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Purpose
Students recognize
that hazardous waste may have far-reaching impacts on ecosystems
and these impacts are not always easy to identify. Students
become familiar with several types of tests used to measure
the environmental effects of hazardous waste pollution. In
the process, they learn that no single assessment procedure
is applicable to all ecosystems and no single test is adequate
to assess pollution impacts on an entire ecosystem. They examine
a case study and discuss the limitations of current ecosystem
assessment methods for establishing cause-and-effect relationships,
especially for mixtures of chemicals in the environment.
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Background
The impact of hazardous
waste on the environment is thought to be widespread and in some areas
severe. Establishing cause-and-effect relationships between exposure
and ecosystem damage is a major challenge. An ecosystem-such
as a marsh-is a highly complex structure, consisting of all living organisms
in a given area and their interactions not only among themselves but
also with the environment. Even a mature ecosystem-one that has
achieved stability over time-is constantly adapting to changes. Some
of these changes are due to natural influences such as animal migration
patterns, weather, erosion, and sedimentation. Other changes, however,
are the result of habitat encroachment and human pollution. This pollution
is often in the form of complex mixtures of chemicals in widely varying
concentrations.
Ecosystems are complex and
dynamic (ever changing). This makes linking any one effect to a specific
cause very difficult. Conditions cannot be controlled sufficiently to
allow the effects of individual pollutants to be observed. Only recently
have scientists begun to focus attention on finding ways to determine
the major effects of hazardous waste on ecosystems.
Researchers have built laboratory
models of ecosystems to study environmental pollution in controlled
settings. But models can provide only simple representations of real
ecosystems that contain thousands of living species. They cannot provide
adequate measures of the diversity of species and the complexity of
the relationships among all the living organisms that make ecosystems
unique.
There is no single best
strategy or design for assessing environmental pollution that is appropriate
for every situation. The characteristics of the area and the specific
objectives and issues of concern must be considered in determining how
to proceed. Nevertheless, scientists generally use four major categories
of tests to assess the impact of hazardous waste on ecosystems:
- Chemical and physical
tests to measure contaminant levels, pH, oxygen levels, and other
environmental conditions
- Toxicity tests
to determine if the pollution can or is causing adverse biological
effects in ecosystem species
- Biomarkers to indicate
actual exposure
- Field surveys.
These ecological assessments
are important tools in Federal and state government efforts to clean
up hazardous waste contamination under the Superfund Program.
For additonal information
on ecosystems and pollution, see the Suggested Reading list found at
the end of the Haz-Ed materials. Other Haz-Ed materials related to this
topic include Fact Flash 2: The Superfund
Cleanup Program.
Preparation
- Assemble the following
materials:
- Copies for each student
of Student Handout 1, Major
Categories of Tests for Ecological Assessment, found at the
end of this activity
- Copies for each student
of Student Handout 2, Case Study:
Tidal Bay Ecological Assessment.
- Read the student handouts
to prepare your lecture.
- Distribute copies of
Student Handout 2 and assign students
to read the case study for homework.
NOTE: In this lesson,
students will encounter a large number of scientific terms and phrases.
Depending on the grade level in which you use the lesson and the skill
level of your students, you may need to spend extra time introducing
unfamiliar vocabulary and preparing your students for this lesson. Many
terms are defined in the Glossary found at the end of the Haz-Ed materials,
but a textbook may be helpful.
Procedure
Class #1
- Ask the class to define
an ecosystem. Then ask the class to distinguish between an ecosystem
and the environment. (An ecosystem is a specialized community, including
all the component organisms, that forms an interacting system, for
example, a marsh. An environment is the totality of conditions surrounding
an organism.)
- Organize the students
in groups of 3 or 4 students each, and ask each group to write down
how hazardous waste released into the environment can affect plants
(flora) or animals (fauna) in an ecosystem. Ask them to list as many
possibilities as they can think of in 10 minutes.
- Ask each group how many
ideas they wrote down.
- Ask the group with the
fewest ideas to lead off by naming one effect of pollution. Ask the
rest of the class if they agree that the named effect can result from
pollution. Ask those who agree to explain why they agree. Ask those
who do not agree to explain their reasons.
Continue the discussion
by asking each group in turn to add to the list. Have a student
write the ideas on the chalkboard as they are mentioned. Some possible
effects that could be listed include:
- Gaps in vital food
chains or nutrient cycles
- Reproductive problems
(such as eggshell thinning or loss of nesting materials)
- Developmental effects
(such as malformed chick beaks)
- Tumors (such as fish
tumors)
- Critical organ damage
(such as liver, kidney, or skin lesions)
- Immune system dysfunction
(leading to, for example, viral infections in dolphins)
- altered individual
or population growth rates
- Changes in population
and community organization
- Loss of total biomass
(flora and fauna)
- Relative loss of taxa
or species abundance in defined areas (such as fish kills, amphibian
mortality, macroinvertebrate depletion)
- Loss of species diversity.
Some of the students'
ideas may overlap because one adverse ecosystem effect can impact
another since life in ecosystems is interdependent. For example,
reproductive and developmental problems in birds may ultimately
cause a decrease in species abundance and diversity, which in turn
may alter community organization.
Some of the students'
ideas may overlap because one adverse ecosystem effect can impact
another since life in ecosystems is interdependent. For example,
reproductive and developmental problems in birds may ultimately
cause a decrease in species abundance and diversity, which in turn
may alter community organization.
Students may not come
up with many ideas. In a way this reflects the current state-of-the-science
in ecological assessment. It can be difficult to determine what
is occurring or could occur in an ecosystem as a result of pollution;
effects are often very subtle unlike those of habitat loss, which
usually tend to be very obvious.
Leave the list on the
chalkboard.
- Distribute the Student
Handout, Major Categories of Tests for Ecological Assessment. You
may wish to have students discuss the various types of tests in more
depth before proceeding.
- Ask the students which
category or categories of tests could be used to study each of the
adverse effects listed on the chalkboard. Have students explain the
choices they make. Encourage the rest of the class to comment on various
answers.
Class #2
- Give each student a copy
of the Student Handout, Case Study: Tidal
Bay Ecological Assessment. Give them about 30 minutes to read it in
class or as homework. Questions and guidelines are in the text to
assist them in their review. Ask them to answer as many of the questions
as possible.
- For the remainder of the
class period, discuss the case study and the students' answers to
the questions. An Instructor's Answer Key is
included at the end of this lesson for your use.
Extensions (Optional)
- After allowing each group
to add to the list of ideas about how hazardous waste can adversely
effect ecosystems, ask students to rank the items according to their
importance. Have them discuss their choices.
- Point out during the discussion
that sometimes investigators limit ecological components of concern
to commercially important species (e.g., blue crabs in the Chesapeake
Bay). Have students discuss how this might influence public attitudes
with respect to proposed environmental regulations or legislation.
Instructor's
Answer Key - Handout 1
Case Study: Tidal Bay Ecological Assessment
- What are the benefits
of comparing contaminant concentrations and biological impacts in
Tidal Bay sediments with those of a reference area?
By expressing all chemical
and biological measures as changes (increases or decreases) relative
to a "normal" ecosystem (Shipshape), comparisons can be made that
provide a sound basis for identifying and quantifying effects. Comparing
results with a reference area allows investigators to determine
not only what is not "normal" in the study area, but also how much
weight to place on the changes.
- What are some of the
limitations (problems) associated with the use of a reference area
and with the choice of Shipshape Inlet as this area?
Shipshape Inlet differs
in sediment type from Tidal Bay, and although it may be the least
polluted area of those studied in the basin, it is hardly a pristine
environment unaltered by urbanization and industrialization. Furthermore,
comparing a complex biological response such as benthic macroinvertebrate
community change with a reference site requires reducing the data
to a single value(s), which results in a substantial loss of data.
- Can you think of another
approach that would work?
If Tidal Bay contained
only one or possibly two specific wastes, the contaminant concentrations
and biological measures of their impact on the ecosystem could be
compared with toxicity and risk levels published in the literature
or in government databases. It is not known, however, how complex
chemical mixtures interact to possibly increase or decrease the
effects of individual chemicals. Further, the exact combination
of chemicals in Tidal Bay may be unique. So, under these circumstances,
the use of a reference area is probably the best choice.
- What impact do you
think the presence of multiple types of hazardous waste will have
on the ability of investigators to establish a cause-and-effect relationship
between specific chemicals and adverse biological changes in Tidal
Bay?
Ideally, characterization
of ecological impacts from hazardous waste is supported by definitive
cause-and-effect relationships between specific chemicals and biological
endpoints. Almost no information is available for establishing cause
and effect for chemical mixtures, however, so they will not be able
to determine specific cause and effect relationships. In lieu of
a standardized approach for assessing ecological impacts of complex
chemical mixtures, the Tidal Bay investigators developed relative
measures of effect based on the reference area.
- Do you feel these measurements
are relevant to this aquatic ecosystem?
A number of biological
measures are used to quantify the pollution impact on Tidal Bay.
These include several toxicity tests, benthic community composition,
and fish histopathology. All of these measures can be justified
on ecological grounds. For example, amphipods are crustaceans that
reside in Tidal Bay and are important prey for higher trophic-level
species like fish. Also, they are relatively sensitive to toxic
chemicals and are highly likely to be exposed to contaminants because
they burrow in and feed on sediment material.
Oysters also are considered
useful indicators of ecological effects because they are very sensitive
to toxic chemicals. The oyster test is a standardized test of developmental
effects, which provides a broader view of adverse effects than lethality
tests alone.
Benthic macroinvertebrate
species also are valuable indicators of toxicity because they live
in direct contact with sediments, are relatively stationary, and
are important components of aquatic food chains. Many fish and crab
that live near the sediment feed on benthic organisms and are exposed
to contaminants through the food chain.
Note: although
the investigators avoided limiting ecological components of concern
to commercially important species or to those selected for the sake
of political expediency, the ecological significance of the effects
observed in the bioassay tests is not explained in terms of the
entire ecosystem of Tidal Bay.
- Are these measurements
likely to furnish the kind of data required to fulfill the purpose
of the assessment? If not, how would you change the approach?
The use of multiple chemical
and biological tests (such as sediment chemistry, sediment toxicity,
benthic macroinvertebrate assemblages, tissue residues resulting
from bioaccumulation, and fish liver histopathology) provides a
powerful weight-of-evidence approach to identify pollution problems
in an ecosystem. They also provide the kind of data needed to define
the extent of hazardous waste contamination in estuarine sediments
and the magnitude of damage to benthic organisms and fish.
- Investigators characterized
degradation of benthic macroinvertebrate communities in terms of a
decrease in the abundance of total amphipods, molluscs, polychaetes,
or total macrofauna. Many conditions can influence the overall abundance
of benthic macroinvertebrates, including an algae bloom that depletes
oxygen in the water. Did the investigators' report consider all factors
that could have altered macroinvertebrate numbers?
While some species may
decrease in abundance due to chemical pollutants, other, more pollution-tolerant
species are likely to increase. This makes changes in abundance at
a major taxon level or at the total macrofauna level an unreliable
indicator of ecosystem health. Generally speaking, a chemical pollution
problem is the only condition that will render a waterway totally
devoid of macroinvertebrates. However, the investigators did not study
the levels of macroinvertebrate species in detail, possibly because
of the extra costs involved. Precise and careful analyses of macroinvertebrate
samples is time consuming and expensive. Also, they did not report
looking at other possible causes for macroinvertebrate depletion.
- Could apparent effects
thresholds be determined for bioaccumulation and histopathology in
fish? Why do you suppose investigators did not do this?
Apparent effects thresholds
could have been established for bioaccumulation and histopathology
in fish, but the purpose of the apparent effects thresholds was
to rank specific problem areas within the bay. The fish indicators
reflect a wide area of conditions. Also, there is a lot of uncertainty
associated with how much hazardous waste the fish have been exposed
to in the water and food chain and for how long. Thus, it is difficult
to link the bioaccumulation and histopathology data directly to
chemical concentrations in specific sediment samples.
- What are some major
strengths of the apparent effects thresholds and what are some limitations?
The apparent effects
thresholds method is a plausible approach for dealing with problems
created by contamination and uncertain cause-and-effect relationships.
It uses empirical relationships to get around difficulties like
bioavailability and synergistic and antagonistic relationships among
chemical mixtures. The approach is limited for several reasons:
it does not describe cause-and-effect relationships, it is site-specific
(specific to certain areas), does not take into account data on
bioavailability of chemicals in organ tissues, and lacks independent
validation.
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