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Monitoring and Assessing Water Quality |  |
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Appendix D
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Table D-1. Advantages, disadvantages, and alternatives to using algal assemblages.
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Nutrient enrichment from human activities generally leads to increased biomass of algae in lakes. Measurement of algal and other plant biomass, or a surrogate (e.g., chlorophyll a), is a good indicator of eutrophication and is clearly related to biological integrity, meeting the first criterion for successful metrics. Trophic state is an expression of the production of a lake ecosystem. We use Carlson and Simpson’s (1996) definition of trophic state based solely on biomass and operationally measured by variables that estimate biomass.
Carlson’s Trophic State Index (TSI) (Carlson 1977) is the most widely used index for eutrophication in lake monitoring programs, and is based on epilimnetic chlorophyll a concentration, total phosphorus concentration, and Secchi depth. Index values range from below 0 (ultraoligotrophic) to above 100 (hypereutrophic). Each of three measures (chlorophyll, total P, Secchi) is used to calculate an independent index, and the indices can be compared to identify whether algal growth is limited by phosphorus, light, or other nutrients. Chlorophyll is the most accurate and is the preferred indicator of trophic state. Total phosphorus and Secchi depth indices are also used as surrogates in the absence of chlorophyll data, and can be used to identify factors contributing to algal growth when all three are measured (Carlson and Simpson 1996). Measurement of primary productivity is not recommended because it is expensive to measure and frequently difficult to interpret.
Other indices have been developed and might be appropriate for different lake ecoregions in the country. A nitrogen index can be included to identify nitrogen limitation (Carlson 1992, Kratzer and Brezonik 1981). Other trophic state models (e.g., Dillon and Rigler 1974, Larsen and Mercier 1976, Vollenweider 1975) use annual phosphorus loading rates or retention fractions, and rely on measurements of nutrient concentrations rather than the biological response to nutrient loading. See Carlson and Simpson (1996) for a complete discussion of the trophic state concept.
Both algae and macrophytes contribute to a lake’s plant biomass, therefore, metrics for both algal and macrophyte biomass are preferred for whole-lake trophic state (Canfield et al. 1983, Carlson and Simpson 1996) (see section 4.2 for macrophyte biomass).
The Tennessee Valley Authority (TVA) uses chlorophyll a concentrations for one of its reservoir assessment metrics. The metric is based on the mean growing season water column concentration and a single maximum concentration (Dycus and Meinert 1994, TVA 1995). Reservoirs are considered mesotrophic or oligotrophic, based on natural watershed geochemistry and expected chlorophyll a concentrations. Low, moderate, and high mean concentrations of chlorophyll a are rated “good”,” fair,” and “poor,” respectively, with differing definition of these three categories for mesotrophic and oligotrophic reservoirs. A very high single-sample maximum (> 30 mg/L) reduces the rating by one class. Thus, a good rating implies chlorophyll concentrations within the range expected and no extreme blooms. In addition, for mesotrophic reservoirs unusually low concentrations (< 3 mg/L) are rated “fair.” Also, if this low concentration occurred despite sufficient phosphorus, it was considered an indication of limitations other than nutrients and resulted in a poor rating.
Many different levels of algal monitoring and assessment exist. Metrics based on indicator taxa can be quite simple, such as qualitative estimates of relative dominance of algal divisions (Table D-2). For example, dominance by diatoms might be rated “good”, and dominance by cyanobacteria might be rated “poor,” requiring only a rapid, qualitative estimate of the relative abundances of diatoms and cyanobacteria. Indicator genera could also be used, For example, abundant populations of the cyanobacteria Oscillatoria or Anabaena indicate eutrophication (e.g., Edmondson and Lehman 1981). Certain diatoms and chrysophytes are sensitive to pH and dissolved aluminum (Charles and Whitehead 1986, Smol et al. 1984).
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Table D-2. Potential algal metrics.
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Algal assemblage data, consisting of taxonomic identifications and abundance (relative or absolute) of each taxa, can be analyzed in two ways:(1) by determining assemblage metrics based on species structure, or (2) by multivariate assemblage analysis. Simplified field and laboratory procedures are possible for some (but not all) of the species structure metrics.
Due to the high temporal variability of plankton, several samples during the growing season might be needed for accurate assemblage analysis.
Assemblage metrics, as defined and used in assessment of biological integrity, rely on the comparison of a metric to a reference value. Assemblage metrics possible for use in algal analysis include (Bahls 1993):
Spatial Variability - Phytoplankton can be patchily distributed in a lake, affecting the variability of a sampling program. Most phytoplankton patchiness is the result of water motion and identifiable water masses, such as Langmuir circulation, vertical stratification, and embayments with limited exchange to open water due to morphometry or submerged vegetation. Effects of these can be minimized by taking vertically integrated samples in mid-lake, with a vertical tow, a pump, or a series of bottle samples.
Temporal Variability - The largest single disadvantage of phytoplankton sampling, including biomass and chlorophyll a measurement, is temporal variability. The algal assemblage seasonal succession cycles are only general, and their exact timing and composition are not predictable (Reynolds 1984). The variability is best controlled with repeated sampling (typically monthly or weekly) using a minimum of 10 samples to obtain either an annual average or an index period average (e.g., growing season, spring overturn, peak biomass) (Knowlton and Jones 1989).
Diatoms and chrysophytes preserved in lake sediments are integrators of lake history and make it possible to infer changes in other biotic assemblages (Charles et al. 1994, Dixit et al. 1992). Environmental variables, such as alkalinity, aluminum, dissolved organic carbon, salinity, nickel. conductivity, calcium, total nitrogen, total phosphorus, Secchi transparency, and trophic state have been inferred using diatom-based predictive models (Charles et al. 1994, Dixit et al. 1992, Fritz 1990).
The diatom fossil record can aid in establishing reference conditions. See Appendix C for methods. Surface sediments represent recent or current lake conditions and usually integrate the assemblage over 1 or more years (Dixit et al. 1992). Presettlement conditions may be characterized by sediment cores of 0.5 to 1.0m depths (Charles et al. 1994). Dating sediment cores is possible using pollen or radioactivity of 210Pb (radon decay product).
Periphyton Spatial Variability - Periphyton abundance and species composition might be variable around the periphery of a lake owing to differences in water quality, local variation of runoff from the shore, differences in substrate, and other factors. Periphyton may be scraped from natural substrates, or artificial substrates may be deployed for periphyton colonization (Kentucky DEP 1993, Bahls 1993, Florida DEP 1996, Oklahoma CC 1993). A composite sample from several substrates at several sites should remove most of the effects of local spatial variability.
Temporal Variability - Like phytoplankton, periphyton are subject to changing water chemistry and seasonal succession. Sampling during an index period in a time of relative stability might remove most of the confounding effects of time.
Response of Metrics - Although periphyton have been used successfully in streams (e.g., Bahls 1993, Patrick 1949), their application as lake indicators is relatively new. Metrics of periphytic diatoms have shown promise for bioassessment, based on investigation of undisturbed reference lakes in Montana (Gerritsen and Bowman 1994), but actual response to disturbance or pollution is as yet unknown. Periphyton are considered an experimental assemblage for lake assessment because of limited information on response to stressors.
Aquatic plants respond to nutrients, light, toxic contaminants, salt, and management. A lack of macrophytes might indicate water quality problems due to herbicides, salinization, or excessive turbidity. Submerged and floating macrophytes respond to nutrients in the sediment (Barko et al. 1992), and an overabundance of submerged or floating leaved plants can be an indicator of excess nutrients. Exotic species (e.g., Eurasian water milfoil) often become dominant and cause weed problems under eutrophic conditions. In addition, submerged macrophytes are sensitive to shading by turbidity and by dense periphyton growth. Many species are sensitive to phytotoxins, such as copper and herbicides.
Submerged macrophytes are extensively managed. Exotic species frequently dominate eutrophic lakes, and control attempts include harvesting, herbicides, and grass carp. Natural macrophytes are managed where they are thought to interfere with recreation.
Extreme eutrophication in shallow lakes may have alternate stable states: one dominated by macrophytes, the other by phytoplankton (Scheffer et al. 1992). Management of such lakes to promote the macrophyte dominated state includes removal of planktivorous fish and introduction of macrophytes and piscivorous gamefish (Hosper et al. 1992).
Macrophytes respond more slowly to environmental changes than do phytoplankton or zooplankton and might be better integrators of overall environmental conditions (Table D-3). This would allow a single sampling event per year, during the time of maximum abundance of macrophytes. Both floating leaved and emergent plants are easily assessed from aerial photographs, which permit estimates of total area covered and percent cover (density) within stands. For the purposes of lake assessment, emergent vegetation (i.e., semi-terrestrial) is lake habitat, but floating and submerged vegetation are lake biota.
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Table D-3. Advantages, disadvantages, and alternatives to using macrophyte assemblages.
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Extent and Percent Cover - Extent and percent cover of rooted vegetation are easily obtained from rapid surveys or remote sensing (aerial or satellite imaging). These methods have been used successfully to monitor the status and trends of submerged vegetation in estuaries (e.g., Orth and Moore 1983). Extent of both floating-leaved and emergent vegetation can be estimated from aerial photos or from shorezone surveys. Wetlands can also be estimated from maps developed by the National Wetlands Inventory (NWI), although these would not indicate the extent of littoral emergent vegetation in most lakes. When compared to expected or reference values, the extent and percent cover of macrophytes and emergents provide an assessment of the overall integrity of the lake system. Loss of emergents and wetlands on a lake margin indicates lost wildlife habitat and possibly increased nutrient and sediment input. Nuisance weed problems might indicate eutrophication, and loss of native macrophytes (compared to reference) might indicate excess turbidity or toxic contamination.
Spatial Variability - With suitable substrate and sufficient light, macrophytes colonize the littoral areas in lakes and reservoirs. Spatial variability of cover and extent within these areas can be a result of one or more of the factors listed below:
Vegetation functional measurements such as net growth, primary productivity, etc., are time consuming and require repeated monitoring at different times in the growing season. It is not clear that the information gained from functional measurements is any better for assessment and management purposes than remote, wide-scale measurements.
Assemblage Metrics - Identification of taxa and relative abundance counts or biomass estimates of each allow calculation of similar assemblage metrics described for the algae assemblages (Table D-4).
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Table D-4. Potential macrophyte metrics.
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To minimize effects of variability, several sites are sampled in a lake and combined into a composite sample.
Temporal Variability - Aquatic macrophyte assemblages on the whole are usually at maximum cover and extent in midsummer. Temporal variability is avoided by sampling the macrophyte assemblage at approximately the same time every year. Interannual variability of macrophyte cover can be high (Scheffer et al. 1992); if so, total vegetated area may not be an effective metric.
Research Needs - It is generally accepted that macrophytes respond to nutrients by expanding their extent and cover. Research is needed to determine which species respond to contaminants such as acid, metals, organics, and salinity. Macrophytes might respond to individual contaminants or only a combination of contaminants. They might respond to contaminants only at extreme levels or conditions.
Benthic invertebrate assemblages in lakes correspond to particular habitat types and can be classified according to the three basic habitats of lake bottom: littoral, sublittoral, and profundal. The littoral habitat of lakes usually supports larger and more diverse populations of benthic invertebrates than do the sublittoral and profundal habitats (Moore 1981, Wiederholm 1984). The vegetation and substrate heterogeneity of the littoral habitat provide an abundance of microhabitats occupied by a varied fauna, which in turn enhances invertebrate production. The littoral habitat is also highly variable due to seasonal influences, land use patterns, riparian variation, and direct climatic effects producing high-energy areas. The epifauna species composition, number of individuals, areal extent, and growth form vary with the species composition of the macrophyte beds, making it difficult to determine the benthic status accurately.
The sublittoral habitat, below the area of dense macrophyte beds, but above typical thermoclines, lacks the heterogeneity of the littoral habitat; However, it is also less subject to littoral habitat variables and influences. The sublittoral habitat is rarely exposed to severe hypoxia but might also lack the sensitivity to toxic effects that is found in the profundal habitat. The sublittoral habitat supports diverse infaunal populations, and standardized sampling is easy to implement because a constant depth and substrate can be selected for sampling. Therefore, the sublittoral habitat is the preferred habitat for surveying the benthic assemblage in most regions.
The profundal habitat, in the hypolimnion of stratified lakes, is more homogeneous due to a lack of habitat and food heterogeneity, and hypoxia and anoxia in moderately to highly productive lakes are common. The profundal habitat is usually dominated by three main groups of benthic organisms including chironomid larvae, oligochaete worms, and phantom midge larvae (Chaoborus) (Wiederholm 1984). Many species of chironomids and tubificid oligochaetes are tolerant to low dissolved oxygen, such that these become the dominant profundal invertebrates in lakes with hypoxic hypolimnia. As hypoxia becomes more severe tubificids can become dominant over chironomids (Hergenrader and Lessig 1980). In cases of prolonged anoxia, the profundal assemblage might disappear entirely. If hypoxia is rare in reference lakes of the region, and if toxic sediments are suspected to occur in some lakes, then the profundal habitat might be preferred for the region.
Benthic macroinvertebrates are moderately long-lived and are in constant contact with lake sediments. Contamination and toxicity of sediments will therefore affect those benthic organisms which are sensitive to them (Wiederholm 1984). Acidification of lakes is accompanied by shifts in the composition of benthic assemblages to dominance by species tolerant of acidic conditions (Perry and Troelstrup 1988, Schindler et al. 1989). Effects of rapid sedimentation are less well-known but appear to cause shifts toward lower abundances and oligotrophic species assemblages as well as more motile species (Masters 1992, Wiederholm 1984).
Benthic macroinvertebrates are present year-round and are often abundant, yet not very motile. However, the benthos integrate environmental conditions at the sampling point (Table D-5). To date, TVA, EMAP, and several states (Florida, Oklahoma, North Dakota) have surveyed benthos as part of lake bioassessment in the United States. Developmental work by TVA, USEPA, and several states is likely to refine metrics based on macroinvertebrates.
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Table D-5. Advantages, disadvantages, and alternatives to using macroinvertebrate assemblages.
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Primary emphasis in the past has been placed on chironomids and oligochaetes as indicators of lake trophic status. Several indices and classification systems have been developed for lake trophic state using chironomid and oligochaete assemblages as indicators (e.g., Naumann 1932). The trophic indices, most of which were developed for lakes of northern Europe, rely on relative abundances of chironomid species, the ratio of tolerant to intolerant tubificid oligochaetes, or the ratio of oligochaetes to chironomids (reviewed in Wiederholm 1980). Ratios are unstable metrics because numerator and denominator are independent (Barbour et al. 1992); proportions or percentage metrics work better.
TVA is using benthic macroinvertebrate composition as one of five assessment indicators in reservoirs (Dycus and Meinert 1992, Dycus and Meinert 1993, Dycus and Meinert 1994). TVA benthic composition metrics evaluate richness, composition, abundance, and indicator taxa. The condition of macroinvertebrate assemblages in TVA reservoirs is strongly associated with hypoxia in the reservoirs (after Dycus and Meinert 1992). The EMAP surface waters pilot project is also using benthic macroinvertebrates for assessing the biological condition of lakes and has found that number of taxa among benthic macroinvertebrates corresponds to level of disturbance in a watershed (USEPA 1993a).
Lake benthic metrics that are responsive to stresses, are in general, similar to stream invertebrate metrics (Table D-6). Metrics used successfully by TVA in assessing reservoirs include (TVA 1994, TVA 1995):
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Table D-6. Potential benthic metrics.
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Invertebrate metrics demonstrated to respond to stresses in Florida lakes include ( FDEP 1994, Gerritsen and White 1997):
Biological assessment using benthic macroinvertebrates must focus on a subset of assemblages (defined by habitat and season) to avoid costly sampling of all assemblages. Assemblage composition is affected by substrate, macrophytes, depth, and season. The optimal assemblage for reasons of cost, variability, and interpretation appears to be the sublittoral assemblage of epifauna and infauna. The littoral assemblage is highly variable and costly to sample, and the profundal assemblage might be uniformly impacted by hypoxia in many regions of the country. Hypoxia might be natural in deep, mesotrophic lakes or in warm water lakes. If hypoxia is an expected profundal condition, sublittoral benthos is the preferred assemblage. If hypoxia is rare or not expected in the reference condition, profundal benthic sampling might be preferred.
Spatial Variability - To account for spatial variability within the sampling area of a lake, at least three grabs must be taken. The grabs can be combined into a composite sample to save money, but valuable information is lost. For example, data on spatial variability is lost, but more importantly effects of one sample with a very large density of a single taxon will be more significant in a composite sample than in the average of individual samples. In large lakes or lakes with heterogenous bottom substrate, five or more sites might need to be sampled. Selection of the epifauna or infauna for sampling will depend on the major substrate type present and the overall objectives of the biosurvey. For example, with sediment problem the benthic infauna would be the appropriate part of the assemblage to sample. If the major substrate type present is hard substrate or vegetation, the epifauna should be sampled.
Toxic or contaminated sediments are more likely to be a stress on profundal invertebrates because sediments accumulate in the deep, depositional areas and infaunal oligochaetes might be more sensitive to toxicity than are other invertebrates. However, the sublittoral habitat has certain advantages for sampling macrobenthos because it is subject to hypoxia less frequently than the profundal habitat and because the sublittoral area typically has greater number of taxa, including some mayflies and caddisflies than the profundal area.
The issue of seasonality needs further investigation to determine the most effective index period for sampling or the sampling frequency. Sampling period can be either during the most stressful period or during a time after recruitment when the populations have stabilized. The selected period should be of the least consequence to the identification and sampling process, especially if the sampling is designed for volunteer monitoring groups. For example, samples taken right after recruitment will have early instars that are difficult to identify. If more than one period is designated, the appropriate sampling frequency needs to be established.
The sampling area should focus on the most predominant substrate available and the metrics should be developed independent of microhabitat variation. The type of sampling gear will depend on the substrate being sampled as each substrate has its own optimal sampling gear. Standardized sampling techniques for each gear type should be implemented to allow for the comparison of data. Processing of samples should be standardized by using a standard net size of 595 mm (No. 30 mesh).
The objective is to adequately characterize the sampling unit which is a single lake, embayment, or lake basin. Heterogeneity within a sampling unit (lake) is not of interest in bioassessment. Samples from several sites are combined into a single composite for analysis and characterization of the lake. To get a representative sample of benthic invertebrates, it is necessary to sample at several locations, such as, three to five areas of the sublittoral zone around the lake. Sampling at each site might also consist of several grab, which can be composited to save money.
Research Needs - Six recommendations for further study were identified during the development of this document by the Benthic Workgroup:
Lake zooplankton consist primarily of crustaceans, rotifers, and, to a lesser extent, semi-planktonic insect larvae of the genus Chaoborus. Many zooplankton species found in north temperate lakes are cosmopolitan or wide-ranging in their distribution (Hutchinson 1967). There is a strong positive relationship between the number of crustacean zooplankton species and lake surface area (Dodson 1992, Fryer 1985), and weaker positive relationships between number of species and lake productivity, and the number of neighboring lakes (Dodson 1992).
More than any other assemblage, zooplankton structure and function are controlled externally by both higher and lower trophic levels (fish predators and algal food) and internally by planktonic predators (Lewis 1979, Zaret 1980, Carpenter et al. 1987) (Table D-7). Zooplankton composition and abundance are variable in time with numbers changing one to three orders of magnitude within weeks. The complexity of open water zooplankton dynamics is in part due to trophic interactions taking place in a three-dimensional environment of reduced structure (Gerritsen 1980).
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Table D-7. Advantages, disadvantages, and alternatives to using zooplankton assemblages.
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The trophic cascade can be modified by nutrient enrichment and internal interactions (Carpenter et al. 1987) and can in turn affect physical characteristics such as light penetration and temperature (Mazumder et al. 1990).
Zooplankton indicators that have been investigated rely on measurement of plankton size structure, and trophic categories (Stemberger and Lazorchak 1994) (Table D-8). From the ecological interactions listed above, zooplankton body size is a potential indicator of the presence or absence of planktivorous forage fish, and of the absence or presence, respectively, of large piscivores. Use of zooplankton body size as an indicator (Mills and Schiavone 1982, Mills et al. 1987, O’Gorman et al. 1991) showed that mean zooplankton body size can predict populations of yellow perch and migration of alewives. Furthermore, dominance by large, visible Daphnia species (e.g., D. pulex, D. galeata) indicates the presence of large piscivores, circumneutral pH, and the absence of blue-green algal blooms (Edmondson and Litt 1982, Mills et al. 1987).
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Table D-8. Potential zooplankton metrics.
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Several zooplankton species, especially some of the larger predators and Daphnids, are sensitive to acidification, and acidic lakes have fewer zooplankton taxa than circumneutral lakes (Baker and Christensen 1991). Large Daphnia (> 1 mm) are used as an indicator of trophic balance in operational biomanipulation in Europe (Hosper et al. 1992, Hosper and Meijer 1993), and lakes with large Daphnia have lower chlorophyll concentrations than comparable lakes without (Mazumder 1994).
The EMAP Surface Waters program is testing selected zooplankton metrics in New England lakes. EMAP zooplankton sampling consists of a single vertical tow at the deepest point of a lake, using a dual (bongo) net, with a fine (48mm) net and a coarse (202mm) net (USEPA 1994a, USEPA 1994b).
Spatial Variability - Zooplankton are subject to many of the same water movements that affect phytoplankton. In addition, many species perform diurnal vertical migration. Integrated sampling of the mid-lake water column with a vertical or oblique tow is usually sufficient for relative abundances of zooplankton species. To avoid possible effects of vertical migration, samples should not be taken near dawn or dusk.
Index Period - Zooplankton assemblages are not stable in time undergoing seasonal succession. To the extent that assemblages are seasonally predictable, they can be sampled within an index period. Mid-summer or mid-winter are relatively stable periods. Midsummer is preferred to coincide with other assemblages.
Research Needs - Although preliminary results from EMAP are encouraging, the responsiveness and reliability of many zooplankton-based metrics are not yet well known. Response of zooplankton metrics to stressors, needs to be tested in different regions of the country. Seasonal variability and predictability of zooplankton assemblages needs to be analyzed to determine optimal index periods and the minimum number of samples required to characterize a lake.
Fish populations are powerful structuring forces on other lake assemblages through feeding interactions (trophic cascades). Abundant populations of piscivorous fish reduce planktivorous forage fish species, releasing predatory zooplankton from predation, resulting in dominance by large-bodied, suspension-feeding zooplankton (e.g., Brooks and Dodson 1965, O’Brien 1979). The large suspension-feeding zooplankton can in turn reduce phytoplankton abundance, increasing water clarity and altering the thermal structure of the lake (Mazumder et al. 1990). The trophic cascade also influences, and is influenced by, nutrient dynamics (Carpenter et al. 1987).
It is well known that fish production is tied to lake primary production (e.g., Oglesby 1977, Ryder et al. 1974). In fact, oligotrophic lakes are often fertilized by fishery agencies to enhance sport fish production. In addition, there are regional, geographic differences in fish abundance that are not explained by trophic state (Nürnberg 1996). Moderate to severe eutrophication reduces and might eliminate desirable sport fish due to loss of habitat, poor water quality, and food web simplification (NRC 1992). Fish are highly dependent on habitat for spawning and for refuge. Some species (e.g., yellow perch, most salmonids) spawn in streams; others require clean rock or gravel habitat in the lake. Submerged vegetation provides cover for both forage fish and piscivores, and recolonization of littoral areas by macrophytes increases sportfish abundance, as well as improving water quality.
More than any other assemblage, fish are subject to management which can confound assessment efforts (Table D-9). Exotic piscivorous sport fish (e.g., striped bass, Pacific salmon) are widespread in lakes throughout the United States, and many of these populations are maintained by regular stocking. Stocking to maintain a population results in an artificially large population - especially juveniles - with resultant trophic cascade effects on zooplankton and phytoplankton. This problem is especially pronounced in “put and take” fisheries, where large numbers of hatchery-reared adults are released for a fishing season and decimate invertebrate assemblages during the season. In general, if exotic piscivorous species reproduce naturally, biological integrity is less likely to be affected.
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Table D-9. Advantages, disadvantages, and alternatives to using fish assemblages.
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Assemblage Composition and Abundance - Measurements of fish assemblage composition and relative abundance can be incorporated into several metrics, including the Index of Biotic Integrity (IBI), an index of several assemblage-level metrics and their variations, and multivariate assemblage analysis. Field measurements for these are the relative abundances of species in the habitat.
Index of Biotic Integrity (IBI) - The Index of Biotic Integrity (IBI) incorporates attributes of fish assemblages to evaluate human effects on a stream and its watershed (Karr 1991, Karr et al. 1986). Those attributes cover the range of ecological levels from the individual through population, community, and ecosystem. IBI consists of 6 to 12 measures, or metrics, in 4 broad categories: species composition, trophic composition, fish abundance, and condition (Karr 1991). A site is assigned scores for the resemblance of each metric to the reference (unimpacted or least impacted) condition expected for that area. Total scores of all metrics result in an overall score for the site.
As with other multimetric indices, component metrics of IBI require adaptation and calibration to the geographic regions in which they will be applied, thus incorporating biogeographic variation of assemblages and systems into the assessment (Karr 1991). This may include deletion or replacement of selected IBI metrics and is done with the development of a reference site data base. Local adaptations of IBI for streams have been developed for several regions of the United States (Karr 1991, Leonard and Orth 1986, Miller et al. 1988, Steedman 1988).
Although lakes and reservoirs differ in physical attributes from rivers and streams (the former being more homogenous), the valued attributes, or biological integrity, of fish assemblages apply equally. These attributes include species composition, trophic composition, abundance, and condition. Differences between lake and stream habitats lie in the expectations for the attributes and will be reflected in reference site data. An index used by TVA on its reservoirs is based on 12 metrics and is called the Reservoir Fish Assemblage Index, or RFAI (Jennings et al. 1995, Hickman and McDonough 1996) (Table D-10). The status of this index is discussed under Research Needs.
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Table D-10. Fish assemblage metrics under investigation by TVA. After Dycus and Meinert (1994) and Hickman and McDonough (1996).
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The major problem in applying IBI to lakes is obtaining representative samples of fish assemblages in lakes. Quantitative sampling in lakes is not as reliable as that in streams because of lake morphology, bottom types, and gear efficiency. Modification of IBI for lakes may include use of relative abundances based on subsamples from constant-effort sampling. Sampling gear and protocols for different habitats of lakes will need to be standardized.
Qualitative Screening - Widespread familiarity with the condition of sport and forage fish in natural resource agencies permits qualitative screening assessment using expert knowledge of local and state fisheries experts (USEPA 1989b). The intent is to serve as a screening tool and to maximize the use of existing knowledge of fish assemblages with a questionnaire polling state fish biologists and university ichthyologists believed knowledgeable about the fish assemblages in lakes of concern. Unlike field surveys, questionnaires can provide information about tainting or fish tissue contamination and historical trends and conditions. Disadvantages of questionnaires include inaccuracy caused by hasty responses, a desire to report conditions as better or worse than they are, and insufficient knowledge.
Contaminants in Fish Tissue - Contaminant concentrations in fish tissue have been monitored to assess the extent of environmental contamination and to estimate risks to human health from consuming fish. Contaminant concentration is an excellent indicator of health risk, but it is not an indicator of biological integrity.
Pathology - Pathological abnormalities (lesions, tumors, growth anomalies) of fish are monitored as overall indicators of environmental degradation, including effects of severe eutrophication, sediment contamination, and acidification. Significant rates of pathology typically occur only in the most severely polluted habitats and in populations of nonmigratory, bottom-feeding fish. Pathology can be incorporated into multimetric indices, such as IBI (Dionne and Karr 1992).
The major problem in developing fish indices for lakes is obtaining representative samples of fish assemblages in lakes. Quantitative sampling in lakes is not as reliable as in streams because of lake morphology, bottom types, and variable gear efficiency. Modification of IBI for lakes can include use of relative abundances based on subsamples from constant-effort sampling. Sampling gear and protocols for different lake habitats will need to be standardized.
Spatial and Temporal Variability - Fish are highly mobile and respond rapidly to gradients in physical habitat and water chemistry. They actively avoid harmful conditions. Physical and chemical parameters that affect fish spatial distribution include:
Many fish seek specific habitats for activities such as feeding, resting, and spawning. Their movement between habitats is dependent on time of day and season. Although fish populations are relatively stable compared to smaller, shorter-lived plankton and benthos, fish mobility and behavior make fish difficult to sample.
Index Period - Sampling during the spring coincides with optimal biological conditions and may show recovery from environmental stress periods. However, to avoid spring spawning, sampling is usually conducted in late summer and early fall. Seasonal changes in the relative abundances of the fish assemblage occur primarily during reproductive periods and (for some species) the spring and fall migratory periods. If fish sampling is required during this period, then changes in relative abundance will be important. Mid to late summer is often a time of oxygen stress and should show the greatest effects from environmental stress.
Sampling Gear - Obtaining both qualitative and quantitative data on fish populations is limited by gear selectivity and the fish mobility (USEPA 1992b). All sampling gear is selective. The habitat or portion of habitat sampled and efficiency of gear for a particular species in one area does not necessarily apply to different species nor to the same species in another area. Temporal and spatial changes in relative abundance of a species can be assessed under a given set of conditions if those species are readily collected with a particular kind of gear.
Electrofishing is the technique used most often by agencies that monitor fish assemblages. The EMAP Surface Water Northeast Lake Pilot Survey found electrofishing the most effective single-gear technique (USEPA 1994a, USEPA 1994b). The RFAI for TVA reservoirs includes electrofishing as a collection technique (Hickman and McDonough 1996). Other considerations with respect to electrofishing are:
Seining, an active sampling technique, can be used in the littoral areas (straight seines). Haul seines and trawls are used in deeper open water areas. Seining or trawling is not effective in areas with bottom obstructions that can tear or foul the net. Although the results are expressed as number of fish captured per unit effort, quantitative seining is very difficult. This method is more useful in determining the variety of fish rather than the number of fish inhabiting the water.
Athough gill nets are a passive technique with several disadvantages, they might be the most appropriate gear type for sampling deep sublittoral habitats. Gill nets are size-selective, depending on mesh size and do not obtain representative samples of the total population. They are most effective on lake herring, trout, lake whitefish, yellow perch, walleyes, and northern pike (USEPA 1992b). There is a high mortality rate of fish caught in gill nets and occasional mortality of nontarget species such as turtles, muskrats, beavers, and diving waterfowl. Trap and fyke nets are effective in shallow areas. Like gill nets, they are also passive and do not obtain a representative sample of the total population. Meador et al. (1993) and Weaver (1993) recommended a multi-gear approach that takes advantage of differences in gear selectivity and efficiency to achieve a more accurate representation of the fish assemblage structure. TVA uses shoreline electrofishing for the shallow littoral zone and experimental gill nets for the sublittoral/limnetic zone (Hickman and McDonough 1996).
Research Needs - TVA has been actively developing assessment tools for its reservoirs for several years. The move to a multimetric approach for reservoir fish began in 1990. Successive steps in this development process have brought continued improvement to the RFAI. Potential improvements in the fish indices include using a simple random sampling design rather than a fixed station design to enhance statistical validity with little increase in variability. Use of the index in reservoirs or other river systems is necessary to test its performance under a wider range of conditions than is available in the Tennessee River. Correlation with known human-induced impacts remains a critical need before general acceptance of the fish index as a reliable method to address reservoir environmental quality.
A related issue is the effect of game fish management on IBI or other fish assemblage metric scores. Nearly all lakes are stocked or have been stocked in the past, and these practices can affect the biological assemblages in a lake. Stocking lakes with large piscivores is also used in biomanipulation to improve water clarity of eutrophic lakes (e.g., Hosper et al. 1992).
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
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