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Case Study:Tidal Bay Ecological Assessment

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.


  1. 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 science.
  2. Read through the entire case study first, and then in a sentence or two answer each question.



This ecological 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 Area

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.

Chemical Pollutants

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 Shipshape Inlet.

  1. What are the benefits of comparing contaminant concentrations and biological impacts in Tidal Bay sediments with those of a reference area?

  2. 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?

  3. Can you think of another approach that would work?

  4. 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:

  1. Toxicity tests using sediment species, population abundances, and community indicators (species richness and community similarity)

  2. Biomarkers for tissue residues of contaminants and fish histopathology (microscopic examinations of specific tissues and organs to detect chemical injury)

  3. 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 sediment material.

    Although oysters 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.
  1. Do you feel these measurements are relevant to this aquatic ecosystem?

  2. 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?

  3. Investigators 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 numbers?


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 area.

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 reference concentrations.

Histopathological analyses 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.

Characterizing and 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:

  1. 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.

  2. 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.
  1. Could apparent effects thresholds be determined for bioaccumulation and histopathology in fish? Why do you suppose investigators did not do this?

  2. What are some major strengths of the apparent effects thresholds and what are some limitations?

  3. Name one point you learned that you feel is most interesting.

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