 |
||
|
Monitoring and Assessing Water Quality |  |
 |
||
|
|
|
|
|||||||||
|
|
Chapter 3
|
|
Figure 3-2. Biological assemblages used for lake assessments. |
The proposed assessment of lake condition is accomplished with additive indices that integrate the habitat and biological scores. The process produces up to three habitat scores, and three or more biological index scores. The scores reduce the complexity of a lake to an understandable level for guiding appropriate remediation or other management actions.
Biological assessment of lakes is implemented in tiers corresponding to the level of effort required. Each suggested tier includes both biological and habitat components. The tiered approach for lake bioassessment developed here allows customization of the methodology to the user’s needs, questions, and resources available. Tier 1 focuses on sampling trophic state indicators, and Tier 2 focuses on sampling biological assemblages for composition and structure indicators (Figure 3-3 Table 3-1). Each tier is further divided into single- and multiple-visit sampling, A and B, respectively. Tier 1A and 1B are the same except that Tier 1B requires several samples during the growing season to obtain seasonal averages of chlorophyll a and nutrient concentrations.
Tier 2A consists of biological assemblages that integrate lake conditions and are sampled during an index period. Tier 2B consists of assemblages with individuals that are short-lived, and hence do not integrate over time. Tier 2B assemblages are sampled repeatedly during the growing season to obtain seasonal averages.
Because chlorophyll and nutrient concentrations are highly variable, Tier 1A, which is sampled only during an index period, may fail to characterize an individual lake. Tier 1A is appropriate for characterizing a region or a class of lakes, especially if many lakes are to be sampled. For characterizing the trophic state of individual lakes with confidence, Tier 1B is preferred.
Both Tier 2A and 2B sample biological assemblages to estimate indicators of species structure, trophic structure, and function. Tier 2B requires multiple visits and analysis, but does not necessarily obtain better or more precise information than Tier 2A.
Because there is tremendous variation in the physical, chemical, and biological characteristics of lakes nationwide, the first step in defining reference conditions is to classify lakes so that comparisons can be made within, not across, classes. Classification of natural lakes should reflect the inherent properties of lakes independent of human influence and therefore must be made on the basis of measurements that do not change as a result of human activities.
A second requirement of classification is that it should reflect differences in the biota of the classes. A deep lake might have a fish assemblage different from that of a shallow lake, and classification should distinguish between the two types of systems. Several lake classifications have been proposed (e.g., Hutchinson 1957, Leach and Herron 1992); however, only a handful of lake classes would be present in a single region. Relevant lake classes must be determined by existing information and the professional judgment of scientists familiar with lakes of the region.
Five elements, detailed in Section 4.2, may be used to establish reference conditions for lake biological assessment:
Expert consensus is required for developing reference conditions. Reference conditions developed from empirical data are preferred: such as biosurveys, sites, paleolimnology, or historical data.
A biological survey provides the best current information about the biota for the system of concern as a real world reflection of biological integrity. This information is essential to determining the reference condition and subsequent biological criteria. There are two approaches for characterizing reference conditions from a biological survey:
Site based - Selection of minimally impaired or most natural sites in a region; or
Condition based - Setting reference conditions as the best available ambient biological conditions.
Paleolimnology is the microscopic examination of sediment cores to provide an accurate record of the relative abundance of certain organisms (primarily diatoms) over the history of natural lakes. The advantage of paleolimnology is that any lake with an accurate sedimentary record can be a reference site regardless of the severity of present-day pollution. Thus, a truly representative sample of lake reference sites can be drawn. With some exceptions, paleolimnology is generally not applicable to impoundments.
A panel of diverse regional experts involved in the determination of the reference condition and the derivation of the biocriteria is the best approach to thoroughly and objectively assimilate the above information. With a carefully selected and balanced panel, all of the nuances of the local ecology as well as the best interests of the jurisdiction can be equated to the designated uses of the waterbody in designing the most protective criteria possible. This approach also reduces the risk of making insufficiently informed decisions inherent in data interpretation by just one or a few like-minded people.
Throughout this document
where differences between lakes and reservoirs dictate alternative
methods, strategies, etc., an icon appears, directing the reader to
reservoir-specific information.
The methodology described in this document is intended for both reservoirs and natural lakes. Because reservoirs are entirely artificial environments, “natural reference condition” has no meaning. Reservoirs, created by the damming of a stream, have characteristics of both rivers and lakes (Thornton 1990a). Reservoirs are divided into three zones (riverine, transitional, and lacustrine), which correspond to flowing, river-like conditions; transition to lake conditions; and nonflowing, lake-like conditions near the dam, respectively. With expected life spans ranging from one to several decades, reservoirs are more ephemeral than most natural lakes and have several physical characteristics not shared with natural lakes. The lakes most like reservoirs are those formed by natural dams in stream valleys (e.g., beaver dams, terminal moraines, landslides).
Reservoirs vary widely in physical characteristics of shape, size, and hydrology. They can range from small shallow impoundments, to deep storage reservoirs, to “run of the river” flow-through reservoirs on large rivers. They are built and managed for widely different purposes, including flood control, navigation, water storage, hydroelectric generation, gamefish production, and others. The management practices in turn affect both physical characteristics (water level variability, stratification) and biota (stocking of fish).
Although no “natural” reservoir reference conditions can exist, the operational determination of reference conditions for reservoirs is the same as that for natural lakes. Reservoirs can be classified according to hydrology, morphometry, management objectives, and other factors. Age of the reservoir will be important in determining the assessment expectations of the reservoir.
Historical data are important because they provide insight to past conditions essential to knowing what may be achievable, especially for degraded or significantly altered systems.
Comparison of the historical record to present reference site data greatly expands the manager’s perspective of the system. However, care must be exercised in making these comparisons when the objectives and survey methods have changed over time.
Ecological models may be used to identify water chemistry reference conditions for reservoirs or for other significantly altered waterbodies. Most reservoirs are less than 50 years old, and there is insufficient empirical evidence to document the expected condition of basins for all regions. Where documentation is available (historical data), extrapolation and model development help qualify the reference condition and may be the best way to derive and calibrate the biocriteria.
Metrics are evaluated for relevance to biological assessment and for response to stress. Expected measurement values vary as a function of regional species pools, regional characteristics (climate, geology, soils, land use, regional scale barriers to colonization), and local site characteristics (habitat factors, including local barriers). A regional approach involving collaboration of neighboring jurisdictions will enhance characterization of reference conditions. Cross-state comparisons can be made more easily if common methods and measurements can be established among states.
Metrics are typically calculated from data collected on single assemblages of lake biota, such as planktonic algae, zooplankton, fish, aquatic plants, and benthic invertebrates. The metrics might include counts, species identifications, ratios, and indices combining several data variables depending on the level of effort, or tier, of the survey.
When performing bioassessment of lakes, individual metrics are assigned scores, usually a number corresponding to good, fair, or poor relative to the values of the measurements in reference conditions (Karr 1991, Karr et al. 1986). This serves to standardize the metrics on the same scale so they can be combined into an additive index. Measurement scores are summed to obtain an index score for each assemblage, such as an IBI or macroinvertebrate community score. Currently, each measurement is weighted equally in the summed index score.
Additive biological indices collapse a great deal of information into a single number. Yet they have been shown to be reliable in detecting impairment of aquatic systems (Fore et al. 1994, Fore et al. 1996, Wallace et al. 1996); they are simple to compute once criteria are established, and they are easily communicated to managers and the public (Gerritsen 1995).
Habitat component scores may give clues to the causes of impairments reflected in biological indices rated fair or poor. Habitat variables that are significantly different from reference conditions are identified as probable causes of impairment, warranting further investigation or remediation. This sort of bioassessment cannot establish cause of impairment; it can only separate probable from improbable causes of impairment. In any given bioassessment, several probable causes might be identified.
Biological data are used to help set biological criteria based on management needs and defined management classes. States may draft general narrative biocriteria early in their program - even before they have designated reference sites or refined their approach to biological surveys. This does not mean that having reference sites and a refined system for conducting surveys is unimportant; it means that a biocriteria program begins with writing into law a statement of intent to protect and manage water resources predicated on an objective or benchmark, for example, “aquatic life shall be as naturally occurs.”
When the objective to restore and protect the biological integrity of the water resources has been formally mandated, then the operational meaning of the statement and the identification of the agency responsible for developing the necessary procedures and regulations can be stipulated as the state’s first steps toward the development of narrative and numeric biological criteria. The key point is that natural or minimally impaired water resource conditions become the criteria for judgement and management.
Although based on the same concept as narrative biocriteria, numeric biocriteria include discrete quantitative values that summarize the status of the biological community and describe the expected condition of this system for different designated water resource uses.
The key distinction between narrative biocriteria supported by a quantitative database and numeric biocriteria is the direct inclusion of a specific value or index in the numeric criteria. This index allows a level of specification to water resource evaluations and regulations not common to narrative criteria. To develop numeric criteria, the resident biota are sampled at minimally impaired sites to establish reference conditions. Attributes of the biota, such as species richness, presence or absence of indicator taxa, and distribution of trophic groups, help establish the normal range of the biological community as it would exist in unimpaired systems.
|
The Tennessee Valley Authority is currently using a multimetric biological assessment methodology on its reservoirs. The Tennessee River watershed drains portions of four ecoregions: Blue Ridge, Central Appalachian Ridge and Valley, Southwestern Appalachians, and Interior Plateau (Omernik 1987) (Figure 3-3). The Tennessee River begins at the confluence of the Holston and French Broad Rivers and receives drainage from the Ridge and Valley and Blue Ridge ecoregions. Downstream, the river drains a small portion of the Southwestern Appalachians and a large part of the Interior Plateau. The main stream carries water from two to four ecoregions. Therefore, dividing the main stream reservoirs by ecoregion does not contribute to a meaningful classification. Figure 3-3 illustrates that the tributary reservoirs can be easily divided by ecoregion. There are several reservoirs with watersheds entirely within the Blue Ridge and Ridge and Valley ecoregions. There is a third, and more dispersed, group of tributary reservoirs in the Interior Plateau. Physical, chemical, and biological indicators were selected to provide information on the health or condition of habitats or ecological compartments. The open water or pelagic area was represented by physical and chemical characteristics of water (including chlorophyll) in midchannel. The shoreline or littoral area was evaluated by sampling the fish community. The bottom or benthic compartment was evaluated using two indicators: quality of surface sediments in midchannel (determined by chemical analysis of sediments) and examination of benthic macroinvertebrates from a transect across the full width of the sample area (including overbanks if present). Three areas were selected for monitoring: the in-flow area, generally riverine in nature, the transition zone or mid-reservoir area where water velocity decreases due to increased cross-sectional area, suspended materials begin to settle, and algal productivity increases due to increase water clarity; and the forebay, the lacustrine area near the dam. Overbanks, basically the floodplain which was inundated when the dam was built, were included in transition zone and forebay areas. Four large embayments (all with drainage areas greater than 500 square miles and surface areas greater than 4500 acres) were included in the Vital Signs Monitoring program. Ecosystem interactions within an embayment are mostly controlled by physical characteristics of the embayment and by activities and characteristics within the embayment watershed, usually with little influence from the main body of the reservoir (Meinert et al. 1992). Sampling frequencies and index periods take into account the expected temporal variation for each indicator. Physical and chemical components vary significantly in the short term so they are monitored monthly from spring to fall. Biological indicators better integrate long-term variations and are sampled once each year. Fish assemblage sampling is conducted in autumn (September-November). Initially, benthic macroinvertebrate sampling was conducted in early spring (February-April) to avoid aquatic insect emergence. The TVA experience showed that a late winter/early spring sampling period is not acceptable for benthic macroinvertebrates because results reflected conditions which occurred the previous year. This causes results from this indicator to be out of synch with the other four indicators. A late fall/early winter collection avoids problems resulting from early spring sampling. The TVA case study is continued in subsequent chapters.
|
Home ~ Preface ~ Chapter 1 ~ Chapter 2
Chapter 3 ~ Chapter 4 ~ Chapter 5 ~ Chapter 6
Chapter 7 ~ Chapter 8 ~ Chapter 9 ~ Chapter 10
Appendix A ~ Appendix B ~ Appendix C ~ Appendix D
Appendix E ~ Appendix F ~ Appendix G
|
|
|
|
||
|
|
