560
D. I. Walker, G. A. Kendrick, and A. J. McComb
resilience to large spatial-scale losses in the seagrass
assemblages over the same 30-year period.
Patch dynamics studies of temperate Australian seagrasses have been limited (Clarke, 1987;
Kendrick et al., 1999). Elsewhere in the world, patch
dynamics studies have predominantly focussed on
fast growing seagrass species like Zostera, Cymodocea, Thalassia, and Halodule (Vidondo et al., 1997;
Ramage and Schiel, 1999; Turner et al., 1999; Robbins and Bell, 2000). These studies have all demonstrated that seagrass patches may move several meters within a year and at rates faster than that recorded
for the growth of individual rhizomes. Large-scale
seagrass losses continue to occur as a result of natural processes (such as sand inundation) and human
activities (presently mainly associated with eutrophication and, on Success Bank, shell-sand dredging).
V. Case Study 2: Florida Bay
Changes, both declines and recoveries, are also
apparent in Florida Bay. Florida Bay is a large
(19,000 km
2 ), shallow (<2 m depth), carbonate
system dominated by Thalassia testudinum, growing over 90% of subtidal mud banks and basins
(Zieman et al., 1989). Seagrasses in Florida Bay have
been the subject of intensive and consistent studies over the last 15–20 years (Zieman et al. 1989;
Fourqurean et al., 1992a,b, 1995; Hall et al., 1999;
Durako et al., 2002). A mass mortality (die-off) event
in 1987, when 4,000 ha of Thalassia died and 24,000
ha were affected, was attributed to the combination of a protistan pathogen, Labyrinthula (Durako
and Kuss, 1994), sulphide toxicity (Carlson et al.,
1994), elevated water temperatures (Hall et al., 1999)
and salinity increases (to 50 psu) (Zieman et al.,
1999). The salinity increase, following alterations to
the hydrology (canal dredging, dike building in the
Everglades) and several years of drought in southern
Florida, appeared to be the major environmental contributor to the die-off of Thalassia (Fourqurean et al.,
1995). Thalassia-dominated subtidal areas were initially replaced by Halodule wrightii (Fourqurean and
Robblee, 1999). Following the return to more normal rainfall conditions in the early 1990s, all seagrasses were then affected by phytoplankton and
algal blooms that reduced light climate available for
seagrass growth and lead to losses of not just Thalassia and Halodule, but also of Syringodium filiforme
(Hall et al., 1999).
Subsequent monitoring has demonstrated extensive changes in seagrass populations across Florida
Bay (Durako et al., 2002). For example, chronic turbidity in western Florida Bay leads to an increase
in distribution and abundance of low-light adapted
species such as Halodule wrightii and Halophila engelmanii (Durako et al., 2002).
These changes have been documented by extensive systematic random sampling using the
Braun-Blanquet frequency/abundance technique
(Mueller-Dombois and Ellenberg, 1974), a rapid
semi-quantitative visual assessment, at over 300 sites
in Florida Bay, over a 10-year period (1995–2004).
Spatial trends in the Braun-Blanquet data have been
mapped using a Krigging algorithm (Durako et al.,
2002), and total basin-level species abundances
have been calculated from the krigged grid data
(Table 3). These abundance maps reveal that although Thalassia is still the dominant genus in
Florida Bay, it has not recovered its pre-decline distribution; Halodule abundance has dramatically increased, so that the previous five fold abundance
of Thalassia over Halodule is now only three fold
(Durako et al., 2002). Zieman (1982) suggested
that, historically, Halodule was more abundant and
that Thalassia increases in Northeast Florida Bay,
prior to the die-off, resulted from the alterations to
the hydrology increasing salinities in Florida Bay
(“lagoonization”).
Many studies on the possible mechanisms of seagrass loss, as well as continued monitoring of seagrass and macro-algal distributions in Florida Bay
have been conducted. There is currently an attempt
to manage the hydrology of Florida Bay to minimize the occurrence of hypersaline conditions, as
part of the Comprehensive Everglades Restoration
Program (CERP), a 10-year $8 billion ecosystem
restoration program. One of the goals of CERP is
to sustainably maintain the seagrasses in Florida
Bay, especially Thalassia, via engineering solutions to manage drainage. Restoration efforts in this
changing system will be challenged by the effects
of increasing population pressures on Florida Bay,
especially in terms of increasing water demands.
VI. Species-Specific Recruitment
and Growth Characteristics
Clearly, species-specific traits in clonal growth and
seedling recruitment play a large role in the timing
D. I. Walker, G. A. Kendrick, and A. J. McComb
resilience to large spatial-scale losses in the seagrass
assemblages over the same 30-year period.
Patch dynamics studies of temperate Australian seagrasses have been limited (Clarke, 1987;
Kendrick et al., 1999). Elsewhere in the world, patch
dynamics studies have predominantly focussed on
fast growing seagrass species like Zostera, Cymodocea, Thalassia, and Halodule (Vidondo et al., 1997;
Ramage and Schiel, 1999; Turner et al., 1999; Robbins and Bell, 2000). These studies have all demonstrated that seagrass patches may move several meters within a year and at rates faster than that recorded
for the growth of individual rhizomes. Large-scale
seagrass losses continue to occur as a result of natural processes (such as sand inundation) and human
activities (presently mainly associated with eutrophication and, on Success Bank, shell-sand dredging).
V. Case Study 2: Florida Bay
Changes, both declines and recoveries, are also
apparent in Florida Bay. Florida Bay is a large
(19,000 km
2 ), shallow (<2 m depth), carbonate
system dominated by Thalassia testudinum, growing over 90% of subtidal mud banks and basins
(Zieman et al., 1989). Seagrasses in Florida Bay have
been the subject of intensive and consistent studies over the last 15–20 years (Zieman et al. 1989;
Fourqurean et al., 1992a,b, 1995; Hall et al., 1999;
Durako et al., 2002). A mass mortality (die-off) event
in 1987, when 4,000 ha of Thalassia died and 24,000
ha were affected, was attributed to the combination of a protistan pathogen, Labyrinthula (Durako
and Kuss, 1994), sulphide toxicity (Carlson et al.,
1994), elevated water temperatures (Hall et al., 1999)
and salinity increases (to 50 psu) (Zieman et al.,
1999). The salinity increase, following alterations to
the hydrology (canal dredging, dike building in the
Everglades) and several years of drought in southern
Florida, appeared to be the major environmental contributor to the die-off of Thalassia (Fourqurean et al.,
1995). Thalassia-dominated subtidal areas were initially replaced by Halodule wrightii (Fourqurean and
Robblee, 1999). Following the return to more normal rainfall conditions in the early 1990s, all seagrasses were then affected by phytoplankton and
algal blooms that reduced light climate available for
seagrass growth and lead to losses of not just Thalassia and Halodule, but also of Syringodium filiforme
(Hall et al., 1999).
Subsequent monitoring has demonstrated extensive changes in seagrass populations across Florida
Bay (Durako et al., 2002). For example, chronic turbidity in western Florida Bay leads to an increase
in distribution and abundance of low-light adapted
species such as Halodule wrightii and Halophila engelmanii (Durako et al., 2002).
These changes have been documented by extensive systematic random sampling using the
Braun-Blanquet frequency/abundance technique
(Mueller-Dombois and Ellenberg, 1974), a rapid
semi-quantitative visual assessment, at over 300 sites
in Florida Bay, over a 10-year period (1995–2004).
Spatial trends in the Braun-Blanquet data have been
mapped using a Krigging algorithm (Durako et al.,
2002), and total basin-level species abundances
have been calculated from the krigged grid data
(Table 3). These abundance maps reveal that although Thalassia is still the dominant genus in
Florida Bay, it has not recovered its pre-decline distribution; Halodule abundance has dramatically increased, so that the previous five fold abundance
of Thalassia over Halodule is now only three fold
(Durako et al., 2002). Zieman (1982) suggested
that, historically, Halodule was more abundant and
that Thalassia increases in Northeast Florida Bay,
prior to the die-off, resulted from the alterations to
the hydrology increasing salinities in Florida Bay
(“lagoonization”).
Many studies on the possible mechanisms of seagrass loss, as well as continued monitoring of seagrass and macro-algal distributions in Florida Bay
have been conducted. There is currently an attempt
to manage the hydrology of Florida Bay to minimize the occurrence of hypersaline conditions, as
part of the Comprehensive Everglades Restoration
Program (CERP), a 10-year $8 billion ecosystem
restoration program. One of the goals of CERP is
to sustainably maintain the seagrasses in Florida
Bay, especially Thalassia, via engineering solutions to manage drainage. Restoration efforts in this
changing system will be challenged by the effects
of increasing population pressures on Florida Bay,
especially in terms of increasing water demands.
VI. Species-Specific Recruitment
and Growth Characteristics
Clearly, species-specific traits in clonal growth and
seedling recruitment play a large role in the timing