562
D. I. Walker, G. A. Kendrick, and A. J. McComb
and extent of seagrass recovery. The following section summarizes our present knowledge on seedling
recruitment and clonal growth for a selection of seagrass species and complements the more detailed
presentation in Chapters 5 and 11.
Rates of loss/recovery are strongly linked to lifespan, clonal growth, and seedling recruitment of
specific species of seagrasses. The temporal and spatial scales of rates of plant growth determine human perceptions of observed patterns of changes
in seagrass landscapes. For example, Zostera regrowth is often rapid, in months to years (Baldwin
and Lovvorn, 1994), as is Halodule (Williams, 1988;
Robbins and Bell, 2000) whereas Posidonia species
only regrow at the scale of decades (Shepherd et al.,
1989; Kirkman and Kuo, 1996; Meehan and West,
2000). Long-lived meadow forming genera such as
Posidonia and Thalassia are recorded as not returning from disturbance. Recovery is often a process
that takes decades (Duarte, 1995), but the loss process is perceived as being more rapid, given our often
poor ability to detect initial declines in seagrasses.
Despite human perceptions, scales of decline for seagrasses are comparable with scales of their recovery if the original cause of loss is removed. Unfortunately, for Posidonia species this would require
that perturbations occurred less than once a century (Duarte 1995), which does not correspond with
the observed rates of loss (see Cambridge and McComb, 1994; Silberstein et al., 1996; (Western Australia) Pergent and Pergent-Martini, 1991; Marba
and Duarte, 1997 (Mediterranean)).
VII. Can Seagrass Management
be Proactive?
Effective management of seagrasses requires detailed understanding of their distributions, growth
rates, and rates of change under natural conditions,
integrating information on rates of seedling recruitment, clonal expansion, and recovery of habitat conditions, and consideration of the changes of these
rates during recovery. In addition, knowledge of
threshold values of decline drivers is essential to
define effective management polices to prevent irreversible, at least at human timescale, seagrass losses.
Few areas of the world have this detailed knowledge.
Seagrass protection requires at least prevention of
their destruction by coastal engineering works or
other physical impacts (see Duarte et al., 2003 for a
review), and, more importantly, reduction of organic
and nutrient inputs as well as maintenance of light
(at 20% of surface irradiance, Dennison et al., 1993)
to ensure an adequate light climate.
Seagrass management is inadequate on a global
scale. Over the last two decades, the loss of seagrass
from direct and indirect human impacts amounts to
18% of the documented seagrass area (Green and
Short, 2003). Seagrass losses occur as development
pressure on coastlines increases. As well as direct
impacts, use and development of catchments creates indirect impacts on otherwise highly regulated
seagrass areas. Effectiveness of protection of seagrass in marine protected areas is still unclear. Assessments of recovery of Posidonia oceanica beds
in Cabrera Archipelago National Park (Spain) concluded that while regulation has improved the status
of seagrasses in the park, recovery is a very slow process requiring centuries to be completed, and will be
threatened by organic inputs from visitors to the park
(Marb` a et al., 2002). Documenting such responses
requires effective monitoring programs.
Landscape scale mapping studies (see also Bell
et al., Chapter 26) require the setting of appropriate
time and space scales for observing recovery in seagrass meadows from known recruitment and growth
rates of the specific seagrass species being studied.
Unfortunately, many managers of coastal environments regard seagrass distributions as being fixed
and static over a human lifespan whereas the evidence suggests they are not (see Chapters 24, 25,
and 26).
Seagrass can be viewed as migrating through shallow subtidal or intertidal habitat (see Chapters 5, 11,
and 26), and as the boundaries of seagrass meadows and patches are not static, coastal habitat may
be regarded as potential seagrass even when there
is no seagrass present (see Chapters 25, 26). Human impacts, particularly resulting in eutrophication
and increased sedimentation, will increase seagrass
losses worldwide (Duarte et al., 2003). Potential seagrass habitat is now being recognised by legislation
in some countries, and if management is effective,
the rates of seagrass recolonisation, as documented
in this chapter, can proceed. The balance, however,
is still toward seagrass decline.
References
Alcoverro T, Duarte CM and Romero J (1995) Annual growth
dynamics of Posidonia oceanica: Contribution of large-scale
D. I. Walker, G. A. Kendrick, and A. J. McComb
and extent of seagrass recovery. The following section summarizes our present knowledge on seedling
recruitment and clonal growth for a selection of seagrass species and complements the more detailed
presentation in Chapters 5 and 11.
Rates of loss/recovery are strongly linked to lifespan, clonal growth, and seedling recruitment of
specific species of seagrasses. The temporal and spatial scales of rates of plant growth determine human perceptions of observed patterns of changes
in seagrass landscapes. For example, Zostera regrowth is often rapid, in months to years (Baldwin
and Lovvorn, 1994), as is Halodule (Williams, 1988;
Robbins and Bell, 2000) whereas Posidonia species
only regrow at the scale of decades (Shepherd et al.,
1989; Kirkman and Kuo, 1996; Meehan and West,
2000). Long-lived meadow forming genera such as
Posidonia and Thalassia are recorded as not returning from disturbance. Recovery is often a process
that takes decades (Duarte, 1995), but the loss process is perceived as being more rapid, given our often
poor ability to detect initial declines in seagrasses.
Despite human perceptions, scales of decline for seagrasses are comparable with scales of their recovery if the original cause of loss is removed. Unfortunately, for Posidonia species this would require
that perturbations occurred less than once a century (Duarte 1995), which does not correspond with
the observed rates of loss (see Cambridge and McComb, 1994; Silberstein et al., 1996; (Western Australia) Pergent and Pergent-Martini, 1991; Marba
and Duarte, 1997 (Mediterranean)).
VII. Can Seagrass Management
be Proactive?
Effective management of seagrasses requires detailed understanding of their distributions, growth
rates, and rates of change under natural conditions,
integrating information on rates of seedling recruitment, clonal expansion, and recovery of habitat conditions, and consideration of the changes of these
rates during recovery. In addition, knowledge of
threshold values of decline drivers is essential to
define effective management polices to prevent irreversible, at least at human timescale, seagrass losses.
Few areas of the world have this detailed knowledge.
Seagrass protection requires at least prevention of
their destruction by coastal engineering works or
other physical impacts (see Duarte et al., 2003 for a
review), and, more importantly, reduction of organic
and nutrient inputs as well as maintenance of light
(at 20% of surface irradiance, Dennison et al., 1993)
to ensure an adequate light climate.
Seagrass management is inadequate on a global
scale. Over the last two decades, the loss of seagrass
from direct and indirect human impacts amounts to
18% of the documented seagrass area (Green and
Short, 2003). Seagrass losses occur as development
pressure on coastlines increases. As well as direct
impacts, use and development of catchments creates indirect impacts on otherwise highly regulated
seagrass areas. Effectiveness of protection of seagrass in marine protected areas is still unclear. Assessments of recovery of Posidonia oceanica beds
in Cabrera Archipelago National Park (Spain) concluded that while regulation has improved the status
of seagrasses in the park, recovery is a very slow process requiring centuries to be completed, and will be
threatened by organic inputs from visitors to the park
(Marb` a et al., 2002). Documenting such responses
requires effective monitoring programs.
Landscape scale mapping studies (see also Bell
et al., Chapter 26) require the setting of appropriate
time and space scales for observing recovery in seagrass meadows from known recruitment and growth
rates of the specific seagrass species being studied.
Unfortunately, many managers of coastal environments regard seagrass distributions as being fixed
and static over a human lifespan whereas the evidence suggests they are not (see Chapters 24, 25,
and 26).
Seagrass can be viewed as migrating through shallow subtidal or intertidal habitat (see Chapters 5, 11,
and 26), and as the boundaries of seagrass meadows and patches are not static, coastal habitat may
be regarded as potential seagrass even when there
is no seagrass present (see Chapters 25, 26). Human impacts, particularly resulting in eutrophication
and increased sedimentation, will increase seagrass
losses worldwide (Duarte et al., 2003). Potential seagrass habitat is now being recognised by legislation
in some countries, and if management is effective,
the rates of seagrass recolonisation, as documented
in this chapter, can proceed. The balance, however,
is still toward seagrass decline.
References
Alcoverro T, Duarte CM and Romero J (1995) Annual growth
dynamics of Posidonia oceanica: Contribution of large-scale