 |
||
|
Monitoring and Assessing Water Quality |  |
 |
||
|
|
|
|
|||||||||
|
|
Chapter 6
|
|
Table 6-1. Potential algal trophic state metrics.
|
Trophic state assessment is relatively inexpensive. Sample collection requires approximately 10 minutes on station and can be done by a single person. Filtration of chlorophyll samples requires another 10 to 15 minutes in the field. Chlorophyll, nutrients, and other water quality chemical analyses are standard and costs are well established in each region.
Macrophytes form an integral part of the littoral zone of many lakes, providing cover for fish and substrate for invertebrates. From a human perspective, overabundant macrophytes (or weedy conditions) can interfere with lake access by fouling equipment, interfering with recreational activities, and detracting from aesthetic appeal. A conspicuous lack of native macrophytes in habitats where they are expected to occur can result in reduced population of sport and forage fish and waterfowl (Crowder and Painter 1991). Potential macrophyte metrics are listed in Table 6-2.
|
Table 6-2. Potential macrophyte metrics.
|
Submerged macrophyte analysis, including an estimate of total percent cover and identification of dominant species, requires approximately 1 to 2 hours in the field for a 300- to 500-acre lake. There is no laboratory analysis.
For the same size lake, macrophyte density or biomass measurements would require 2 to 4 hours in the field to collect samples, and to sort and weigh by species. Stem counts would likely require a longer time. Again, there is no laboratory analysis.
Phytoplankton cells continually grow and die, and dead cells sink to the bottom. One group of algae, the diatoms, have shells (called frustules) made of silica (glass), which are preserved when the dead cells fall to the lake bottom. The preserved diatoms provide an integrated record of the diatom assemblage in the lake. A sample of the top 1 to 2cm of lake sediment contains a representative sample of diatoms from the most recent 1 to 3 years. Sedimented diatoms can be sampled at any time and will always yield a sample representative of the most recent years. Potential sedimented diatom metrics are listed in Table 6-3.
|
Table 6-3. Potential sediment diatom metrics.
|
Sedimented diatoms are sampled rapidly in the field. A sample requires approximately 1 hour to prepare and 2 to 4 hours to count and identify 300 to 500 cells.
Benthic macroinvertebrates are long-term indicators of environmental quality; they integrate water, sediment, and habitat qualities (USEPA 1989b, USEPA 1990d). Macroinverte-brate species have sensitive life stages that respond to stress and integrate effects of both short-term and long-term environmental stressors. Classification of benthos according to their relative sensitivity to pollution and their functional feeding group level differentiates effects on ecological health in response to organic or toxic perturbations. Potential metrics are listed in Table 6-4.
|
Table 6-4. Potential benthic macroinvertebrates metrics.
|
Macroinvertebrates are sampled from the predominant substrate available in the sublittoral zone. The type of sampling gear will depend on the substrate being sampled: each substrate has its own optimal sampling gear (Chapter 7).
A benthic sample, consisting of several grabs, requires 2 to 4 hours in the field. Sorting, counting, and identifying 100 organisms to species requires approximately 4 to 6 hours in the laboratory.
Fish assemblages include species that represent a variety of trophic levels (omnivores, herbivores, insectivores, planktivores, piscivores), and that exhibit a range of tolerance to water quality or habitat degradation. Fish are long-lived and integrate short-term temporal environmental changes, and also integrate effects of lower trophic levels (e.g., primary producers and benthic macroinvertebrates); thus, fish assemblage structure is reflective of integrated environmental health. Of all biological components of lakes, fish probably receive the greatest public attention because of sport and commercial fishing and attendant concerns regarding fish production success and safety for human consumption.
Fish are the most difficult and time consuming of all assemblages to sample; are wide-ranging and might not reflect local conditions in large lakes; and are actively and intensively managed by stocking and angling. Each feasible gear type suitable for their sampling in lakes is highly selective (USEPA 1994a, USEPA 1994b). Unbiased sampling methods such as explosives, rotenone, and draining a lake are generally too destructive. Because of lake fish assemblage sampling method bias, the use of a combination of more than one gear type is recommended (Chapter 7). Potential fish metrics are provided in Table 6-5. Among the most promising measurements are indicators of fish health (external gross pathology) and fish tissue contamination.
|
Table 6-5. Potential fish metrics.
|
Fish populations are generally nonrandomly distributed and clumped in response to habitat variables; therefore, the choice of sampling methods and equipment, index period, and sampling frequency depend upon waterbody physical characteristics and specific study objectives. An understanding of the attributes and/or biases of sampling equipment and methods used in fish assemblage surveys is essential in order to draw valid conclusions from the data. The relative labor intensity of fish sampling techniques varies greatly, depending on the specific method chosen and the abundance and diversity of the catch. For example, passive techniques (e.g., trap nets, gill nets, etc.) generally require a deployment and capture cycle of many hours (e.g., overnight sets) to several days; and the processing of catches from either passive or active sampling techniques may require several hours depending on local abundances and method efficiencies.
Phytoplankton assemblage data, consisting of taxonomic identifications and abundances (relative or absolute) can be analyzed in two ways: by determining assemblage measurements based on species structure or by performing multivariate assemblage analysis. Potential phytoplankton metrics are listed in Table 6-6.
|
Table 6-6. Potential phytoplankton metrics.
|
The recommended approach is to sample the phytoplankton assemblage and to count and identify cells to order or genus. Simplified field and laboratory procedures are possible for measurements based on higher taxonomic levels such as division or order. Identification to species is considered supplemental at this time because it is not clear that the information gained represents a substantial improvement over higher levels of taxonomy.
The phytoplankton assemblage requires 4 to 10 samples during the growing season to obtain a seasonal average of the phytoplankton assemblage. The exact number will need to be determined from preliminary or existing data sets.
An integrated water column sample of phytoplankton requires approximately 10 minutes to collect. Laboratory identification and counting of 300 to 500 cells requires 1 to 4 hours in the laboratory.
In most lakes, zooplankton are the central trophic link between primary producers and fish. Zooplankton are ubiquitous in all lakes and are quickly and easily sampled in the field. Zooplankton species richness is reduced under chemical stresses (Baker and Christensen 1991), and abundant large Daphnia are associated with clear lakes with healthy sport fish populations (Mazumder 1994). Trophic structure measurements require knowledge of feeding of zooplankton species—trophic links and complexity measures require the most detailed knowledge. Potential zooplankton metrics are shown in Table 6-7.
|
Table 6-7. Potential zooplankton metrics.
|
Zooplankton are sampled with vertical or oblique tows, using a plankton net equipped with a 7:1 reducing cone (DeBernardi 1984). The recommended approach is to sample 4 to 6 times during a growing season to obtain seasonal averages.
A zooplankton sample can be collected in approximately 10 to 30 minutes in the field. Identification and counting of 100 to 200 organisms requires approximately 1 to 2 hours. Six samples in a growing season per lake thus requires six trips and 6 to 12 laboratory hours.
Periphyton, the algae growing on solid substrates (rock, wood, sediment, macrophytes), have a long history of use in bioassessment of streams (Patrick 1949). Diatoms are often the group of choice among periphytic algae. Ecology of periphyton is much like other algal assemblages: they respond to nutrient enrichment; they are cropped by grazers; and their species composition is affected by pH, metal concentrations, trace elements, and contaminants. In addition, periphyton are affected by the physical and chemical characteristics of their substrate. Like phytoplankton, periphyton are subject to changing water chemistry and seasonal succession. Several sampling periods may be necessary to characterize lake periphyton.
Whereas periphyton have been used successfully in streams (Bahls 1993, Patrick 1949), their application as lake indicators is relatively new. Measurements of periphytic diatoms have shown promise for bioassessment, based on investigation of undisturbed reference lakes in Montana (Gerritsen and Bowman 1994), but actual responses to disturbance or pollution are as yet unknown.
Analysis of a periphyton sample requires 2 to 6 hours, similar to diatoms and phytoplankton.
|
Case Study: Florida Metric Selection and Index Development Of 32 potential macroinvertebrate metrics examined, 9 were selected as candidate metrics for an invertebrate index for Florida lakes. Responsive metrics are shown in Figure 6-1. Most metrics have different values among the three lake types, with sandy-bottom lakes having the greatest macroinvertebrate number of taxa (and other related metrics), the greatest proportion of OET (Odonata, Ephemeroptera, Trichoptera) organisms, and the greatest proportions of filter feeders and surface gatherers (Fig. 6-1).
Figure 6-1. Metrics related to algal production, Florida lakes (from Florida DEP 1994). a: algal density (cells/ml) b: chlorophyll a c: chlorophyll TSI Several metrics were correlated with each other. The Shannon index was strongly correlated with total taxa and with dominance. Graphic examination of the relationships among the metrics showed that the Shannon-total taxa relationship was not entirely linear, and the Shannon-dominance relationship had large and asymmetric variance in the middle of the range (Fig. 6-2). Because of the variance and non-linearity of the relationships, all of the candidate metrics were retained for inclusion in a potential index.
Figure 6-2. Relationship of two highly correlated attributes, Shannon diversity and percent dominance (from Gerritsen and White 1997). Reference and test lakes also differed in water column measurements (Fig. 6-3). Test lakes as a group had higher chlorophyll concentrations and reduced Secchi transparency, and higher total phosphorus than the corresponding reference lakes, showing increased trophic state in the test lakes. Total Kjeldahl nitrogen was higher in sandy and mud-muck test lakes, and algal growth potential was increased in the mud-muck lakes.
Figure 6-3. Distribution of nutrients, secchi depth, and chlorophyll a in Florida reference and test labs. Eight invertebrate metrics appear responsive to lake stressors, and can be used in a lake invertebrate index (number of taxa, Shannon-Wiener diversity, Hulbert index, OET taxa, percent dominance, percent filterers, percent OET, and percent gatherers). Two trophic state indicators (Secchi depth and chlorophyll a concentration) also had characteristic values under reference conditions, and are best used in conjunction with an invertebrate index to determine status of a lake. |
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
|
|
|
|
||
|
|
