Chapter 24 Human Impacts on Seagrasses
587
sufficiently sensitive to detect an environmentally
realistic dose, the outcome is merely an example of
another dose that ‘can’ kill a seagrass; however, it
is unlikely that actual exposure will be sufficiently
concentrated to reach this endpoint. This is why we
must be looking for the most sensitive endpoint using environmentally realistic doses.
Similarly, the frequency and duration of exposure
to various contaminants deserves some attention. For
example, experimental regimes simulating chronic
exposure to one stressor (e.g. high turbidity) followed by a series of pulse events with assorted contaminants (e.g. herbicides and heavy metals) would
be informative. Many studies showed seasonal differences in the various responses measured, so determining whether such scenarios affect seagrass health
and survival more during summer or winter conditions is also important.
There is a definite need to assess a wider range
of species (specifically from low nutrient environments) to nutrient enhancement and toxicity. This
should include chronic low level and acute pulse exposures. In addition, more work needs to be done on
the mechanisms of nutrient metabolism, exclusion
and uptake, as well as methods to reduce nutrient
toxicity in seagrasses.
Incorporation of the interactive stress of environmental factors (like temperature, salinity, and light)
is needed to evaluate the relative contributions of
contaminants, freshwater, and turbidity following
storm events. More research is required on the specific mechanisms of mortality in response to sedimentation, including the relative contributions of
blocking light and inhibition of gas exchange toward
burial stress and eventual mortality.
Recovery is rarely considered. Equally important
is the lack of data on chronic and sub-lethal impacts,
as most of the literature deals with acute toxicity. As
indicated in most of the sections, we are currently
unaware of the impact of pre-exposure, interaction
of factors (Bowmer, 1986), multiple pulses or cotolerance.
Genetic and population influence on contaminant
tolerance has not been fully investigated. Chronic
low-level exposure may influence genetic selection
for more tolerant genotypes or encourage enhanced
phenotypic tolerance. Several studies have shown
spatial separation of a species results in differences
in tolerance to contaminants, and this could be linked
to subtle genetic differences. Recent developments
in molecular biology mean that we will soon see a
range of genetic biomarkers available for identifying
proteins that are damaged by anthropogenic contaminants.
V. Summary
This chapter has reviewed the role of anthropogenic
pollutants in past extensive declines of seagrasses.
With increasing awareness of the sensitivity of seagrasses to many pollutants, it appears that effective
management of catchment processes can facilitate
seagrass recovery. Some factors are well known, as
are many of the important processes, while many
others remain poorly understood. Yet even the wellresearched contaminants (such as metals) do not
have adequate safe-level guidelines for decisionmakers. The culture of determining threshold levels and performing toxicological tests has not yet
been applied to seagrass management, but this may
be an effective route to take. Furthermore, we need
to move our knowledge from the laboratory into the
field, to provide data for models to predict the impact
of realistic contaminant loads, and we need to test
the generality of these results across a wider range of
species in temperate and tropical environments. The
interactive effects of different anthropogenic stressors as well as their interaction with natural stressors
such as temperature and salinity are not well understood and should be the focus of research efforts.
Sediment impacts on seagrass are being increasingly
observed in many systems. Management efforts to
control sediment inputs can potentially have a variety of positive effects on seagrass systems, because
non-point source inputs of nutrients and other contaminants will potentially also be reduced.
References
Abal EG, Loneragan N, Bowen P, Perry CJ, Udy JW and Dennison WC (1994) Physiological and morphological responses of
the seagrass Zostera capricorni Aschers. to light. J Exp Mar
Biol Ecol 178: 113–129
Agawin NSR, Duarte CM and Fortes MD (1996) Nutrient limitation of Philippine seagrasses (Cape Bolinao, NW Philippines):
In situ experimental evidence. Mar Ecol Prog Ser 138: 233–
243
Alexander RB, Smith RA, Schwarz GE, Preston SD, Brakebill
JW, Srinivasan R and Pacheco PA (2001) Atmospheric nitrogen flux from the watersheds of major estuaries of the United
States: An application of the SPARROW watershed model.
In: Valigura RA, Alexander RB, Castro MS, Meyers TP, Paerl
587
sufficiently sensitive to detect an environmentally
realistic dose, the outcome is merely an example of
another dose that ‘can’ kill a seagrass; however, it
is unlikely that actual exposure will be sufficiently
concentrated to reach this endpoint. This is why we
must be looking for the most sensitive endpoint using environmentally realistic doses.
Similarly, the frequency and duration of exposure
to various contaminants deserves some attention. For
example, experimental regimes simulating chronic
exposure to one stressor (e.g. high turbidity) followed by a series of pulse events with assorted contaminants (e.g. herbicides and heavy metals) would
be informative. Many studies showed seasonal differences in the various responses measured, so determining whether such scenarios affect seagrass health
and survival more during summer or winter conditions is also important.
There is a definite need to assess a wider range
of species (specifically from low nutrient environments) to nutrient enhancement and toxicity. This
should include chronic low level and acute pulse exposures. In addition, more work needs to be done on
the mechanisms of nutrient metabolism, exclusion
and uptake, as well as methods to reduce nutrient
toxicity in seagrasses.
Incorporation of the interactive stress of environmental factors (like temperature, salinity, and light)
is needed to evaluate the relative contributions of
contaminants, freshwater, and turbidity following
storm events. More research is required on the specific mechanisms of mortality in response to sedimentation, including the relative contributions of
blocking light and inhibition of gas exchange toward
burial stress and eventual mortality.
Recovery is rarely considered. Equally important
is the lack of data on chronic and sub-lethal impacts,
as most of the literature deals with acute toxicity. As
indicated in most of the sections, we are currently
unaware of the impact of pre-exposure, interaction
of factors (Bowmer, 1986), multiple pulses or cotolerance.
Genetic and population influence on contaminant
tolerance has not been fully investigated. Chronic
low-level exposure may influence genetic selection
for more tolerant genotypes or encourage enhanced
phenotypic tolerance. Several studies have shown
spatial separation of a species results in differences
in tolerance to contaminants, and this could be linked
to subtle genetic differences. Recent developments
in molecular biology mean that we will soon see a
range of genetic biomarkers available for identifying
proteins that are damaged by anthropogenic contaminants.
V. Summary
This chapter has reviewed the role of anthropogenic
pollutants in past extensive declines of seagrasses.
With increasing awareness of the sensitivity of seagrasses to many pollutants, it appears that effective
management of catchment processes can facilitate
seagrass recovery. Some factors are well known, as
are many of the important processes, while many
others remain poorly understood. Yet even the wellresearched contaminants (such as metals) do not
have adequate safe-level guidelines for decisionmakers. The culture of determining threshold levels and performing toxicological tests has not yet
been applied to seagrass management, but this may
be an effective route to take. Furthermore, we need
to move our knowledge from the laboratory into the
field, to provide data for models to predict the impact
of realistic contaminant loads, and we need to test
the generality of these results across a wider range of
species in temperate and tropical environments. The
interactive effects of different anthropogenic stressors as well as their interaction with natural stressors
such as temperature and salinity are not well understood and should be the focus of research efforts.
Sediment impacts on seagrass are being increasingly
observed in many systems. Management efforts to
control sediment inputs can potentially have a variety of positive effects on seagrass systems, because
non-point source inputs of nutrients and other contaminants will potentially also be reduced.
References
Abal EG, Loneragan N, Bowen P, Perry CJ, Udy JW and Dennison WC (1994) Physiological and morphological responses of
the seagrass Zostera capricorni Aschers. to light. J Exp Mar
Biol Ecol 178: 113–129
Agawin NSR, Duarte CM and Fortes MD (1996) Nutrient limitation of Philippine seagrasses (Cape Bolinao, NW Philippines):
In situ experimental evidence. Mar Ecol Prog Ser 138: 233–
243
Alexander RB, Smith RA, Schwarz GE, Preston SD, Brakebill
JW, Srinivasan R and Pacheco PA (2001) Atmospheric nitrogen flux from the watersheds of major estuaries of the United
States: An application of the SPARROW watershed model.
In: Valigura RA, Alexander RB, Castro MS, Meyers TP, Paerl