Chapter 26 A Landscape Perspective
637
In seagrass systems, the pathogenic stage of infection by protists of the genus Labyrinthula causes
lesions easily visible as dark longitudinal bands
on seagrass blades (Muehlstein et al., 1991; Burdick et al., 1993). In some cases Labyrinthula spp.
does not cause noticeable harm to the seagrass host
(Vergeer and den Hartog, 1994). But in some locations disease incidence may be high and lesion
cover extensive (Burdick et al., 1993). Ralph and
Short (2002) recently provided convincing data that
Labyrinthula spp. is a primary pathogen of Z. marina and that infection by this saprophytic ‘slime
mold’ (sensu Muehlstein et al., 1988) may operate
as a modifier of landscapes by increasing the mortality rate of seagrass shoots or ramets and reducing
the overall photosynthetic capacity of the seagrass
plant (see also Moore and Short, Chapter 16).
Seagrass die-offs in the past have been linked to
outbreaks of Labyrinthula spp. Infection of the temperate seagrass Z. marina by Labyrinthula zosterae
is thought to have caused the 1930s ‘wasting disease’ that dramatically reduced seagrass cover in the
entire North Atlantic (DeJonge and DeJonge 1992;
Duarte et al., Chapter 11) and is suspected to be a primary cause in the decline of some seagrass patches
more recently (Short et al., 1986; Short et al., 1988;
Burdick et al., 1993). Occasionally, complete local
landscapes may be lost and/or replaced with new
structural elements (e.g. loss of Z. marina in the
northeastern USA and its replacement by macroalgae; Short et al., 1988). Studies comparing the temporal spread of infection with rates of change in
spatial patterns, along with information on environmental conditions (physical parameters), may
help identify what factors (e.g. salinity, temperature)
might be associated with highly contagious events of
extensive lesion cover. A crucial challenge is to be
able to identify under what conditions outbreaks of
disease may (and do) occur in seagrass systems, and
to link such changes to changes in the landscape
mosaic.
C. Algal Blooms and Macroalgae
Algal blooms, including invasions by some rhizophytic macroalgae of the genus Caulerpa, may also
alter seagrass landscape structure. Such blooms have
become common events in selected geographic locations. For example, Caulerpa taxifolia was recently introduced into new areas of the Mediterranean where it has established both persistent and
expanding populations (Meinesz and Hesse, 1991;
Kenworthy et al., Chapter 25). This rapidly growing
species can spread across long distances by fragmentation and may outcompete other vegetation in the
shallow water habitats. Caulerpa taxifolia is thought
to be competitively successful because it (1) extends
laterally across the substrate as well as branching upward into a large, dense mat without requiring a solid
structure for attachment, and (2) it can survive and
thrive in the low light conditions present in the subcanopy of other vegetation (Boudouresque, 1997).
Caulerpa species also contain large amounts of secondary metabolites and are highly unpalatable to
most herbivores (Lemee et al., 1996), thereby reducing the potential for control by grazing. Secondary
metabolites may also severely limit epiphyte attachment. In combination, reduced herbivory and epiphyte load may facilitate growth and fragmentation
of Caulerpa, and thus increase landscape coverage.
Detailed impacts on seagrass systems caused by
the increasing dominance of Caulerpa species are
starting to be recognized, especially in European
waters. For example, researchers have reported that
C. racemosa can displace seagrass by overgrowing seagrass blades as well as invade bare areas
more quickly than seagrass (Ceccherelli et al., 2000).
Close examination of Caulerpa in seagrass beds
has revealed that these macroalga can overgrow rhizomes of seagrasses (De Villele and Verlaque, 1995).
Additionally, C. taxifolia has been reported to cause
chlorosis and decreased abundance and longevity of
Posidonia oceanica, and invasion of seagrass beds
has also been followed by deterioration of rhizomes
and roots with a corresponding increase of hypoxia
in sediments (De Villele and Verlaque, 1995). The
appearance of invasive C. taxifolia on the west coast
of the USA has recently sounded an alarm call as
well. However, rapid and extreme efforts to eradicate
C. taxifolia upon arrival have thus far prevented the
predicted smothering of California coastal ecosystems predicted by Jousson et al. (2000). Assessments
of the large scale dynamics in the spatial structure of
a seagrass ecosystem when influenced by macroalgal
blooms would be most effective using a landscape
approach.
Further interaction of seagrasses and macroalgae has been seen in the creation of gaps linked to
macroalgal accumulation disturbances, which may
lead to increased heterogeneity of faunal components within seagrass landscapes. Using an experimental approach, Holmquist (1997) created gaps
637
In seagrass systems, the pathogenic stage of infection by protists of the genus Labyrinthula causes
lesions easily visible as dark longitudinal bands
on seagrass blades (Muehlstein et al., 1991; Burdick et al., 1993). In some cases Labyrinthula spp.
does not cause noticeable harm to the seagrass host
(Vergeer and den Hartog, 1994). But in some locations disease incidence may be high and lesion
cover extensive (Burdick et al., 1993). Ralph and
Short (2002) recently provided convincing data that
Labyrinthula spp. is a primary pathogen of Z. marina and that infection by this saprophytic ‘slime
mold’ (sensu Muehlstein et al., 1988) may operate
as a modifier of landscapes by increasing the mortality rate of seagrass shoots or ramets and reducing
the overall photosynthetic capacity of the seagrass
plant (see also Moore and Short, Chapter 16).
Seagrass die-offs in the past have been linked to
outbreaks of Labyrinthula spp. Infection of the temperate seagrass Z. marina by Labyrinthula zosterae
is thought to have caused the 1930s ‘wasting disease’ that dramatically reduced seagrass cover in the
entire North Atlantic (DeJonge and DeJonge 1992;
Duarte et al., Chapter 11) and is suspected to be a primary cause in the decline of some seagrass patches
more recently (Short et al., 1986; Short et al., 1988;
Burdick et al., 1993). Occasionally, complete local
landscapes may be lost and/or replaced with new
structural elements (e.g. loss of Z. marina in the
northeastern USA and its replacement by macroalgae; Short et al., 1988). Studies comparing the temporal spread of infection with rates of change in
spatial patterns, along with information on environmental conditions (physical parameters), may
help identify what factors (e.g. salinity, temperature)
might be associated with highly contagious events of
extensive lesion cover. A crucial challenge is to be
able to identify under what conditions outbreaks of
disease may (and do) occur in seagrass systems, and
to link such changes to changes in the landscape
mosaic.
C. Algal Blooms and Macroalgae
Algal blooms, including invasions by some rhizophytic macroalgae of the genus Caulerpa, may also
alter seagrass landscape structure. Such blooms have
become common events in selected geographic locations. For example, Caulerpa taxifolia was recently introduced into new areas of the Mediterranean where it has established both persistent and
expanding populations (Meinesz and Hesse, 1991;
Kenworthy et al., Chapter 25). This rapidly growing
species can spread across long distances by fragmentation and may outcompete other vegetation in the
shallow water habitats. Caulerpa taxifolia is thought
to be competitively successful because it (1) extends
laterally across the substrate as well as branching upward into a large, dense mat without requiring a solid
structure for attachment, and (2) it can survive and
thrive in the low light conditions present in the subcanopy of other vegetation (Boudouresque, 1997).
Caulerpa species also contain large amounts of secondary metabolites and are highly unpalatable to
most herbivores (Lemee et al., 1996), thereby reducing the potential for control by grazing. Secondary
metabolites may also severely limit epiphyte attachment. In combination, reduced herbivory and epiphyte load may facilitate growth and fragmentation
of Caulerpa, and thus increase landscape coverage.
Detailed impacts on seagrass systems caused by
the increasing dominance of Caulerpa species are
starting to be recognized, especially in European
waters. For example, researchers have reported that
C. racemosa can displace seagrass by overgrowing seagrass blades as well as invade bare areas
more quickly than seagrass (Ceccherelli et al., 2000).
Close examination of Caulerpa in seagrass beds
has revealed that these macroalga can overgrow rhizomes of seagrasses (De Villele and Verlaque, 1995).
Additionally, C. taxifolia has been reported to cause
chlorosis and decreased abundance and longevity of
Posidonia oceanica, and invasion of seagrass beds
has also been followed by deterioration of rhizomes
and roots with a corresponding increase of hypoxia
in sediments (De Villele and Verlaque, 1995). The
appearance of invasive C. taxifolia on the west coast
of the USA has recently sounded an alarm call as
well. However, rapid and extreme efforts to eradicate
C. taxifolia upon arrival have thus far prevented the
predicted smothering of California coastal ecosystems predicted by Jousson et al. (2000). Assessments
of the large scale dynamics in the spatial structure of
a seagrass ecosystem when influenced by macroalgal
blooms would be most effective using a landscape
approach.
Further interaction of seagrasses and macroalgae has been seen in the creation of gaps linked to
macroalgal accumulation disturbances, which may
lead to increased heterogeneity of faunal components within seagrass landscapes. Using an experimental approach, Holmquist (1997) created gaps