Case Study: Tidal Bay Ecological Assessment
This case study
is an example of how one scientific group attempted to document
the impact of a mixture of organic compounds and metals on an
estuary, fictitiously named Tidal Bay. although there is no single
best strategy or design for ecological assessments that is appropriate
for every ecosystem, the assessment techniques and lessons learned
in this case study have implications for measuring the impact
of pollutants in other ecosystems where water-fresh, tidal, or
marine-is contaminated or threatened.
- Critique this
case study using the questions provided. You may not understand
all of the detail provided; for example, you probably will not
be familiar with all the animal species and chemicals. This
should not limit your ability to see the logic underlying the
investigation and the strengths and weaknesses of the approach.
In the process, you will discover a lot about environmental
- Read through
the entire case study first, and then in a sentence or two answer
assessment was conducted for the purpose of defining the extent
of hazardous waste contamination in the tidal sediments (soil,
stones, or other materials deposited by tidal waters) of Tidal
Bay and to measure the magnitude of existing biological damage
to benthic (bottom-dwelling) organisms and fish. It was not intended
to be a risk assessment since it did not investigate the future
of the ecosystem.
Concerns about the potential ecological and human health effects
of hazardous waste in Tidal Bay focus on exposure of aquatic organisms
to contaminated marine sediments. The sediments support a variety
of benthic organisms that can be directly influenced by sediment
contamination. Benthic macroinvertebrate species, such as shrimp,
are valuable indicators of toxicity because they live in direct
contact with sediments, stay close to their homes, and are important
parts of aquatic food chains. Many fish and crabs that live in
or near the sediment feed on benthic organisms and are exposed
to contaminants through the food chain. Therefore, if tests on
these benthic macroinvertebrates do not reveal negative effects
caused by polluted tidal; sediments, it is unlikely that other
biological groups, such as fish or plankton, are affected by these
pollutants. For example, if the shrimp that live in the sediment
are tested and have nothing wrong with them, the crabs and fish
will probably be fine too, since they eat the shrimp.
Description of Area
The study area is a bay formed by a river delta made up of seven
minor waterways, associated shorelines, and water at depths less
than 60 feet below low tide. Tidal Bay is in a heavily industrialized
area at the south end of a large basin. Industrial and municipal
sources, such as a pulp mill, petroleum refineries, chemical manufacturers,
aluminum processors, and a shipbuilding and repair yard are located
on filled-in tideflats. A municipal sewage treatment plant discharges
into the river upstream of the bay.
Selection of Reference
A reference area, Shipshape Inlet, was selected to compare against
the contaminated sites in Tidal Bay. Chemical and biological measures
taken in Tidal Bay are compared to this reference site. Shipshape
Inlet was chosen because it is associated with the same large
basin that includes Tidal Bay and has some of the lowest levels
of the contaminants of concern in the basin. Also, an extensive
amount of chemical and biological data are already available on
Shipshape Inlet. The range of sediment types in Shipshape Inlet,
however, does not include the fine-grained sediments characteristic
of the Tidal Bay waterways.
Routine chemical tests for about 150 chemicals were completed
on over 190 samples of surface and subsurface sediments collected
from areas of the bay. Chemicals detected in more than two-thirds
of the surface sediments include phenol, 4-methylphenol, polycyclic
aromatic hydrocarbons (PAHs), 1,4-dichlorobenzene, polychlorinated
biphenyls (PCBs), dibenzofuran, and metals.
The chemicals present
in Tidal Bay at higher concentrations than those in Shipshape
Inlet are causing the greatest concern. Twelve chemicals or chemical
groups were at concentrations greater than 100 times and less
than 1,000 times those in Shipshape Inlet. Nine chemicals or chemical
groups were at concentrations greater than 1,000 times those in
- What are
the benefits of comparing contaminant concentrations and
biological impacts in Tidal Bay sediments with those of
a reference area?
- 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?
- Can you
think of another approach that would work?
- 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 (negative) biological changes in Tidal Bay?
To assess the health and condition of the selected animals (benthic
macroinvertebrates and fish), several measurement endpoints were
evaluated. These included:
- Toxicity tests using
sediment species, population abundances, and community indicators
(species richness and community similarity)
- Biomarkers for tissue
residues of contaminants and fish histopathology (microscopic
examinations of specific tissues and organs to detect chemical
- Chemical tests of
contaminants in the sediments. The sediment toxicity tests were
conducted in the laboratory using amphipods, oysters, or bacteria,
and field-collected sediment samples with known chemical concentrations.
Bioassays were repeated using the same sediment samples that were
diluted to lesser contaminant levels.
The amphipod toxicity
test measures death rates in a crustacean that resides in Tidal
Bay and is an important prey for higher species like fish. Amphipods
are relatively sensitive to toxic chemicals and are likely to
be exposed to contaminants because they burrow in and feed on
do not live in Tidal Bay, they reside in other areas of the
basin, and oyster embryos and larvae are very sensitive to toxic
chemicals. The oyster toxicity test measures the occurrence
of developmental abnormalities in larvae (and embryos) exposed
to Tidal Bay sediments for 48 hours.
Abundances of benthic
macroinvertebrates were determined from field-collected samples.
Community indicators involved counting species richness and
the amount of major taxa such as crustaceans and molluscs. Only
decreases in abundances of major taxa in Shipshape Inlet were
used to identify and rank problem areas in the bay. Bioaccumulation
(contaminant concentrations in muscle tissue) of English sole
(fish) and Dungeness crab were measured as biomarkers of exposure.
Because contaminants were detected infrequently in the crab
muscle tissue, only the English sole data were used to identify
and rank exposure levels. Histopathological tests were conducted
on the livers of English sole.
The magnitude of
exposure was determined by the chemical concentrations of contaminants
in sediments. Because sediments represent a sink for pollution
(that is, pollutants tend to accumulate in sediments), organisms
that live in it or on it are continuously exposed.
|A number of
measurements were used to quantify contaminant impact on the
ecosystem. These include several bioassay species, benthic
community composition, bioaccumulation, and fish histopathology.
- Do you
feel these measurements are relevant to this aquatic ecosystem?
- Are these
measurements likely to give the kind of data required
to fulfill the purpose of the assessment? If not, how
would you change the approach?
characterized degradation of benthic macroinvertebrate
communities in terms of a decrease in the abundance of
total amphipods, molluscs, polychaetes, or total macrofauna.
However, many conditions can influence the overall abundance
of benthic macroinvertebrates including an algae bloom
that depletes oxygen in the water. Did the investigators
consider all factors that could have altered macroinvertebrate
The analysis of the ecological effects and exposure data involved
mainly statistical comparisons of test results from Tidal Bay
and the reference area. For example, Tidal Bay sediments from
18 of 52 tested areas induced significant, acute lethality in
amphipods as compared with the reference area sediments. Significant
elevations in oyster larvae abnormalities occurred in sediments
from 15 of 52 areas tested compared with sediments from the reference
area. Significant decreases in the abundance of total taxa and
the abundance of polychaetes, molluscs, and crustaceans occurred
in 18 of 50 areas tested in Tidal Bay compared to the reference
Concentrations of most
metals in the muscle tissue of English sole were less than 2 times
the average reference concentrations, but concentrations of copper
in the Tidal Bay fish tissue were 3 to 9 times higher than average
reference concentrations. Polychlorinated biphenyls (PCBs) were
detected in all fish and crab sampled. Lead and mercury were elevated
in Dungeness crab with maximum concentrations about 5 times the
revealed the presence of liver abnormalities that were significant
in terms of number in Tidal Bay compared to the reference area.
The incidence of liver lesions was greatest in fish from areas
with the highest concentrations of sediment-associated contamination.
Ranking Problem Areas
The original data from the toxicity tests, abundances, and biomarkers
were used to evaluate the increases in contamination or negative
effects to determine if these changes were statistically significant.
They were also used to evaluate quantitative relationships among
these variables. However, because single-chemical relationships
between exposure and effects could not be established (that is,
a one-to-one relationship could not be proved), two methods were
used to characterize and express the ecological impacts:
- Biological Indicators.
Using both exposure (chemical concentration) and effects data
(from toxicity tests, macroinvertebrate abundances, and biomarkers),
investigators developed ratios between the effects in Tidal Bay
and those found at the reference site, Shipshape Inlet. The ratios,
or biological indicators, were used in describing the overall
impact of contamination on the ecosystem.
- Apparent Effects Thresholds.
Because biological effects data were not available for all portions
of the study area, a method was developed to estimate thresholds
of chemical concentrations above which biological effects would
be expected. These are called apparent effects thresholds. Threshold
concentrations of contaminants were estimated using data generated
from the amphipod mortality toxicity test, oyster larvae abnormality
toxicity test, and macroinvertebrate abundances. These measurements
were selected because of their sensitivity to sediment contamination,
availability of standard test protocols, and ecological relevance.
The apparent effects thresholds were compared with measured concentrations
of sediment contaminants. The apparent effects thresholds indicate
the potential for adverse ecological effects in Tidal Bay.
- Could apparent
effects thresholds be determined for bioaccumulation and
histopathology in fish? Why do you suppose investigators
did not do this?
- What are
some major strengths of the apparent effects thresholds
and what are some limitations?
- Name one
point you learned that you feel is most interesting.