species,
meadows, and clones, may in extreme cases persist for centuries or millennia (Reusch et al., 1999;
Hemminga and Duarte, 2000). Hence, studies on
temporal dynamics of seagrasses tend to focus on
different attributes depending on the time scale of
interest. Seasonal studies often involve a small spatial scale and focus on attributes such as shoot density or biomass while long-term studies generally
involve large spatial scales with focus on population
attributes such as presence/absence or area cover.
The following sections give examples of changes
in abundance of seagrasses on seasonal and interannual time scales and discuss long-term perspectives. For further discussions on landscape dynamics of seagrass meadows, the reader is referred to the
chapters by Walker et al., Chapter 23 and Bell et al.,
Chapter 26.
B. Seasonal Fluctuations
The biomass of seagrasses may change markedly
over an annual cycle. A large-scale compilation of
data from 14 seagrass species shows that, on average, 70% of the intra-annual variability in biomass
of seagrasses reflects seasonal responses (Duarte,
1989). As seasonal variability in seagrass biomass
is mainly regulated by changes in light and temperature associated with the solar cycle (Sand-Jensen,
1975; Perez and Romero, 1992; Alcoverro et al.,
1995), it changes with latitude. In fact, there seems
to be a latitude-dependent upper boundary to seasonal biomass variability rather than a simple linear coupling between the two parameters (Fig. 6;
Duarte, 1989). Hence, temperate seagrass communities tend to show greater seasonality but also a
wider range of seasonal responses than tropical and
subtropical communities, which maintain a more stable biomass throughout the year. However, there is
still substantial seasonal variability in some tropical
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
landscape dominated by a superior competitor. The
blowouts in seagrass meadows are very similar to the
wind-induced migrating wind-throws responsible
for the ‘wave-regenerated’ evergreen high-latitude
forests, in which gaps in the forest generated move
slowly upwind at a rate of 1–3 m year
−1 as old
trees succumb to wind fall and younger trees recruit into the space cleared by the wind falls (e.g.
Cooper, 1913; Sprugel, 1976). As gap formation
and closure are not synchronized in the meadow,
a mosaic of different stages of gap dynamics may
be encountered in a meadow, maintaining a mosaic
of species diversity in the meadow (Duarte et al.,
2000). There are, of course, exceptions to this simplified successional pattern, as pioneer species may
sometimes develop strategies, such as the formation
of a three-dimensional canopy, preventing their exclusion (Fourqurean et al., 1995).
The closure of gaps is primarily dependent on
clonal processes, through the extension of rhizomes
of the plants at the periphery of the patches onto the
gap, as demonstrated by multiple examinations of
gap dynamics, including experimental approaches
(e.g. Williams, 1987; Rasheed, 1999), as well as observations of recovery of gaps following disturbance,
such as those produced by propellers (e.g. Andorfer
and Dawes, 2002; Kenworthy et al., 2002).
VI. Dynamics of Seagrass Meadows
at Different Time Scales
A. Disturbance
As seagrass meadows provide a variety of ecosystem services, there is much focus on the range and
time scales of their variability. At a given site, this
variability reflects the frequency and magnitude of
disturbances relative to the capacity of the species
to resist and recover. Disturbances can be natural or human-induced and are defined here as factors preventing seagrasses from reaching their maximum potential abundance. Natural disturbances
most commonly responsible for seagrass loss include extreme climatic events (such as hurricanes)
and biological interaction such as diseases, grazing,
and bioturbation, while the most common humaninduced disturbances are eutrophication, leading to
reduced water clarity and quality, and dredging,
filling, and certain fishing practices causing direct
physical damage (see review by Short and WyllieEcheverria, 1996). Changes in light conditions, temperature, and water level, due to climate changes, are
also likely to affect the world’s seagrass meadows
both directly and indirectly and cause large-scale
variations, but this aspect is not treated separately
here (for further discussion see Walker et al., Chapter 23, and Ralph et al., Chapter 24 and Kenworthy
et al., Chapter 25). Tolerance toward disturbances as
well as growth and recolonization potentials differ
among species and various seagrass species therefore show different temporal and spatial dynamics.
While individual seagrass shoots have a life
span of weeks or decades depending on species,
meadows, and clones, may in extreme cases persist for centuries or millennia (Reusch et al., 1999;
Hemminga and Duarte, 2000). Hence, studies on
temporal dynamics of seagrasses tend to focus on
different attributes depending on the time scale of
interest. Seasonal studies often involve a small spatial scale and focus on attributes such as shoot density or biomass while long-term studies generally
involve large spatial scales with focus on population
attributes such as presence/absence or area cover.
The following sections give examples of changes
in abundance of seagrasses on seasonal and interannual time scales and discuss long-term perspectives. For further discussions on landscape dynamics of seagrass meadows, the reader is referred to the
chapters by Walker et al., Chapter 23 and Bell et al.,
Chapter 26.
B. Seasonal Fluctuations
The biomass of seagrasses may change markedly
over an annual cycle. A large-scale compilation of
data from 14 seagrass species shows that, on average, 70% of the intra-annual variability in biomass
of seagrasses reflects seasonal responses (Duarte,
1989). As seasonal variability in seagrass biomass
is mainly regulated by changes in light and temperature associated with the solar cycle (Sand-Jensen,
1975; Perez and Romero, 1992; Alcoverro et al.,
1995), it changes with latitude. In fact, there seems
to be a latitude-dependent upper boundary to seasonal biomass variability rather than a simple linear coupling between the two parameters (Fig. 6;
Duarte, 1989). Hence, temperate seagrass communities tend to show greater seasonality but also a
wider range of seasonal responses than tropical and
subtropical communities, which maintain a more stable biomass throughout the year. However, there is
still substantial seasonal variability in some tropical