Chapter 25 Seagrass Conservation Biology
607
2. Multiple Stressors
A comprehensive research agenda is needed to predict the response of the seagrass biome to multiple stressors, especially when considering the
consequences of global climate change, resource extraction by fisheries, and the growing human population in the coastal zone (Short and Neckles,
1999; Section IV, Ralph et al., Chapter 24). The selection and application of appropriate management
regimes depends on the ability to distinguish natural from human stressors (sensu Jameson et al.,
2002). For example, there is strong evidence suggesting that increases in nutrient loading and siltation, two stressors often co-occurring in disturbed
systems, can lead to long-term shifts in taxonomic
composition of seagrass communities by placing
extraordinary physiological demands on the biota
(Borum, 1985; Tomasko and Lapointe, 1991; Nixon,
1995; Fourqurean et al., 1995; Short and Burdick,
1996; Terrados et al., 1998; see also Walker et al.,
Chapter 23 and Ralph et al., Chapter 24). Many seagrasses have complex life histories which are affected by different stressors over their lifetime and
the distribution, abundance and species composition
of the biome results from many different factors operating at varying spatial and temporal scales, and
seldom isolated from one another.
In the Mediterranean, the input of nutrients resulting from aquaculture activities was responsible of
significant declines of a P. oceanica meadow around
fish farming facilities (Pergent et al., 1999; Cancemi
et al., 2000; Dimech et al., 2000). Prior to the placement of the fish farm, these meadows were considered ‘healthy and productive’ but appear to have been
negatively affected by a single human stressor, the
introduction of aquaculture. Thus, if healthy meadows are so readily affected, it will be important in
the future to understand how beds already stressed
respond to additional stressors such as aquaculture
practices.
Mass mortality of seagrasses over very large areas of an entire subtropical estuary in south Florida,
USA (Florida Bay; 25.00 N; 80.45 W) fragmented
the seagrass system resulting in a cascade of impacts affecting both the benthic and pelagic communities in a much larger geographical region (Roblee
et al., 1991; Fourqurean and Roblee, 1999; Walker
et al., Chapter 23). The causes of this mass mortality were hypothesized to result from a combination
of factors including water diversion, eutrophication
and an unusually advanced community succession
in the absence of physical disturbances associated
with an abnormally low recurrence of hurricanes
in south Florida. This example demonstrated that
the magnitude and complexity of these multiple factors must be taken into account in developing a plan
for restoration of Florida Bay, because it is located
within a dynamic and interconnected system of uplands, wetlands, estuaries, reef tract, and continental
shelf (Jameson et al., 2002). The south Florida region
is one of the most densely populated coastal zones in
North America where human stressors are numerous
and widespread. Landscape scale studies integrating biology, physical sciences, and socio-economics
will be needed to forecast the future of this seagrass
biome under different management and restoration
scenarios incorporating multiple stressors (see Bell
et al., Chapter 26).
Studies in the vicinity of urbanized estuaries and
industrial areas have documented reduced plant fitness from heavy metal contamination associated
with industrial development (Lyngby and Brix,
1984; Pergent-Martini, 1998; Ralph and Burchett,
1998; Capiomont et al., 2000; Prange and Dennison, 2000; Ralph et al., Chapter 24), and although
metals are probably not biomagnified (Ward, 1989),
transfer of metal toxicity to consumers has been
suggested (Prange and Dennison, 2000). Seagrass
biomes are exposed to a wide range of pollutants
that need to be factored into the equation of multiple stressors, especially when the affects cascade
up and down food webs or are confounded by the
absence of grazer species lost through overfishing.
These stressors include nonpoint source pollution,
waste treatment, agricultural as well as industrial
practices and toxic substances found in fertilizers,
pesticides, herbicides, and fungicides.
3. Scale Factors
The conservation and management of seagrasses
requires approaches at multiple scales (Fonseca,
1996; Virnstein, 2000). Every attempt to describe the
biome by mapping, monitoring or process-oriented
studies must consider the effect of scale on the
proposed research question. Linkages and connectivity between seagrasses and other species and
with adjacent systems should be an integral part of
this research, including the construction of models
robust enough to forecast change at local, regional,
607
2. Multiple Stressors
A comprehensive research agenda is needed to predict the response of the seagrass biome to multiple stressors, especially when considering the
consequences of global climate change, resource extraction by fisheries, and the growing human population in the coastal zone (Short and Neckles,
1999; Section IV, Ralph et al., Chapter 24). The selection and application of appropriate management
regimes depends on the ability to distinguish natural from human stressors (sensu Jameson et al.,
2002). For example, there is strong evidence suggesting that increases in nutrient loading and siltation, two stressors often co-occurring in disturbed
systems, can lead to long-term shifts in taxonomic
composition of seagrass communities by placing
extraordinary physiological demands on the biota
(Borum, 1985; Tomasko and Lapointe, 1991; Nixon,
1995; Fourqurean et al., 1995; Short and Burdick,
1996; Terrados et al., 1998; see also Walker et al.,
Chapter 23 and Ralph et al., Chapter 24). Many seagrasses have complex life histories which are affected by different stressors over their lifetime and
the distribution, abundance and species composition
of the biome results from many different factors operating at varying spatial and temporal scales, and
seldom isolated from one another.
In the Mediterranean, the input of nutrients resulting from aquaculture activities was responsible of
significant declines of a P. oceanica meadow around
fish farming facilities (Pergent et al., 1999; Cancemi
et al., 2000; Dimech et al., 2000). Prior to the placement of the fish farm, these meadows were considered ‘healthy and productive’ but appear to have been
negatively affected by a single human stressor, the
introduction of aquaculture. Thus, if healthy meadows are so readily affected, it will be important in
the future to understand how beds already stressed
respond to additional stressors such as aquaculture
practices.
Mass mortality of seagrasses over very large areas of an entire subtropical estuary in south Florida,
USA (Florida Bay; 25.00 N; 80.45 W) fragmented
the seagrass system resulting in a cascade of impacts affecting both the benthic and pelagic communities in a much larger geographical region (Roblee
et al., 1991; Fourqurean and Roblee, 1999; Walker
et al., Chapter 23). The causes of this mass mortality were hypothesized to result from a combination
of factors including water diversion, eutrophication
and an unusually advanced community succession
in the absence of physical disturbances associated
with an abnormally low recurrence of hurricanes
in south Florida. This example demonstrated that
the magnitude and complexity of these multiple factors must be taken into account in developing a plan
for restoration of Florida Bay, because it is located
within a dynamic and interconnected system of uplands, wetlands, estuaries, reef tract, and continental
shelf (Jameson et al., 2002). The south Florida region
is one of the most densely populated coastal zones in
North America where human stressors are numerous
and widespread. Landscape scale studies integrating biology, physical sciences, and socio-economics
will be needed to forecast the future of this seagrass
biome under different management and restoration
scenarios incorporating multiple stressors (see Bell
et al., Chapter 26).
Studies in the vicinity of urbanized estuaries and
industrial areas have documented reduced plant fitness from heavy metal contamination associated
with industrial development (Lyngby and Brix,
1984; Pergent-Martini, 1998; Ralph and Burchett,
1998; Capiomont et al., 2000; Prange and Dennison, 2000; Ralph et al., Chapter 24), and although
metals are probably not biomagnified (Ward, 1989),
transfer of metal toxicity to consumers has been
suggested (Prange and Dennison, 2000). Seagrass
biomes are exposed to a wide range of pollutants
that need to be factored into the equation of multiple stressors, especially when the affects cascade
up and down food webs or are confounded by the
absence of grazer species lost through overfishing.
These stressors include nonpoint source pollution,
waste treatment, agricultural as well as industrial
practices and toxic substances found in fertilizers,
pesticides, herbicides, and fungicides.
3. Scale Factors
The conservation and management of seagrasses
requires approaches at multiple scales (Fonseca,
1996; Virnstein, 2000). Every attempt to describe the
biome by mapping, monitoring or process-oriented
studies must consider the effect of scale on the
proposed research question. Linkages and connectivity between seagrasses and other species and
with adjacent systems should be an integral part of
this research, including the construction of models
robust enough to forecast change at local, regional,