584
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
5. Recovery from Petrochemical Exposure
It seems laboratory experiments portray the impact
of PAHs to be much more serious than field-based
experiments or actual spills. There are numerous examples of oil spills having no significant impact on
the seagrasses after several years. One year after the
Gulf War oil spill, leaf morphology and growth indicators suggested three species of seagrass remained
in good health (Kenworthy et al., 1993). Dean et al.
(1998) found that Z. marina showed little detrimental
effects 1 year after the Exxon Valdez oil spill. These
types of impact assessments are usually based on
comparing the impacted site to a reference site, as
the region is rarely surveyed before the spill. This
limits the scope of interpretation of this style of investigation. However, recovery of deeper seagrasses
exposed to the WSF alone is generally much more
successful than recovery of smothered intertidal seagrasses.
III. Management of Anthropogenic
Pollutants
Information on safe pollutant loads is urgently
needed for coastal resource managers to make sound
decisions. Peters et al. (1997) recommend that
benchmarks for risk assessment, baseline monitoring criteria and effective management strategies are
needed to protect seagrasses. In this way, managers
need to be able to access and synthesize information on contaminants and stressors, to predict risk
to the ecosystem. To predict risk they must know
the magnitude of the stressors for which a detrimental effect can be monitored. We need to provide
an indication of the levels of tolerance (ideally under
in situ conditions) and indicate the ecological consequences (death, reduce photosynthesis, biochemical damage) of the exposure to a known contaminant. Much of this type of data is slowly becoming
available; however, it may not be in a form that is
readily accessible for synthesis into risk assessment
protocols.
Due to the complex nature of seagrass decline, it
is rare to find clear examples of causality, and therefore it is difficult to have direct evidence of the impact
of contamination on seagrass health (Bester, 2000).
Additionally, in many coastal systems the stressors
co-vary (Moore et al., 1996). This suggests that a
comprehensive approach to management is needed,
where all potential contaminants are addressed. In
many cases the solutions proposed may address more
than one factor. For example, improving catchment
basin retention of sediment runoff will help to improve water clarity through reductions in suspended
sediments as well as reducing a significant proportion of nutrient inputs.
A. Construction of Pollutant Loading Models
In response to demands of resource managers, numerical models are needed to assess the susceptibility of seagrass to nutrient enrichment and other
contaminants. For example, nutrient loading models have been developed to determine the sources
and quantities of nutrients coming into estuarine
and nearshore marine ecosystems. Numerous studies have shown that most estuaries are strongly nitrogen limited, although in some locations, such as
eastern and central portions of Florida Bay (Florida,
USA; Fourqurean et al., 1992) or Shark Bay (Western Australia; Smith and Atkinson, 1983), available
evidence suggests that phosphorus is the principal
nutrient limiting primary production. As a result,
nutrient loading models typically focus on nitrogen
as the nutrient of concern.
A variety of techniques exist for determining the
sources and quantities of pollutants entering estuarine and nearshore marine ecosystems. These efforts
range from complex models that examine rainfall,
land use, soil types, elevation changes, etc. to simple spreadsheet-type models that predict pollutant
loads as a function of land use categories alone. For
many estuaries, a significant percentage of the watershed is gauged for stream flow measurements. If
water quality data are collected at gauge locations,
then direct estimates of pollutant loads can be made
(flow x concentration = load).
In many coastal ecosystems (e.g. Tampa Bay,
Florida, USA; Chesapeake Bay, USA) significant
point sources of nutrients from wastewater treatment plants and/or industrial facilities occur downstream of any stream flow gauge locations. Often,
point sources discharge directly into estuarine waters, or into the lower tidal reaches of tributaries to
these estuaries. Consequently, development of pollutant loading estimates requires knowledge of the
quantity of water discharged from all relevant point
sources, as well as information on the quality of the
discharged effluent.
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
5. Recovery from Petrochemical Exposure
It seems laboratory experiments portray the impact
of PAHs to be much more serious than field-based
experiments or actual spills. There are numerous examples of oil spills having no significant impact on
the seagrasses after several years. One year after the
Gulf War oil spill, leaf morphology and growth indicators suggested three species of seagrass remained
in good health (Kenworthy et al., 1993). Dean et al.
(1998) found that Z. marina showed little detrimental
effects 1 year after the Exxon Valdez oil spill. These
types of impact assessments are usually based on
comparing the impacted site to a reference site, as
the region is rarely surveyed before the spill. This
limits the scope of interpretation of this style of investigation. However, recovery of deeper seagrasses
exposed to the WSF alone is generally much more
successful than recovery of smothered intertidal seagrasses.
III. Management of Anthropogenic
Pollutants
Information on safe pollutant loads is urgently
needed for coastal resource managers to make sound
decisions. Peters et al. (1997) recommend that
benchmarks for risk assessment, baseline monitoring criteria and effective management strategies are
needed to protect seagrasses. In this way, managers
need to be able to access and synthesize information on contaminants and stressors, to predict risk
to the ecosystem. To predict risk they must know
the magnitude of the stressors for which a detrimental effect can be monitored. We need to provide
an indication of the levels of tolerance (ideally under
in situ conditions) and indicate the ecological consequences (death, reduce photosynthesis, biochemical damage) of the exposure to a known contaminant. Much of this type of data is slowly becoming
available; however, it may not be in a form that is
readily accessible for synthesis into risk assessment
protocols.
Due to the complex nature of seagrass decline, it
is rare to find clear examples of causality, and therefore it is difficult to have direct evidence of the impact
of contamination on seagrass health (Bester, 2000).
Additionally, in many coastal systems the stressors
co-vary (Moore et al., 1996). This suggests that a
comprehensive approach to management is needed,
where all potential contaminants are addressed. In
many cases the solutions proposed may address more
than one factor. For example, improving catchment
basin retention of sediment runoff will help to improve water clarity through reductions in suspended
sediments as well as reducing a significant proportion of nutrient inputs.
A. Construction of Pollutant Loading Models
In response to demands of resource managers, numerical models are needed to assess the susceptibility of seagrass to nutrient enrichment and other
contaminants. For example, nutrient loading models have been developed to determine the sources
and quantities of nutrients coming into estuarine
and nearshore marine ecosystems. Numerous studies have shown that most estuaries are strongly nitrogen limited, although in some locations, such as
eastern and central portions of Florida Bay (Florida,
USA; Fourqurean et al., 1992) or Shark Bay (Western Australia; Smith and Atkinson, 1983), available
evidence suggests that phosphorus is the principal
nutrient limiting primary production. As a result,
nutrient loading models typically focus on nitrogen
as the nutrient of concern.
A variety of techniques exist for determining the
sources and quantities of pollutants entering estuarine and nearshore marine ecosystems. These efforts
range from complex models that examine rainfall,
land use, soil types, elevation changes, etc. to simple spreadsheet-type models that predict pollutant
loads as a function of land use categories alone. For
many estuaries, a significant percentage of the watershed is gauged for stream flow measurements. If
water quality data are collected at gauge locations,
then direct estimates of pollutant loads can be made
(flow x concentration = load).
In many coastal ecosystems (e.g. Tampa Bay,
Florida, USA; Chesapeake Bay, USA) significant
point sources of nutrients from wastewater treatment plants and/or industrial facilities occur downstream of any stream flow gauge locations. Often,
point sources discharge directly into estuarine waters, or into the lower tidal reaches of tributaries to
these estuaries. Consequently, development of pollutant loading estimates requires knowledge of the
quantity of water discharged from all relevant point
sources, as well as information on the quality of the
discharged effluent.