 |
||
|
Monitoring and Assessing Water Quality |  |
 |
||
|
|
|
|
|||||||||
|
|
Chapter 7
|
|
Figure 7-1. Tiered sampling structure. |
The desktop screening assessment involves documentation of existing data without any observations in the field (Table 7-1). No assessment can be better than the data that go into it; therefore, desktop screening alone might be unreliable. Its use should be limited to planning for more detailed monitoring and assessment. It incorporates cost and time efficiencies, allowing evaluation of a large number of sites, and identifying potentially affected areas for further investigation using higher tiers. Information is obtained from land use data and from a questionnaire to identify known problems in a lake (Table 7-1).
|
Table 7-1. Desktop screening assessment.
|
The questionnaire identifies existing known problems in lakes, but does not address new problems. An example questionnaire (Figure 7-2) is modeled after one for stream bioassessment (USEPA 1989b). Potential recipients of the questionnaire include regional biologists from natural resource agencies, the Cooperative Extension Service (CES), and academic biologists. Land use, NPDES, and population density data will identify lakes likely to have problems requiring further attention (primarily from eutrophication), but will not estimate biological impairment in the lakes. Components of desktop screening include the following:
|
Figure 7-2. Example of desktop screening questionnaire.
|
Land Use - Land use information indicates the relative level of anthropogenic stresses in a lake watershed, especially nonpoint sources of pollutants. Many states estimate land use from satellite images.
Discharges - USEPA maintains a data base of NPDES discharges and their receiving waters.
Algae - Questions on the history of nuisance algal blooms and perceived problems with high turbidity due to algae are included in the questionnaire (Figure 7-2).
Macrophyte Survey - Local professionals knowledgeable of the macrophytes in the lake(s) are canvassed for existing data and information (Figure 7-2). The questionnaire can provide the following information:
Fish Assemblage - Local professionals knowledgeable about fish assemblages can provide the following information:
Based on responses to the questionnaire, perceived levels of impairment can be judged from the three biological assemblages: algae, macrophytes, and fish. The three evaluations are kept separate. Perception of a problem, or a substantial departure from expected conditions, earns a rating of “impaired” for the respective assemblage. The land use information is used to identify potential stressors on a lake.
Tier 1 requires sampling of primary producers to assess trophic state and aquatic macrophytes. It can be done with a single visit during an index period when the objective is a synoptic survey and screening of many lakes (Tier 1A). Tier 1A is only appropriate for regional assessments—it cannot be used to assess single lakes. More precise estimates for single lakes can be made with Tier 1B, comprising several sampling visits to determine growing season averages. Tier 1 consists of the Desktop Screening land use survey, lake physical habitat, water chemistry (dissolved oxygen, nutrient concentrations, conductivity, alkalinity, pH), Secchi depth, chlorophyll a concentration, and a submerged macrophyte survey (Table 7-2). The survey enables:
|
Table 7-2. Tier 1: Trophic state and macrophyte sampling.
|
Tier 1A consists of sampling during an index period, typically mid to late summer for trophic state variables (e.g., chlorophyll a, Secchi depth, nutrients). Tier 1A is adequate for characterization of lakes in a region, when many lakes must be sampled to develop the characterization and assessment. Tier 1A will yield a good characterization of a region or a population of lakes, but precise characterization of individual lakes, for site-specific management, will require Tier 1B, with more frequent sampling. Tier 1B takes into account the changes in chlorophyll and nutrients that can occur in a short time and is used to estimate seasonal averages of the variables by sampling several times during the growing season. Trophic State Indices (TSI) are calculated from the seasonal average estimates of chlorophyll, Secchi depth, and nutrients. The number of sampling visits required depends on the temporal variation in the lake and the desired precision of the estimated seasonal average. Monthly sampling appears to be adequate for most purposes (Knowlton and Jones 1989).
Design of a sampling program inevitably requires compromises to answer the intended questions in a reasonable time and at a reasonable cost. In lake biosurveys, the unit of interest (sampling unit) may be the whole lake, a lake basin, a tributary arm, or an embayment. In some situations, the unit of interest may be an area of the lake receiving discharges or runoff. The object of sampling is to characterize the sampling units with sufficient precision and accuracy to meet the needs of the program.
Sample sites are selected to be representative of the lake. Single sites have traditionally been located in the middle of the lake, usually over the deepest area. For unbiased characterization, multiple sites should be selected randomly. Sampling may be stratified by zones, e.g., littoral, pelagic, and inflows; or riverine, transitional, and lacustrine (Figure 7-3). Estimation of mean values for the whole lake should be weighted by the relative area or volume of each zone. Figure 7-4 shows an example of sampling locations for all tiers in a relatively simple lake (natural or impoundment).
|
Figure 7-3. Sampling zones in large or complex lakes (large reservoirs, multi-basin lakes).
Figure 7-4. Integrated sampling, Tiers 1 and 2.
|
Lakes may be characterized by single or multiple sample sites in each lake, depending on the objectives of the survey.
If the objective is to characterize a large population of lakes, as in a statewide survey, then a single sample per lake is most cost-effective. A single site is typically chosen as the midpoint of the central basin of the lake, and is usually sufficient to prioritize lakes within a region.
Large riverine reservoirs have
known gradients of nutrients and productivity from the river inflow to
the dam (Kennedy and Walker 1990), and a single site is not
appropriate. Large reservoirs would require a minimum of three sites,
corresponding to the riverine, transitional, and lacustrine zones,
respectively (Figure 7-3).
If turbidity, nutrients, and algae are known to be variable across the surface of a lake, then multiple sample sites are required (Figure 7-4).
If gradients are known to
occur, as in many large reservoirs, then sampling should be stratified
by zones. For example, in a reservoir one could define the three
reservoir zones (riverine, transitional, lacustrine) as sampling
strata, and take two or more samples from each zone.
The exact number of sampling sites in a lake or lake zone is determined by the spatial variability of nutrients, turbidity, and chlorophyll; and the desired precision. In general, within a basin or reservoir zone, variation in time is larger than variation in space (Knowlton and Jones 1989). Thus, chlorophyll sampled 2 weeks apart may differ by several fold, but samples on the same day 500m apart are likely to differ much less.
If precise characterization of individual lakes is an objective of the biological survey then it is more cost-effective to sample repeatedly during the growing season (Tier 1B) than to sample multiple sites at a single time (Tier 1A).
Composite samples are taken from several sites in a lake or lake zone, and combined into a single sample for laboratory analysis. For example, water samples may be taken from four sites in a lake, and poured into a single clean bucket. The composite sample is subsampled for chlorophyll a and nutrients. Secchi depth temperature, and DO are measured at each of the four sites. Care must be taken that the methods and volume sampled are the same at each site. Composite samples characterize the lake better than a single sample and they save laboratory analysis costs. The principal disadvantage of composite samples is that they do not allow estimation of spatial variability within a lake.
|
Table 7-3. Sampling summary for chlorophyll, water quality, and phytoplankton.
|
The Tier 1 Trophic State Indices (TSI) are estimated from Secchi depth, chlorophyll a, and nutrient concentrations. Field methods for Secchi transparency and chlorophyll a are outlined below and summarized in Table 7-4.
|
Table 7-4. Tier 2A: Routine biological sampling.
|
Secchi Depth (SD)
Secchi depth is a measure of transparency. Turbidity caused by suspended sediments and algae decreases Secchi depth.
Sampling Location - Secchi disk transparency can be measured at one or more representative locations.
Frequency - Tiers 1A and 2A: single determination, midsummer. Tiers 1B and 2B: 6 to 10 samples during the growing season (e.g., March through October).
Sampling Procedure - Readings are obtained with a 20cm plastic or metal Secchi disk that is divided into black and white quadrants on a nonstretchable line, calibrated in centimeters. The disk is lowered into the water until it disappears from view, then is raised slowly to the point where it reappears. Secchi depth is the average of the two depths.
Observations are made from the sunny side of the boat or dock, during midday, without sunglasses, and as close as possible to the water in order to reduce glare.
Data Analysis - Secchi depth can be used in deter-mining trophic state along with chlorophyll a.
Chlorophyll a sampling and analysis follow standard protocols (USEPA 1994a, USEPA 1994b).
Presampling - Samples must be collected in a clean container, without using acid washes or phosphorus detergents. Before sample collection, bottles and collectors should always be double or triple rinsed with the lake water to be sampled.
Sample Location - One location or several representative locations for composite sample.
Frequency - The same as Secchi depth.
Depth - Chlorophyll a concentration may be estimated from surface samples taken at 0.5m, from integrated epilimnion samples, or from integrated water column samples. Half-meter surface samples require the least equipment and can be taken by hand; epilimnion and integrated water column samples are taken with a flexible hose.
Sampling Procedure - Surface sample, 0.5m. A rinsed, l-liter sample bottle is inverted and held at depth (arm’s length) by hand, turned up to fill, and brought to the surface.
Hose sample - A flexible hose is an easy method to obtain an integrated sample over the whole water column or over a defined portion, such as the epilimnion. The weighted end of a plastic hose is lowered to a given depth. The upper end is stoppered or clamped at the surface, and the weighted end is hauled to the surface with an attached line. The hose is emptied into a clean sample bucket, and chlorophyll and chemical subsamples can be drawn from the integrated sample. The hose may be lowered to 1m above the bottom for a water column sample, to the metalimnion, to twice Secchi depth as an estimate of the photic zone, or to a fixed depth (e.g., 5m). Each standard depth method has its own advantages and disadvantages (Carlson and Simpson 1996). Consistency of sampling method is more important than selecting the “best” standard depth.
Water samples are filtered for chlorophyll a extraction. A “rule of thumb” for the quantity to filter is 100ml for every foot of Secchi depth (330ml for every meter; D. Canfield, personal communication). Samples are vacuum-filtered on glass-fiber paper, and the filter papers are stored frozen in the dark. Detailed instructions for filtering and analysis are in APHA (1992) and USEPA (1994a, 1994b).
Samples of water for chemical analysis are collected in the same manner as chlorophyll samples. Sampler bottles should be cleaned in a phosphate-free detergent prior to use and rinsed two to three times in lake water in the field. Samples may need to be preserved or filtered in the field depending upon which chemicals are to be analyzed.
Dissolved Oxygen and Temperature Profiles - A dissolved oxygen/temperature electrode (EPA Method 360.1) is used to measure both dissolved oxygen and temperature. Using the electrode, dissolved oxygen and temperature may be measured at 0.5m intervals to produce dissolved oxygen and temperature profiles. Dissolved oxygen electrodes should be calibrated against standard chemical titration methods before and after field use.
pH, Alkalinity and Acid Neutralizing Capacity - A calibrated pH meter may be used to determine pH. Acid neutralizing capacity is important to the ability of a waterbody to resist changes in pH due to addition of acid and is based upon the alkalinity of the water and dissociated organic compounds present. Carbonates, bicarbonates and hydroxides are the major contributors to alkalinity which is determined using calorimetric titration methods (APHA 1992). For more precise determination of acid neutralizing capacity, the Gran plot method is used (USEPA 1987a).
Total Dissolved Solids (TDS) - Total dissolved solids consist of inorganic solutes such as nitrates, sulfates, and carbonates, and organic substances dissolved in water (APHA 1992). TDS is measured by first filtering a measured volume of sample water through a filter, and weighing the dried residue. See APHA (1992) for specific methods.
Algal Growth Potential Test (AGPT) - Because nutrients are not always present in a form available to algae, direct chemical measurements may not be predictive of the actual potential for algal growth. The Algal Growth Potential Test (also know as a biostimulation study, APHA 1992) was developed to directly measure in a standardized way the potential of waters to support algal growth.
Total Nitrogen - Nitrogen is an important plant nutrient and may serve as a limiting factor in some waters, especially subtropical lakes. Total nitrogen is a combination of nitrate/nitrite nitrogen and total Kjeldahl nitrogen (organic and reduced nitrogen). Total Kjeldahl nitrogen is measured using a digestion technique that converts organic nitrogen to ammonia and includes any other ammonia present in the sample. Nitrate plus nitrite is measured with standard colorimeter methods (APHA 1992).
Total Phosphorus - Phosphorus is a limiting nutrient in many fresh waters. Total phosphorus can be analyzed using the automated procedure outlined in USEPA Method 365.1.
Trophic state determinations provide a method for determining whether increased nutrients or sediments (loading) are causing changes in a lake. Carlson’s TSI uses Secchi depth, chlorophyll a, and total phosphorus, each producing an independent measure of trophic state (Carlson 1977). Index values range from approximately 0 (ultraoligotrophic) to 100 (hypereutrophic). The index is scaled so that TSI = 0 represents a Secchi transparency of 64 m. Each halving of transparency represents an increase of 10 TSI units. For example, TSI of 50 represents a transparency of 2m, the approximate division between oligotrophic and eutrophic lakes (USEPA 1990b). A TSI is calculated from each of Secchi depth (SD), chlorophyll concentration (Chl), and total phosphorus concentration (TP) (Carlson 1977, Carlson and Simpson 1996).
TSI(Chl) = 30.6 + 9.8 In(Chl)
TSI(TP) = 4.15 + 14.42l In(TP)
TSI(SD) = 60-14.4l In(SD)
Trophic state indices are used to infer trophic state of a lake and whether algal growth is nutrient limited or light limited. If the three indices are approximately equal, then phosphorus limits algal growth. If the three are not equal, then other interpretations exist (Carlson and Simpson 1996). A trophic state index has also been developed for total nitrogen (TN) (Kratzer and Brezonik 1981, Carlson 1992):
TSI(TN) = 54.45 + 14.43 In(TN)
For a more complete discussion of trophic state indices and their interpretation, see Carlson (1992) and Carlson and Simpson (1996).
The Tier 1 macrophyte survey is a visual estimate of percent cover of submerged and floating macrophytes in shallow water, and identification of the most dominant species and weedy or exotic species. The survey can be done with aerial photographs (if available); a visual whole-lake survey in small lakes, or examination of transects in large lakes. Three to ten transects should be sufficient for most lakes or embayments too large to survey in their entirety. Large lakes with known differences within the lake should be sampled by lake zone; for example, the shallow riverine zones of a reservoir may have greater macrophyte cover than the lacustrine zone.
To avoid bias, transects should be selected randomly within each lake zone. A method of selecting transects is to divide the shore into equal segments (corresponding to the number of transects). A point is selected randomly in each segment as the starting point for transects. Transects are perpendicular to shore to deeper water.
Total vegetative cover is estimated visually. The presence of algae mats and epiphytes should be noted. Cover might be difficult to estimate in turbid waters. Vegetation samples may be collected with a rake and total abundance estimated from the material raked in (ordinal scale: sparse, moderate, abundant). The most dominant species, and any weedy or exotic species, are identified.
Tier 2A sampling requires two or more lake biotic assemblages: macrophytes, sedimented diatoms, fish, or macrobenthos (Table 7-4). Tier 1 variables, including DO, chlorophyll a, and Secchi depth, are also critic components of the Tier 2A survey. Tier 2A may be built on either Tier 1A or 1B. Macrophytes are the easiest of these assemblages to identify and count in the field (using wet weight instead of relative abundance). Sedimented diatoms are also relatively easy to sample, although identification and enumeration must be done in the laboratory. The choice of which plant assemblage to sample clearly depends on the importance of the assemblage in lakes of the region - diatoms would be the choice in regions where macrophytes are minor components of the lake system.
The habitat components of the Tier 2A survey build on the desktop screening and Tier 1 habitat assessment and also include a semi-quantitative shore zone habitat evaluation (Table 5-3). Tier 2A requires estimates of shorezone land use, riparian vegetation, emergent macrophyte extent and cover, and floating macrophyte extent and cover at several transects from the shore.
The Tier 2A faunal component consists of the benthic macroinvertebrates. Macroinvertebrates are sampled from the sublittoral zone, below the floating macrophyte zone, yet above the thermocline to avoid sampling predominantly anoxic areas. Tier 2A sampling typically consists of a single visit during an index period. Benthic macroinvertebrates may optionally be sampled more frequently to obtain growing season averages. Macrophytes are best sampled mid- to late in the growing season when plant biomass is near its annual maximum. Sedimented diatoms, which represent sedimentation of at least a year or more, may be sampled at any time.
Tier 2A allows more precise detection and identification of problems and potential causes than Tier 1, as well as detection of biological effects on the biotic assemblages selected for assessment.
Tier 2A sampling of macrophytes and benthic macroinvertebrates, and the shorezone habitat are surveyed along 3 to 10 transects perpendicular to the shoreline (Figure 7-4). Transects are the same as the Tier 1 macrophyte transects: the lake (or lake zone) shoreline is divided into equal length segments corresponding to the number of transects, and a transect start point is randomly selected in each segment.
Each transect is extended visually on the lake shorezone, and the condition of the shorezone is determined. Shorezone measurements include riparian vegetation cover estimates, lake bank substrate and erosion, and human modifications. Figure 7-5 is an example scoring sheet for habitat measurements showing how the variables are scored.
|
Figure 7-5. Example scoring sheet for shorezone habitat. |
Tier 2A macrophyte sampling is more systematic and detailed than Tier 1. The objective is to obtain relative abundances of macrophyte taxa to develop assemblage measurements. Relative abundance can be estimated by stem counts (number of stems of each species) or biomass. Biomass is preferred because a stem does not correspond to an individual plant, and biomass is a good indicator of species dominance in the habitat. An alternative to relative abundance is scoring presence and absence of species in quadrat.
One to four macrophyte sampling locations are established on each transect within depth zones between shorezone emergent and the unvegetated, sublittoral bottom. For example, location may be identified in 0-1m depth, 1-2m, 2-3m, and 3-4m depth (Weber et al. 1995).
Stem counts - May be done with the transect method, by counting stems touching a line held on the transect. Stems may also be counted in quadrants, where all stems within a 1/4 m2 quadrat are counted and identified. Stem counts may require diving in water deeper than 1 m. One or more sampling stations (for quadrat sampling) are selected on each transect between the emergent macrophyte zone and the deepest extent of submerged macrophytes.
Biomass Sampling - The easiest method to estimate macrophyte biomass is with an aquatic weed rake (Table 7-5). At each station on the transect, an aquatic weed rake or thatching rake is dragged a set distance (e.g., 1m) to sample vegetation (Trebitz et al. 1993). Plants from all stations on the lake are identified and sorted by species, and the total of each species collected is weighed (wet weight) to obtain estimates of biomass and proportion of biomass of each species. Algae mats and epiphytic growth on leaves and stems are described. Voucher specimens of each species should be kept for complete identification and for permanent record. Depth is sounded at the lakeward edge of submerged vegetation.
|
Table 7-5. Sampling summary for submerged macrophytes.
|
Aquatic weed rakes are biased against macrophytes that can slip through the tines of the rake. Therefore, a more accurate estimate of biomass would be to clip all plants in the quadrat for wet weight determination. Clip plots would require diving or snorkeling in water more than 1m deep. Biomass can be estimated more accurately by drying the sorted plant material for dry weight determination, at the cost of additional processing.
The weed rake and wet weight determination is likely to be the most cost-effective methodology for most purposes. Although it undersamples certain species, it is likely to be consistent enough to use for biological surveys, as long as the same sampling methodology is used in all lakes.
Presence-Absence - Instead of estimating biomass, species can be scored for presence or absence within quadrants (Weber et al. 1995). Each sampling location along a transect is divided into four quadrants. Each quadrat is sampled with the rake, and each species receives one point for every quadrat in which it occurs.
The macroinvertebrate assemblage beyond the macrophyte zone is sampled with gear appropriate to the bottom type and depth (e.g., Ponar, Ekman grab sampler, dome sampler); and the assemblage is identified and characterized (Table 7-6).
|
Table 7-6. Sampling summary for benthic macroinvertebrates.
|
Sampling Period - Two sampling periods have been identified either the most stressful period (usually late summer) or a period after recruitment (usually early spring) but before major emergence of adult insects.
Sampling Location - Along transects, the sublittoral habitat is recommended as the most appropriate habitat for sampling due to its relatively stable nature.
Sampling Gear - The type of gear will depend on the substrate being sampled (Table 7-7). A standard mesh size of 595 mm (No. 30 mesh) is required.
|
Table 7-7. Benthic macroinvertebrate sampling gear appropriate for major substrate types.
|
Sample Replication - To characterize the macroinvertebrate assemblage, multiple grabs are taken from several sites. Each transect ends in a macroinvertebrate sample site, and two to three grabs are taken at each site. Grabs may be composited into a single composite sample.
Sample Processing - To process the sample, organisms are removed from sticks, rocks, and similar size objects. The remainder of the sample is placed in a tub and mixed into a fine, uniform slurry. After mixing, the slurry is sieved using a U.S. No. 30 sieve (595 um) to remove organic and mineral material. The benthic organisms are retained by the sieve, which can be emptied into a light-color, gridded sorting tray. Grid cells are selected at random and sorted until at least a 100-organism subsample has been counted and identified to the appropriate taxonomic level. The last grid cell is sorted completely until all organisms from the grid are identified to the lowest practical level. Further description of sorting is presented in EPA/440/4-89-001 (USEPA 1989b).
Fish assemblages can be sampled by electrofishing in and/or beyond the macrophyte zone. Sampling effort for fish should be kept constant between transects. Electrofishing is generally the single most cost-effective sampling method for fish (Scott et al. 1992) but it is not effective in deep water. If deep water fish are an endpoint of concern, then gill nets, fish traps, or trawls can be used. A combination of nets and electrofishing often provide a more representative sample of the fish assemblage; however, multiple methods translate to a substantial cost for field effort. A variety of nets may be used to sample littoral and sublittoral areas. Fish sampling methodologies are further outlined in EPA 600/R-92/111 (USEPA 1992b) and Table 7-8.
|
Table 7-8. Sampling summary for fish assemblage.
|
|
Case Study: Florida sampling methods In 1995, FDEP adopted a new sampling protocol to obtain more representative samples of each lake, in part based on results from the earlier samples. Lakes greater than 1000 acres are divided into two or more basins, usually by separating at constriction points or between bathymetrically identifiable basins (Fig. 7-6). The 2-4m sublittoral zone of each lake basin is divided into 12 equal segments, and a grab is taken in each segment with a Petite Ponar or Ekman sampler (0.02m2) (Fig. 7-6). Positions of segments and sampling sites are estimated by eye in the field. The 12 grabs are combined into a single composite sample, which is randomly subsampled to a count of 100 organisms, identified to the lowest practical taxonomic level. Basins (in lakes greater than 1000 acres) are retained as separate sample units. Lakes smaller than 1000 acres are represented by a single 100-organism sample. A second grab sample is taken at each of the 12 stations for sediments, which are likewise combined into a single representative sample.
Figure 7-6. Florida lakes sampling scheme. (The lake is divided into 12 approximately equal segments. A Ponar grab is taken from each segment, at a random location in the 2 to 4 meter depth zone. Water chemistry, chlorophyll, and Secchi depth are measured from the center of the lake.) In fixed organism subsampling, a targeted number of organisms (typically 100 to 500) is identified. If fixed organism subsampling for benthos is conducted in an unbiased manner using a random selection method, the resulting information on richness and relative abundance is comparable among samples. For benthic samples, the targeted number is reached by randomly choosing several fractions or “grids” from a pan; all organisms enclosed within the grids are sorted to avoid bias toward large and easily seen individuals. Ideally, several (four or more) grids are sorted to ensure proper representation. Surface and bottom water chemistry samples, and phytoplankton samples, are taken near the center of each lake. Observations included field measurements and laboratory analyses, and identification of phytoplankton to genus. |
|
Case Study: TVA Benthic Macroinvertebrates Collection Methods Benthic macroinvertebrate assemblage samples were collected in the spring (March and April) at 69 locations on 30 TVA reservoirs. Sample locations were selected in each of the forebay, mid-reservoir, and inflow areas, corresponding to lacustrine, transitional, and marine conditions, respectively (Figure 7-3). At each sample location, a line-of-sight transect was established across the width of the reservoir, and one Ponar grab sample collected at 10 equally spaced locations along this transect. When rocky substrates were encountered, a Peterson dredge was used. Care was taken to collect samples only from the permanently wetted bottom portion of the reservoir (i.e., below the elevation of the minimum winter pool level). Samples were washed in the field, transferred to a labeled collection jar, and fixed with 10 percent buffered formalin solution. Samples were sorted and identified in the field, to the lowest practical taxon, typically genus, and reported as number per square meter. To assess the reproducibility of benthic macroin-vertebrate sampling results, replicate samples were collected at 13 of the 69 sampling locations in 1994, with all types of reservoir locations (i.e., forebay, transition zone, embayment and inflow) included. At each of the replicate sampling locations, the sampling protocol involved collection of a first set of 10 samples, leaving the sampling location, and then returning as near as possible to the original transect site (on the same day) and repeating the collection of a second (replicate) set of 10 samples. Results from sets of replicate samples were evaluated for reproducibility. |
Electrofishing - Multiple habitats are selected in littoral areas for electrofishing. Habitat distinctions are based on substrate (e.g., rocks, sand, clay) and on available cover (e.g., vegetation, woody debris).
Nets - A variety of nets are used to sample littoral and sublittoral areas. It is recommended that trapping nets (gill nets, trammel nets, fyke nets, trapnets) be set for 2 to 5 days with collection once or twice a day.
Sample Processing - Fish samples are processed as recommended in the RBP manual EPA 440/4-89-001 (USEPA 1989b). Sampling duration and area or distance sampled are recorded in order to determine level of effort. Specimens are identified to species, then total numbers and weights, and the incidence of external anomalies is recorded for each. Voucher specimens of each species from each site are preserved in a 10 percent formalin solution, in a labeled jar. The voucher collections are placed with the state ichthyological museum to confirm identifications and to constitute a biological record. This is especially important for uncommon species, for species requiring verification of identification, and for documentation of new distribution records. If kept in a live well, most fish can be identified and counted in the field by trained personnel and returned to the lake alive. Additional information on field methods is presented in Karr et al. (1986) and EPA 600/R-92/111 (1992b).
|
Case Study TVA Fish Collection Methods Shoreline electrofishing samples were collected during daylight hours from inflow, transition, and five forebay zones of most reservoirs from September to mid-November (Figure 7-3). Only one or two zones were sampled on reservoirs where zones were indistinguishable. No inflow zones were sampled in tributary reservoirs. A total of 15 electrofishing transects, each covering 300m of shoreline, was collected from each of the sampled zones. All habitats were sampled in proportion to their occurrence in the zone. Where conditions permitted experimental gill nets were set overnight in each reservoir zone. Excessive current prevented use of gill nets in mainstream inflow areas. In forebay and transition zones, nets were set in all habitat types, alternating mesh sizes toward the shoreline between sets. Total length (mm) and weight (g) were obtained for all sport species and channel catfish. Remaining species captured were enumerated prior to release. During electrofishing, fish observed but not captured were included if positive identification could be made and counts were estimated when high densities of identifiable fish were encountered. Young-of-year fish were counted separately and, as in stream IBI calculations (Karr 1981), were excluded from proportional and abundance metrics (due to sampling inefficiencies for the age group). Only fish examined closely to obtain length and weight measurements were inspected externally for signs of disease, parasites, and anomalies. |
Diatom frustules are preserved in lake sediments that are not disturbed or resuspended. Field sampling for sediment diatoms can be relatively fast. Field methods outlined below and in Table 7-9 are similar to those used in EMAP (USEPA 1994b). Sample Location - Sediment samples are obtained from or near the deepest area of the lake. A single core sample is sufficient (Charles et al. 1994). Sampling and Analysis - Sediment diatoms can be sampled with a corer that is able to reliably sample and retain the top 1 cm of sediment. The top 1 cm of sediment is carefully removed from the sampler and kept at 4°C in a plastic bag. Diatom samples are prepared, enumerated, and identified following the procedure from the EMAP manual (USEPA 1994b).
|
Table 7-9. Sampling summary for sedimented diatoms.
|
Tier 2B consists of phytoplankton and zooplankton sampling in addition to Tier 1B sampling, and is conducted at the same sites and times as Tier 1B (Table 7-10). Sampling frequency may range from three samples during the growing season to monthly samples, depending on the objectives of the program. The number of sampling sites is the same as Tier 1B, and samples may be composited among the sample sites to economize laboratory effort, if within-lake spatial variability is not an issue.
|
Table 7-10. Tier 2B: Water column biological sampling.
|
Phytoplankton are subsampled from the same water sample collected for chlorophyll and nutrients in the Tier 1 sampling protocol. The water sample may be a surface sample or an epilimnion or photic zone hose sample. The large sample is mixed thoroughly before subsamples are taken from it.
A sample of 150 to 500ml is sufficient for phytoplankton. The phytoplankton sample is preserved in the field with Lugol’s solution (APHA 1992). Cells are identified and counted using the Utermohl method on an inverted microscope, or by filtration onto a membrane filter (APHA 1992). The Utermohl method requires settling chambers and an inverted microscope, and the filter method requires a compound microscope and filtering apparatus.
Sampling Procedure - Zooplankton are sampled with a vertical tow at the same sites as phytoplankton, trophic state, and water quality (Table 7-11). Nets of 118mm mesh and 30 cm diameter will sample most crustacean zooplankton. The net should be equipped with a cone to prevent spill and escape of active organisms. Zooplankton are anesthetized with carbonated water, and preserved in 4 percent formaldehyde. After fixing, long-term storage should be in 70 percent ethanol.
|
Table 7-11. Sampling summary for crustacean zooplankton.
|
Analysis - The sample is split until 100 to 200 organisms remain in the subsample. Zooplankton are identified to genus; equipment includes dissecting microscope and keys. Lengths of Daphnia are recorded.
Periphyton should be sampled several times during the growing season: certain species might be dominant depending on the time of year. Field methods are outlined below and summarized in Table 7-12 (after Bahls 1993).
|
Table 7-12. Sampling summary for periphytic diatoms.
|
Sampling Location - A minimum of two random sampling points along each transect is suggested; a determination of greater sampling effort should be based on lake size and professional judgment.
Sample Collection - Collection can be from natural or artificial substrates depending on the preference of the investigation team or agencies. Natural substrates include rocks, logs, macrophytes, and mud. A composite sample of three to five substrates (e.g., fist-sized rocks) is obtained from each sample site. The area scraped from each substrate should be approximately equal. Use a pocket-knife or similar tool for scraping solid substrates. A spoon or large-bore eyedropper can be used for lifting microalgae from mud or silt substrates. Macroalgae can be picked by hand. Epiphytic algae can be dislodged from macroalgae, moss, and aquatic macrophytes by placing a portion of the higher plant in the sample container and shaking vigorously. The moss or macrophyte is then removed and discarded (Bahls 1993).
Sample Preservation - Preserve samples in watertight, unbreakable jars. Water is added from the sample site to cover the sample; then enough Lugol’s solution is added to impart a reddish-brown tint. Artificial substrates can be preserved intact in a suitable container or scraped in the field.
Sample Preparation - Extracellular organic matter is decomposed by oxidation, leaving only the diatom shells (frustules) as described in APHA (1992). Using the cleaned diatoms (frustules), a permanent mount is prepared and a proportional count is made of 300 to 500 cells (APHA 1992). Counts for each species are divided by the total count and multiplied by 100 to obtain percent relative abundance (PRA).
More detailed habitat procedures allow monitoring agencies to focus on specific water and sediment quality problems in a lake, and specific land use practices in the watershed, for identification of probable cause of impairment (Table 7-13). Supplemental habitat components may include: a detailed watershed assessment (soils and geology, detailed land use, agricultural practices); a stream assessment for migratory fish habitat; additional water quality analysis (nutrients, contaminants); and sediment quality (sediment grain size, sediment organic carbon, contaminants, toxicity).
|
Table 7-13. Supplemental components.
|
Tiers 2A and 2B will allow detection of effects of toxic substances on the respective biological assemblages, but will not provide positive identification of toxicity as a probable cause of impairment. Positive identification of contamination and toxicity as a probable cause will require the supplemental survey, particularly habitat contaminant analysis and toxicity assays. The detailed land use measurements in the habitat assessment allow identification of more specific nonpoint source probable causes of impairment. The tiers allow detection of biological effects on at least two assemblages, and hence detection of effects at multiple levels (including cascades of effects).
The diagnostic habitat survey is similar to the Tier 1 and Tier 2 habitat survey but evaluates more detailed components. In searching for probable causes of impairment, land use is broken down into more detailed land use categories, including high- and low-density residential, industrial and commercial transportation, cropland, pasture, orchard, mines, etc. If agriculture is thought to contribute to impairment, then the dominant agricultural practices should be documented, as well as their distribution in the watershed. If the fish assemblage shows impairment (particularly migratory fish), then fish spawning habitat in inflowing streams can be evaluated.
The Sediment Classification Methods Compendium (USEPA 1992f) discusses various aspects of sediment analyses including sample collection and handling, quality assurance/quality control issues, and toxicity testing. In addition, this guide furnishes references for specific methods.
There are three main types of devices used to collect sediment samples. The choice of sampler to be used for a particular study depends upon the nature of the sample needed. Grab samplers and core samplers can be used in toxicity testing and in evaluating chemical and physical properties of the sediment. Additionally, cores can be used in evaluating historical sediment records.
Equipment should be thoroughly cleaned between samples to prevent cross contamination. In some cases, preservation methods such as pH control or addition of chemical preservatives will need to be done. Standard methods for sample handling can be found in ASTM (1990).
Sediment particle size is measured using stacks of different sized sieves. The sediment to be analyzed is first heated to dryness. Samples may need to be stored cold, frozen, or preserved. Then a known weight of dried sediment is poured into a stack of sieves of different sizes to separate the particles. Each size fraction is then weighed and expressed as a percentage of the total dry sample weight.
Chemical analyses that can be measured include metals, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pesticides, and volatile and semivolatile organic pollutants. Metals are typically measured using atomic absorption spectrophotometry. Other constituents should be analyzed using USEPA approved methods (USEPA l991f, ASTM 1990). Although it is not a contaminant, total organic content (TOC) should also be analyzed since it is an important indicator of the bioavailability of nonionic hydrophobic organic pollutants. Likewise, acid volatile sulfides (AVS) are important in determining the bioavailability of metals.
Several approaches are recognized by USEPA for evaluating sediment toxicity. These approaches may be used separately or in combination to provide evidence of toxicity and to generate sediment quality criteria. (USEPA 1994).
Whole (bulk) sediment toxicity testing is a method of evaluating the level of toxicity of a sediment sample. Typically, test organisms are exposed to sediment for 10 to 14 days. Endpoints used are growth and survival. The most often used organisms in freshwater sediment toxicity tests are the amphipod Hyalella azteca and larvae of the midge Chironomus tentans. Other organisms that have been tested include other benthic infauna such as the mayfly Hexagenia spp; and the worms Tubifex tubifex and Lumbriculus variegatus; and two cladocerans, Daphnia magna and Ceriodaphnia dubia. Results of exposure to contaminated sediments is compared with control (uncontaminated) sediments (USEPA 1994j, ASTM 1998, PSEP 1995, Environment Canada 1994).
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
|
|
|
|
||
|
|
