[225] FRAMEWORK FOR ENHANCING THE STATISTICAL DESIGN OF AQUATIC ENVIRONMENTAL STUDIES* F E R N A N D O C A M A C H O and G I A N L. V A S C O T T O Ontario Hydro Research Division, 800 Kipling Ave. Toronto, Ontario M8Z 5S4
(Received April 1990) Abstract. Aquatic environmental studies can be categorized by the breadth of their scope and the types of desired results. The use of this categorization coupled with a clear specification of objectives and a judicious knowledge of the environmental variability should lead to more statistically efficient studies. This paper discusses the types of lacustrine studies commonly encountered in terms of their categorization. It provides examples of how the intrinsic environmental variability can influence their design and stresses the importance of properly stated objectives, the developing of testable hypotheses, the design of robust and powerful studies, and the importance of evaluating the implication of changes as critical factors for conducting effective and efficient environmental studies.
1. Introduction
All of man's activities on this planet, including this very existence, are likely to leave a mark on the environment. This mark or change is often referred to as man's environmental impact. In our era of global overcrowding and massive manipulation of natural resources, a noticeable degradation of environmental quality has taken place which has raised concerns about the future of the planet. Society is realizing that all this change has a cost, and now it must decide what type of costs (ie., environmental changes) are acceptable. To assess this damage, some times a priori, but most often a posteriorL environmental studies are carried out. These studies are referred to as impact studies or environmental assessment studies. The intent of the environmental studies has to be applauded. However, their value may be limited since often they do not meet expectations, partly because of inadequate design, partly because of the inappropriateness of analytical techniques to deal with data which are highly variable and do not conform to main statistical assumptions, and partly because of erroneous ecological assumptions. Several authors have recognized the need for improving the quality of these studies, and have suggested several frameworks for this purpose (see Beanlands and Duinker, 1983, 1984; Rosenberg et al., 1981). These frameworks tend to stress the ecological considerations rather than the statistical aspects. The objective of this paper is to extend the concept of design stressing the importance of * An early version of this paper was presented at the First International Conference on Environmentrics, Cairo, Egypt, April 4-7, 1989, under the title Framework for the Design of Aquatic Environmental Studies.
Environmental Monitoring and Assessment 17: 303-314, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.
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good statistical design in all aquatic environmental studies. This will be done with the help of a conceptualized framework that can be used for simplifying the problems being addressed and prioritizing their components to maximize efficiency. The environmental studies can be broken down into categories and their components so that appropriate statistical considerations can be applied to each of them. The statistical considerations include the recognition of the existence of confounding factors, the need for complex designs, the need for quantitatively defining the desired results, and the need of adequate sample sizes to achieve the desired results. To reduce the scope of the paper to a manageable size, and because much of our group's experience has been with the Great Lakes, it was decided to discuss only the aquatic environmental studies of lacustrine systems. Therefore, many other types of important studies such as river, air, and land pollution, are not considered here. The paper is organized as follows. Section 2 classifies the studies according to the breadth of scope and the type of desired results. Section 3 discusses elements that are common among the types of studies. Section 4 discusses diverse elements found among the studies with the help of an impact assessment study example. Finally, Section 5 presents some conclusions and areas that require further consideration. 2. Classification of the Studies
Environmental aquatic investigations may be classified according to the spatial scope and the type of desired results. This classification is important since it will dictate considerations for the planning, design, and implementation of the studies. In particular, it will help in determining the amount of resources required and provide an indication of the variables that should be measured. 2.1. CLASSIFICATIONBY BREADTHOF SCOPE Based on the spatial scope, the studies can be classified into three broad categories: large scale, partial, and local studies. Large Scale or Whole Lake Studies
These studies are implemented over a large geographical area, usually covering a complete lake. Because of the extent of the sample area, sophisticated equipment may be required to handle the samples. These studies require intensive sampling schemes both in time and resources. To optimize the sampling effort, prior information of the spatial variability of the lake should be used to divide the lake into zones of homogeneous characteristics. It is advised to allocate the sampling effort in direct proportion to the variability of the zones. Data collected in these studies are highly variable, both spatially and temporally. The degree of variability is likely to change with the type of lake and the zones found in it. This variability is increased during periods of unstable weather. Because of the distances covered in the larger lakes, the time spent traveling between sampling locations may have a large effect in the observed variability between samples (particularly for some nutrients, planktonic organisms, and fish) than the actual spatial variability (Esterby, 1986). Therefore, the relative merits of intensive versus extensive sampling should be carefully
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evaluated. In some cases it may be desirable to stratify the lake into zones, allocate the sampling effort as above, and sample each zone independently of the others over as brief a period of time as possible. In general, sophisticated statistical techniques are required to analyze the data collected in large surveys. These techniques should be able to handle multiple variables, temporal and spatial dependency, non-normality of the distribution, and probably unequal time intervals in the sampling period. Partial or Basin Studies
These studies are carried out over a portion of the lake that can be categorized by certain uniform characteristics. Although it may cover a large geographical area, it is usually a small proportion of the total size of the lake. Similar difficulties to those encountered on large scale studies can be found in these studies. However, the efforts required may not be as intensive. Local Studies
These studies concentrate on a small geographical area and are usually associated with impact assessment studies. The samples are taken from near shore sites. Thus, the data are highly variable and may be affected by many other variables such as storms, winds, temperature, etc. In addition, the whole lake may be undergoing changes whose effects must be removed from the area in question. To account for part of the variability, it may be necessary to sample intensively a n d / o r measure covariates that can be used in the analysis. The statistical methods required for these studies depend on the particular application, although tests of homogeneity, particularly ANOVA, are usually employed. 2.2.
C L A S S I F I C A T I O N BY D E S I R E D R E S U L T S
On the basis of the desired results, each of the above classes can be further categorized into four groups: surveys, monitoring programs, assessment studies, and general purpose studies. Surveys
These are, in general, the first type of investigations made on a lake and are used for determining general spatial and temporal characteristics of physical, chemical, and biological variables. Later, the information may be used for making decisions about the environment or for setting baseline limits. Some of the particular applications include the following: (i) The gathering of information on unknown environments. (ii) The determination of the range of conditions. (iii) The gathering of information to identify areas that meet certain conditions. (iv) The determination of the most desirable site for a new industrial development. The duration of the study varies depending on the extent of the survey, but it is desirable that it extends for at least a year to capture annual cycles.
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Monitoring Programs These are generally used for two main purposes: (i) maintain historical records which may be used to identify long-term trends in a variety of parameters; and (ii) ensure that certain environmental criteria are met. In the first case, the monitoring program is basically a reduced survey carried out in a repetitive fashion; nevertheless, it is important to ensure good quality on the sampling programs (Kwiatkowski, 1986). In the second case, the objective of the monitoring program is to gather information to detect a (possible) change with respect to a specified base line. To meet this objective, two important requirements are necessary: (i) a clear understanding of the base line that is considered normal or desirable (this base line should be stable, useful, and real); and (ii) a clear hypothesis of the type of changes that are desirable to detect. In order to meet these requirements, a major effort needs to be made to specify the base line condition, including an assessment of the normal (temporal and spatial) variability of the variables to be studied and an understanding of the causative factors of these changes. This has to be followed by a careful consideration of the changes that are to be detected, including a clear specification of the assumptions to be made. The design should also include considerations of the magnitude of the Type I and Type II errors that should be allowed in the study.
Assessment Studies These are used to investigate if a particular effect is real and to establish, if possible, a direct cause and effect relationship between a given source and the observed change. Perhaps, these types of studies are the most difficult to design for several reasons: (i) they require very specific hypotheses; (ii) there are problems with confounding factors (in particular, the temporal and spatial variability inherent in the collection of the data) which may cause problems in the detection of certain changes; in this case, prior information a n d / o r special designs should be used to control for such factors; (iii) they require complex designs; and (iv) they require a high level of replication. The most common designs used for these studies are the controlled site studies, the pre-operational and post-operational studies and a combination of both. In the controlled site studies, data from the site where an effect is suspected is compared with data from one or more controlsites. These designs are likely to be successful only if a complete assessment of the spatial variability is available for the study. In the pre-operational post-operational studies, data collected prior to a given intervention is compared with data collected after the intervention. Observed differences are assumed to be due to the intervention. The major difficulty in this situation is that the time period used in the study may not be sufficient to allow a complete assessment of the temporal variability of the data (see also Section 4.1). In any case, it is recommended to carry out prior deliberations to decide the
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scope and the type of hypotheses to be tested (or that are reasonable to test) as part of the design of the study (see for example Maher, 1984, Greig et al. 1984). General Knowledge Studies
These concentrate on a specific search for basic or fundamental trends, patterns, and characteristics of the environment. These studies are usually associated with the testing of scientific hypotheses. The intensity of effort depends on the problem being investigated. The design of these studies should agree with the standards of any scientific study. The set of priorities required for these studies may be of a different nature than those used for the previous ones. 3. Common Elements of Design 3.1. METHODOLOGICALCONSIDERATIONS The planning, implementation and analysis of each of the categories described in Section 2 have elements which are common to all, while others are unique to the problem being addressed. Among the common elements the following are basic and should be specified prior to the implementation of the study: (i) a set of clear and concise stated objectives; (ii) a clear idea of the nature of expected results; (iii) a well-designed sampling plan that maximizes the effectiveness and efficiency of the study; (iv) a priori strategy for analyzing the data collected; and (v) a trade off analysis between the extent of the results and the economic issues. It is necessary to stress the importance of these elements, particularly because some of these issues are waived in several studies. Clear and concise objectives: Although this seems to be an obvious requirement for any study, it is surprising how seldom clear, well-defined objectives are presented. It should be pointed out that broad objectives do not suffice. Only concise objectives will define the focus of the study and provide an indication of the scope. Furthermore, they will be useful for developing the hypotheses that the study will address. To illustrate this point, suppose it is stated that the objective of a survey program is to determine the spatial variability of organic carbon in Lake Ontario. This statement, although seemingly clear and concise, left out some important details of the objective. For example, the statement does not specify the time frame desired for the characterization, or the precision required for the estimates. In general, the details required in the statement of the objectives depend on the type of study being implemented. For example, in assessment studies it is necessary to specify the particular hypothesis to be tested. To clarify this point, suppose an assessment study will be implemented to determine the effects of a new power plant in Lake Ontario. Then, it should state, for example, that one of the objectives is to test the hypothesis that discharges of warm water from the station will lead to reductions by some predetermined amounts in
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round whitefish, lake whitefish, and lake trout populations. Note that the specification of the hypothesis focuses its attention to the type of changes that are considered important, and gives an indication of the scope of the expected results. It should be recognized that any anthropogenic activity will result in environmental change. Therefore, to have socially useful studies, it is critical to identify apriori the type of impact that will be considered unacceptable. Furthermore, it is also necessary to identify what would be the significance of such a change to culturally desirable qualities of the system under investigation. It could be said that the purpose in stating clear and concise objectives is to confront the researcher with questions that could be reasonably answered by the study. This is fundamental if adequate results are expected from the study.
Clear Idea of the Nature of Expected Results This element is closely related to the specification of the objectives. In the example given above, the statement of the desired precision for the estimates clearly indicates what is expected from the study. In testing for an hypothesis, a statement of the desired confidence level and thepower level, at a pre-established magnitude of change, are required to specify the type of expected results. Failure to do so may result in a study with a high probability of producing false positives, limiting the use of the results. Note that statements of precision will allow the researcher to foresee (and probably control) the quality of results and the level of effort that is required. Also, they will have a direct impact on the design of the sampling plan.
A Well-Designed Sampling Plan Based on the objectives, together with the nature of the expected results, a sampling plan should be designed to maximize the effectiveness of the study. Principles of sampling design are given elsewhere (see for example Green, 1979). However, it should be noted that the plan should indicate all the variables to be measured, the frequency, the methods, and the sites. It should also assess tile amount of resources required to complete the sampling program. The selection of sites, frequency of sampling, and the variables to be measured are the tasks implicitly considered in the design of environmental studies. As can be seen from the discussion so far, this is only one of the elements of the proposed frame of design.
A Priori Strategy for Analyzing the Data Collected This is a necessary pai"t of a good design. Unless the data present unexpected difficulties, the prior investigation of the analysis strategy will help the researcher to foresee the amount of information that is required to achieve the desired power and to determine the variables that should be included in the sample. Also, this will reduce problems that are usually encountered during the analysis stage, making it possible to obtain more conclusive results. In the selection of the analysis technique, it should be kept in mind that usually the data have undesirable properties, such as lack of independence, non-normality of the distributions, and autocorrelation. The selected method should be able to handle
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these difficulties. An a priori strategy for analyzing the data should be used iteratively with the formulation of the sampling design. Such an iteration will increase the probability of meeting objectives and could result in a more economic and meaningful design.
Trade-off Analysis Between Extent of the Results and Economic Issues On theoretical grounds it would be desirable to carry out studies that are as comprehensive as possible. However, practical and economic limitations usually determine the results that are realistically obtainable. After the proposed sampling plan has been completed, it is necessary to make an evaluation of all the resources and costs that are required to ensure that the study can be carried out. If this is not possible, it would be necessary to reduce the scope and nature of the study. In this exercise the researcher must find out by how much it is possible to reduce the precision of the results without compromising the quality of the results, or which hypotheses will have to be dropped from the study. In any case, after the economic evaluation, the researcher will have a good idea of the type and scope of results that can be reasonably obtained. Often, this type of evaluation will require a setting of priorities for the objectives to be addressed. It will also ensure that critical issues receive sufficient levels of effort at the expense of secondary or ancillary interests. In some situations the economic considerations may reduce the amount of effort to such a degree that it may not be possible to detect the desired level of environmental change due to the associated reduction on the level of statistical power. If such studies are carried out, they will have a greater chance of having false positive results with the danger of creating false assurances. Therefore, it may be desirable not to carry out such studies at all. 3.2. GENERAL KNOWLEDGECONSIDERATIONS Prior knowledge of the environmental properties of the system being studied may reveal other common elements that could impact components of the study to varying degrees. In particular, the morphometric characteristics, the trophic state, and the spatial and temporal variability of the lakes may affect the sampling designs and the properties of the collected data. For example, take a stratified lake, if the study requires sampling of both the epilimnetic and the hypolimnetic zones, the sample effort should be larger in the epilimnetic zone because this has higher spatial and temporal variability. Likewise, sandy, homogeneous shore lines having common sediments and slopes may require less sampling effort than highly variable ones (i.e., mixtures of rock and sand). The near-shore zone, which is affected by storms, seiches, rapid temperature variations, etc., is likely to require more frequent sampling than hypolimnetic zones that are usually affected by well-defined cyclic seasonal events. 4. Diverse Elements of Design - An Impact Assesment Example
Each of the studies described in Section 2 has inherent features that need to be considered
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during the design. Although these influence the five elements of a good design described in Section 3, the greatest impact is in the formulation of the objectives and in the specification of the desired results. An example of a hypothetical impact assessment study will be used to illustrate how the diverse elements can be used during the design, particularly during the unfolding of the issues just mentioned. The goal of the study would be to determine the operational effects of a new generating station located on the shores of the Great Lakes (for a more general information on the ecological issues involved see Greig et al., 1984). A discussion of the current practice is presented in order to contrast the differences that can be obtained with the proposed methodology. 4. l. COMMON APPROACH The most common approach is to use a control site study (see Section 2.2). These studies are based on the principle that if the two sites can be assumed to have similar biological, chemical, and morphologic characteristics, differences observed between the data sets can be attributed to the effects of the intervention. This similarity assumption is fundamental to useful conclusions from the study. However, in practice the sites are often selected only on the basis of morphometric similarities, with the implicit assumption that this is sufficient to ensure similar chemical and biological properties. This assumption is rarely tested and when it is tested, non-significant results are taken as confirmation that the two sites are similar. Unfortunately, given the large variability of the data, particularly the biological, there is a large chance of drawing false positive conclusions (i.e., of not detecting site differences when these exist). This is a real danger in many studies, unless sufficient power is ensured by adequate sampling efforts. If the similarity of the two sites is not reasonably well confirmed (i.e., by assessing the Type I and Type II errors), then the conclusions obtained by the study are of limited use. Sometimes the option is available to initiate the study prior to the construction of the plant. In these cases, pre-operational and post-operational studies can be carried out. The implicit assumption in these studies is that the lake is not changing during the implementation of the study. Under this assumption, data from the pre-operational phase serve as control. Unfortunately, in rapidly expanding industrial areas, this assumption may not be valid making it difficult to identify the effects due to the plant. The major drawback of traditional designs is that the assumptions related to the appropriateness of the controls may not be testable. Considerable improvements have been recently seen, particularly in the areas of placing greater emphasis on the testing of sound ecological hypothesis well bounded in time and space as recommended by Beanland and Duinker (1983, 1984). However, these are often too broad to be statistically testable, because they fail to consider and define what would be an acceptable change. 4.2. PROPOSED APPROACH
Developing objectives Before being able to obtain concise objectives, it is necessary to state a general goal. This
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goal is then broken down into major areas of potential effect. Each of these are analyzed and their potentially significant impacts are identified. In the current human culture, an impact is considered meaningful only if it threatens human health, a valuable resource (fish, recreational use, transportation) or an aesthetic value (the beauty of the Falls). Other ecological effects (i.e., preserving biodiversity) are now gaining in importance. In the case of the example, the goal would be to assess the impact of the new station on the aquatic environment. Many areas of potential effect can be identified. The following three were arbitrarily chosen because they would apply to any generating facility built on the Great Lakes that uses the once-through-cooling process. (i) The effect of removal and transfer of organisms by the intake of cooling water. (ii) The effect of passage of organisms through the plant. (iii) The effects on the thermal discharge of water on the use of area contained within a thermal-envelope defined, for example, by a AT of 1 ~
Development of Hypotheses The next step is to formulate testable hypotheses for each of the identified areas of impact. Methodologies for developing these must be grounded in sound ecological and limnological principles (see for example Greig et al., 1984). The hypothesis must consist of a succinct statement of what is to be tested and the degree of change that is to be detected. Once the hypotheses are formulated, they can be tested by carrying out the appropriate studies. In the example this can be accomplished as follows.
For area (i) Of all the organisms that are transferred and do not go through the plant, the only ones that may be significantly impacted are the fish. These are either completely removed, or as in modern stations, returned by the fish return systems to the vicinity of the discharge. The issue then becomes whether or not the transfer of the organisms have, in one's judgement, unacceptable consequences. What the transfer does is to move organisms from a coot zone to a hot/warm zone over a short period of time. The effects of transfer can be phrased as follows: (i) can the organisms adapt to such a rapid temperature change? (ii) do the stresses encountered in the passage result in significant deleterious effects to the organisms? Once the levels of change considered acceptable are identified, (i.e. is a 10% mortality tolerable?), then a hypothesis can be formulated and addressed in a combination of laboratory experimentation and field verification. While the design of laboratory experiments do not often run into the statistical problems of field studies, the design of field verification programs requires considerable a priori information. To design the sampling program, sampling variability (spatial and temporal) available from either published work or from the result of a prior survey should be used to estimate the replication and frequency of sampling required to have the power necessary to detect the desired change. As a result of these considerations, the researcher may want to test the hypothesis that
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the mortality (up to 24 hr following passage) of the three main commercially important species does not exceed 10%. For Area (ii) The issues associated with the organisms that pass through the plant (i.e., that are entrained) are similar to those associated with the organisms that are transferred, with the exception that the temperature changes are much greater (over 20 ~ and they may be also exposed to pressure changes. The organisms of concern are planktonic or semi-planktonic. A variety of studies have investigated the effect of entrainment on phytoplankton and zooplankton in the Great Lakes, but no major effects have been detected (Dunstall, 1978, 1981). This is probably due to the rapid turn-over rates of the organisms and the high day-to-day variability. The ichthyoplankton, both the fish eggs and larvae, are vulnerable to entrainment. The issues of concern are then: (i) how many are entrained? (ii) what is the viability of the entrained individuals? (iii) what is the effect of passage? (iv) what is the effect of relocation? The necessity of addressing these questions ultimately rests on the ability of assessing the importance of the consequences of the changes. Questions (i) and (ii) can be addressed by a simple collection of intake and discharge samples and measurements of viability. If reduced viability is encountered, then it may be desirable to investigate questions (iii) and (iv) by carrying out laboratory experiments simulating the plant passage. In this case a hypothesis with two components can be formulated. Is a significant proportion (for example a 10%) of the viable available larvae and eggs passing through the near shore zone in the vicinity of the plant being entrained? And, if a significant proportion is entrained, then does a significant proportion of those entrained die within a predetermined period (3 hr) after returning to ambient temperature? As above, the power necessary to test the hypothesis has to be a crucial consideration. For Area (iii) Several effects can be determined a priori. One of these is that warm water fish (gizzard shad) will move to the discharge area during the witner, while cold water species (trout, perch) will leave it during the summer. In general, it can be assumed that the fish have preferred temperatures and distribute themselves along temperature gradients. It is then possible to formulate hypotheses to test the association between fish population and thermal gradient generated by the discharge. For example, the hypothesis that there is a dose-response relationship of the available fish with respect to the spatiotemporal temperature gradient may be formulated. To test this hypothesis it will be necessary to gather data over a grid covering the expected plume of the discharge. Before concluding this section, it is important to note that the overall impact assessment was reduced to a series of small studies designed to address specific hypotheses, which
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dealt with issues that are recognizable in terms of their impact, and are bounded by acceptable and unacceptable changes. The hypotheses must be testable, both ecologically and quantitatively. What is still missing is an evaluation of the actual significance of these impacts in terms of the values that the society places on its resources. Although this is important, it is outside of the scope of the present discussion. 5. C o n c l u s i o n s
A way of approaching environmental studies in an effective and efficient manner has been presented. The method consists of two main components: (i) an identification of the type of study that is to be undertaken for obtaining desired results; and (ii) an adequate design which centers on the clear identification of the desired objectives. With clearly identified objectives, it becomes easier to prioritize and to obtain valuable results. Also, it avoids the need for mining the data in search for possible effects. As illustrated by the example, there are advantages in breaking down large-scale environmental studies into small parts. - The goals of the study are better focused through the formulation of concise objectives, many of these stated as testable hypotheses. - With small objectives, it is possible to design adequate sampling plans that will provide enough data to obtain the required precisions, and in particular, enough power to test the related hypotheses. - The results of the study are easy to interpret. - The researcher does not have to rely on the data to generate the hypotheses to be tested, giving more control over the type of results that can be obtained. - The studies do not have to rely on non-testable assumptions that could limit the usefulness of the results. - The cost of the study can often be reduced because wasted efforts are eliminated. As a result of the ideas presented in the paper, several things have become obvious: (i) There are many common elements among studies which may offer opportunities for better apportioning of the research efforts. (ii) There is a critical need to improve the emphasis placed on the power of the studies carried out, on the formulation of meaningful and testable hypotheses and on the validity of the types of controls that may be used. And, (iii) There is a need to put into perspective the ecological and economic implications of the changes detected and at the same time recognizing that all industrial activities will impose an associated environmental cost whose acceptance or rejection will depend on society's will. References
Bealands, G. E. and Duinker, P. N., 1984: 'An Ecological Framework for Environmental Impact Assessment', Journal of Environmental Management 18, 267-277.
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Beanlands, G. E. and Duinker, P. N.: 1983, 'An Ecological Framework for Environmental Impact Assessment in Canada', Institute for Resource and Environmental Studies. Dalhousie University in cooperation with the Federal Environmental Assessment Review Office. Dunstall, T. G.: 1981, 'Effect of Entrainment on Phytoplankton Primary Production - A Summary of Studies Conducted at Four Ontario Hydro Generating Stations, 1975-1977', Ontario Hydro Research Division Report No. 81-139-K. Dunstall, T. G.: 1978, 'Use of a Sample Grid to Determine the Effect of Once-through Cooling on the Distribution of Zooplankton and Phytoplankton', Ontario Hydro Research Division Report No. 78-257-K. Esterby, S. R.: 1986, 'Spatial Heterogeneity of Water Quality Parameters', in E1 Shaarawai, A. H. and Kwiatkowski, R. E. (eds.), Developments in Water Science, Elsevier. Green, R. H.: 1979, Sampling Design and Statistical Methods for Environmental Biologists, John Wiley & Sons, Toronto. Greig, A. L., Cunningham, G., Everitt, R. R., and Jones, M. L.: 1984, 'Final Report of two Workshops to Consider the Environmental Effects and Monitoring Options for the Darlington NGS', ESSA Environmental and Social Systems Analysts Ltd. Report prepared for Ontario Hydro. Kwiatkowski, R. E.: 1986, 'The Importance of Design Quality Control to a National Monitoring Program', in El Shaarawai, A. H. and Kwiatkowski, R. E. (eds.), in Developments in Water Science, Elsevier. Maher, J. F. B.: 1984, 'Outline of Environmental Pre-operational and Post-operational Studies for Darlington GS', Ontario Hydro, Environmental Studies & Assessment Department, Report No. 84252. Rosenberg, D. M., Resh, V. H. et al.: 1981, 'Recent Trends in Environmental Impact Assessment', Canadian Journal of Fisheries and Aquatic Sciences 38, 591-624.
(Received April 1990) Abstract. Aquatic environmental studies can be categorized by the breadth of their scope and the types of desired results. The use of this categorization coupled with a clear specification of objectives and a judicious knowledge of the environmental variability should lead to more statistically efficient studies. This paper discusses the types of lacustrine studies commonly encountered in terms of their categorization. It provides examples of how the intrinsic environmental variability can influence their design and stresses the importance of properly stated objectives, the developing of testable hypotheses, the design of robust and powerful studies, and the importance of evaluating the implication of changes as critical factors for conducting effective and efficient environmental studies.
1. Introduction
All of man's activities on this planet, including this very existence, are likely to leave a mark on the environment. This mark or change is often referred to as man's environmental impact. In our era of global overcrowding and massive manipulation of natural resources, a noticeable degradation of environmental quality has taken place which has raised concerns about the future of the planet. Society is realizing that all this change has a cost, and now it must decide what type of costs (ie., environmental changes) are acceptable. To assess this damage, some times a priori, but most often a posteriorL environmental studies are carried out. These studies are referred to as impact studies or environmental assessment studies. The intent of the environmental studies has to be applauded. However, their value may be limited since often they do not meet expectations, partly because of inadequate design, partly because of the inappropriateness of analytical techniques to deal with data which are highly variable and do not conform to main statistical assumptions, and partly because of erroneous ecological assumptions. Several authors have recognized the need for improving the quality of these studies, and have suggested several frameworks for this purpose (see Beanlands and Duinker, 1983, 1984; Rosenberg et al., 1981). These frameworks tend to stress the ecological considerations rather than the statistical aspects. The objective of this paper is to extend the concept of design stressing the importance of * An early version of this paper was presented at the First International Conference on Environmentrics, Cairo, Egypt, April 4-7, 1989, under the title Framework for the Design of Aquatic Environmental Studies.
Environmental Monitoring and Assessment 17: 303-314, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.
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good statistical design in all aquatic environmental studies. This will be done with the help of a conceptualized framework that can be used for simplifying the problems being addressed and prioritizing their components to maximize efficiency. The environmental studies can be broken down into categories and their components so that appropriate statistical considerations can be applied to each of them. The statistical considerations include the recognition of the existence of confounding factors, the need for complex designs, the need for quantitatively defining the desired results, and the need of adequate sample sizes to achieve the desired results. To reduce the scope of the paper to a manageable size, and because much of our group's experience has been with the Great Lakes, it was decided to discuss only the aquatic environmental studies of lacustrine systems. Therefore, many other types of important studies such as river, air, and land pollution, are not considered here. The paper is organized as follows. Section 2 classifies the studies according to the breadth of scope and the type of desired results. Section 3 discusses elements that are common among the types of studies. Section 4 discusses diverse elements found among the studies with the help of an impact assessment study example. Finally, Section 5 presents some conclusions and areas that require further consideration. 2. Classification of the Studies
Environmental aquatic investigations may be classified according to the spatial scope and the type of desired results. This classification is important since it will dictate considerations for the planning, design, and implementation of the studies. In particular, it will help in determining the amount of resources required and provide an indication of the variables that should be measured. 2.1. CLASSIFICATIONBY BREADTHOF SCOPE Based on the spatial scope, the studies can be classified into three broad categories: large scale, partial, and local studies. Large Scale or Whole Lake Studies
These studies are implemented over a large geographical area, usually covering a complete lake. Because of the extent of the sample area, sophisticated equipment may be required to handle the samples. These studies require intensive sampling schemes both in time and resources. To optimize the sampling effort, prior information of the spatial variability of the lake should be used to divide the lake into zones of homogeneous characteristics. It is advised to allocate the sampling effort in direct proportion to the variability of the zones. Data collected in these studies are highly variable, both spatially and temporally. The degree of variability is likely to change with the type of lake and the zones found in it. This variability is increased during periods of unstable weather. Because of the distances covered in the larger lakes, the time spent traveling between sampling locations may have a large effect in the observed variability between samples (particularly for some nutrients, planktonic organisms, and fish) than the actual spatial variability (Esterby, 1986). Therefore, the relative merits of intensive versus extensive sampling should be carefully
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evaluated. In some cases it may be desirable to stratify the lake into zones, allocate the sampling effort as above, and sample each zone independently of the others over as brief a period of time as possible. In general, sophisticated statistical techniques are required to analyze the data collected in large surveys. These techniques should be able to handle multiple variables, temporal and spatial dependency, non-normality of the distribution, and probably unequal time intervals in the sampling period. Partial or Basin Studies
These studies are carried out over a portion of the lake that can be categorized by certain uniform characteristics. Although it may cover a large geographical area, it is usually a small proportion of the total size of the lake. Similar difficulties to those encountered on large scale studies can be found in these studies. However, the efforts required may not be as intensive. Local Studies
These studies concentrate on a small geographical area and are usually associated with impact assessment studies. The samples are taken from near shore sites. Thus, the data are highly variable and may be affected by many other variables such as storms, winds, temperature, etc. In addition, the whole lake may be undergoing changes whose effects must be removed from the area in question. To account for part of the variability, it may be necessary to sample intensively a n d / o r measure covariates that can be used in the analysis. The statistical methods required for these studies depend on the particular application, although tests of homogeneity, particularly ANOVA, are usually employed. 2.2.
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On the basis of the desired results, each of the above classes can be further categorized into four groups: surveys, monitoring programs, assessment studies, and general purpose studies. Surveys
These are, in general, the first type of investigations made on a lake and are used for determining general spatial and temporal characteristics of physical, chemical, and biological variables. Later, the information may be used for making decisions about the environment or for setting baseline limits. Some of the particular applications include the following: (i) The gathering of information on unknown environments. (ii) The determination of the range of conditions. (iii) The gathering of information to identify areas that meet certain conditions. (iv) The determination of the most desirable site for a new industrial development. The duration of the study varies depending on the extent of the survey, but it is desirable that it extends for at least a year to capture annual cycles.
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Monitoring Programs These are generally used for two main purposes: (i) maintain historical records which may be used to identify long-term trends in a variety of parameters; and (ii) ensure that certain environmental criteria are met. In the first case, the monitoring program is basically a reduced survey carried out in a repetitive fashion; nevertheless, it is important to ensure good quality on the sampling programs (Kwiatkowski, 1986). In the second case, the objective of the monitoring program is to gather information to detect a (possible) change with respect to a specified base line. To meet this objective, two important requirements are necessary: (i) a clear understanding of the base line that is considered normal or desirable (this base line should be stable, useful, and real); and (ii) a clear hypothesis of the type of changes that are desirable to detect. In order to meet these requirements, a major effort needs to be made to specify the base line condition, including an assessment of the normal (temporal and spatial) variability of the variables to be studied and an understanding of the causative factors of these changes. This has to be followed by a careful consideration of the changes that are to be detected, including a clear specification of the assumptions to be made. The design should also include considerations of the magnitude of the Type I and Type II errors that should be allowed in the study.
Assessment Studies These are used to investigate if a particular effect is real and to establish, if possible, a direct cause and effect relationship between a given source and the observed change. Perhaps, these types of studies are the most difficult to design for several reasons: (i) they require very specific hypotheses; (ii) there are problems with confounding factors (in particular, the temporal and spatial variability inherent in the collection of the data) which may cause problems in the detection of certain changes; in this case, prior information a n d / o r special designs should be used to control for such factors; (iii) they require complex designs; and (iv) they require a high level of replication. The most common designs used for these studies are the controlled site studies, the pre-operational and post-operational studies and a combination of both. In the controlled site studies, data from the site where an effect is suspected is compared with data from one or more controlsites. These designs are likely to be successful only if a complete assessment of the spatial variability is available for the study. In the pre-operational post-operational studies, data collected prior to a given intervention is compared with data collected after the intervention. Observed differences are assumed to be due to the intervention. The major difficulty in this situation is that the time period used in the study may not be sufficient to allow a complete assessment of the temporal variability of the data (see also Section 4.1). In any case, it is recommended to carry out prior deliberations to decide the
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scope and the type of hypotheses to be tested (or that are reasonable to test) as part of the design of the study (see for example Maher, 1984, Greig et al. 1984). General Knowledge Studies
These concentrate on a specific search for basic or fundamental trends, patterns, and characteristics of the environment. These studies are usually associated with the testing of scientific hypotheses. The intensity of effort depends on the problem being investigated. The design of these studies should agree with the standards of any scientific study. The set of priorities required for these studies may be of a different nature than those used for the previous ones. 3. Common Elements of Design 3.1. METHODOLOGICALCONSIDERATIONS The planning, implementation and analysis of each of the categories described in Section 2 have elements which are common to all, while others are unique to the problem being addressed. Among the common elements the following are basic and should be specified prior to the implementation of the study: (i) a set of clear and concise stated objectives; (ii) a clear idea of the nature of expected results; (iii) a well-designed sampling plan that maximizes the effectiveness and efficiency of the study; (iv) a priori strategy for analyzing the data collected; and (v) a trade off analysis between the extent of the results and the economic issues. It is necessary to stress the importance of these elements, particularly because some of these issues are waived in several studies. Clear and concise objectives: Although this seems to be an obvious requirement for any study, it is surprising how seldom clear, well-defined objectives are presented. It should be pointed out that broad objectives do not suffice. Only concise objectives will define the focus of the study and provide an indication of the scope. Furthermore, they will be useful for developing the hypotheses that the study will address. To illustrate this point, suppose it is stated that the objective of a survey program is to determine the spatial variability of organic carbon in Lake Ontario. This statement, although seemingly clear and concise, left out some important details of the objective. For example, the statement does not specify the time frame desired for the characterization, or the precision required for the estimates. In general, the details required in the statement of the objectives depend on the type of study being implemented. For example, in assessment studies it is necessary to specify the particular hypothesis to be tested. To clarify this point, suppose an assessment study will be implemented to determine the effects of a new power plant in Lake Ontario. Then, it should state, for example, that one of the objectives is to test the hypothesis that discharges of warm water from the station will lead to reductions by some predetermined amounts in
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round whitefish, lake whitefish, and lake trout populations. Note that the specification of the hypothesis focuses its attention to the type of changes that are considered important, and gives an indication of the scope of the expected results. It should be recognized that any anthropogenic activity will result in environmental change. Therefore, to have socially useful studies, it is critical to identify apriori the type of impact that will be considered unacceptable. Furthermore, it is also necessary to identify what would be the significance of such a change to culturally desirable qualities of the system under investigation. It could be said that the purpose in stating clear and concise objectives is to confront the researcher with questions that could be reasonably answered by the study. This is fundamental if adequate results are expected from the study.
Clear Idea of the Nature of Expected Results This element is closely related to the specification of the objectives. In the example given above, the statement of the desired precision for the estimates clearly indicates what is expected from the study. In testing for an hypothesis, a statement of the desired confidence level and thepower level, at a pre-established magnitude of change, are required to specify the type of expected results. Failure to do so may result in a study with a high probability of producing false positives, limiting the use of the results. Note that statements of precision will allow the researcher to foresee (and probably control) the quality of results and the level of effort that is required. Also, they will have a direct impact on the design of the sampling plan.
A Well-Designed Sampling Plan Based on the objectives, together with the nature of the expected results, a sampling plan should be designed to maximize the effectiveness of the study. Principles of sampling design are given elsewhere (see for example Green, 1979). However, it should be noted that the plan should indicate all the variables to be measured, the frequency, the methods, and the sites. It should also assess tile amount of resources required to complete the sampling program. The selection of sites, frequency of sampling, and the variables to be measured are the tasks implicitly considered in the design of environmental studies. As can be seen from the discussion so far, this is only one of the elements of the proposed frame of design.
A Priori Strategy for Analyzing the Data Collected This is a necessary pai"t of a good design. Unless the data present unexpected difficulties, the prior investigation of the analysis strategy will help the researcher to foresee the amount of information that is required to achieve the desired power and to determine the variables that should be included in the sample. Also, this will reduce problems that are usually encountered during the analysis stage, making it possible to obtain more conclusive results. In the selection of the analysis technique, it should be kept in mind that usually the data have undesirable properties, such as lack of independence, non-normality of the distributions, and autocorrelation. The selected method should be able to handle
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these difficulties. An a priori strategy for analyzing the data should be used iteratively with the formulation of the sampling design. Such an iteration will increase the probability of meeting objectives and could result in a more economic and meaningful design.
Trade-off Analysis Between Extent of the Results and Economic Issues On theoretical grounds it would be desirable to carry out studies that are as comprehensive as possible. However, practical and economic limitations usually determine the results that are realistically obtainable. After the proposed sampling plan has been completed, it is necessary to make an evaluation of all the resources and costs that are required to ensure that the study can be carried out. If this is not possible, it would be necessary to reduce the scope and nature of the study. In this exercise the researcher must find out by how much it is possible to reduce the precision of the results without compromising the quality of the results, or which hypotheses will have to be dropped from the study. In any case, after the economic evaluation, the researcher will have a good idea of the type and scope of results that can be reasonably obtained. Often, this type of evaluation will require a setting of priorities for the objectives to be addressed. It will also ensure that critical issues receive sufficient levels of effort at the expense of secondary or ancillary interests. In some situations the economic considerations may reduce the amount of effort to such a degree that it may not be possible to detect the desired level of environmental change due to the associated reduction on the level of statistical power. If such studies are carried out, they will have a greater chance of having false positive results with the danger of creating false assurances. Therefore, it may be desirable not to carry out such studies at all. 3.2. GENERAL KNOWLEDGECONSIDERATIONS Prior knowledge of the environmental properties of the system being studied may reveal other common elements that could impact components of the study to varying degrees. In particular, the morphometric characteristics, the trophic state, and the spatial and temporal variability of the lakes may affect the sampling designs and the properties of the collected data. For example, take a stratified lake, if the study requires sampling of both the epilimnetic and the hypolimnetic zones, the sample effort should be larger in the epilimnetic zone because this has higher spatial and temporal variability. Likewise, sandy, homogeneous shore lines having common sediments and slopes may require less sampling effort than highly variable ones (i.e., mixtures of rock and sand). The near-shore zone, which is affected by storms, seiches, rapid temperature variations, etc., is likely to require more frequent sampling than hypolimnetic zones that are usually affected by well-defined cyclic seasonal events. 4. Diverse Elements of Design - An Impact Assesment Example
Each of the studies described in Section 2 has inherent features that need to be considered
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during the design. Although these influence the five elements of a good design described in Section 3, the greatest impact is in the formulation of the objectives and in the specification of the desired results. An example of a hypothetical impact assessment study will be used to illustrate how the diverse elements can be used during the design, particularly during the unfolding of the issues just mentioned. The goal of the study would be to determine the operational effects of a new generating station located on the shores of the Great Lakes (for a more general information on the ecological issues involved see Greig et al., 1984). A discussion of the current practice is presented in order to contrast the differences that can be obtained with the proposed methodology. 4. l. COMMON APPROACH The most common approach is to use a control site study (see Section 2.2). These studies are based on the principle that if the two sites can be assumed to have similar biological, chemical, and morphologic characteristics, differences observed between the data sets can be attributed to the effects of the intervention. This similarity assumption is fundamental to useful conclusions from the study. However, in practice the sites are often selected only on the basis of morphometric similarities, with the implicit assumption that this is sufficient to ensure similar chemical and biological properties. This assumption is rarely tested and when it is tested, non-significant results are taken as confirmation that the two sites are similar. Unfortunately, given the large variability of the data, particularly the biological, there is a large chance of drawing false positive conclusions (i.e., of not detecting site differences when these exist). This is a real danger in many studies, unless sufficient power is ensured by adequate sampling efforts. If the similarity of the two sites is not reasonably well confirmed (i.e., by assessing the Type I and Type II errors), then the conclusions obtained by the study are of limited use. Sometimes the option is available to initiate the study prior to the construction of the plant. In these cases, pre-operational and post-operational studies can be carried out. The implicit assumption in these studies is that the lake is not changing during the implementation of the study. Under this assumption, data from the pre-operational phase serve as control. Unfortunately, in rapidly expanding industrial areas, this assumption may not be valid making it difficult to identify the effects due to the plant. The major drawback of traditional designs is that the assumptions related to the appropriateness of the controls may not be testable. Considerable improvements have been recently seen, particularly in the areas of placing greater emphasis on the testing of sound ecological hypothesis well bounded in time and space as recommended by Beanland and Duinker (1983, 1984). However, these are often too broad to be statistically testable, because they fail to consider and define what would be an acceptable change. 4.2. PROPOSED APPROACH
Developing objectives Before being able to obtain concise objectives, it is necessary to state a general goal. This
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goal is then broken down into major areas of potential effect. Each of these are analyzed and their potentially significant impacts are identified. In the current human culture, an impact is considered meaningful only if it threatens human health, a valuable resource (fish, recreational use, transportation) or an aesthetic value (the beauty of the Falls). Other ecological effects (i.e., preserving biodiversity) are now gaining in importance. In the case of the example, the goal would be to assess the impact of the new station on the aquatic environment. Many areas of potential effect can be identified. The following three were arbitrarily chosen because they would apply to any generating facility built on the Great Lakes that uses the once-through-cooling process. (i) The effect of removal and transfer of organisms by the intake of cooling water. (ii) The effect of passage of organisms through the plant. (iii) The effects on the thermal discharge of water on the use of area contained within a thermal-envelope defined, for example, by a AT of 1 ~
Development of Hypotheses The next step is to formulate testable hypotheses for each of the identified areas of impact. Methodologies for developing these must be grounded in sound ecological and limnological principles (see for example Greig et al., 1984). The hypothesis must consist of a succinct statement of what is to be tested and the degree of change that is to be detected. Once the hypotheses are formulated, they can be tested by carrying out the appropriate studies. In the example this can be accomplished as follows.
For area (i) Of all the organisms that are transferred and do not go through the plant, the only ones that may be significantly impacted are the fish. These are either completely removed, or as in modern stations, returned by the fish return systems to the vicinity of the discharge. The issue then becomes whether or not the transfer of the organisms have, in one's judgement, unacceptable consequences. What the transfer does is to move organisms from a coot zone to a hot/warm zone over a short period of time. The effects of transfer can be phrased as follows: (i) can the organisms adapt to such a rapid temperature change? (ii) do the stresses encountered in the passage result in significant deleterious effects to the organisms? Once the levels of change considered acceptable are identified, (i.e. is a 10% mortality tolerable?), then a hypothesis can be formulated and addressed in a combination of laboratory experimentation and field verification. While the design of laboratory experiments do not often run into the statistical problems of field studies, the design of field verification programs requires considerable a priori information. To design the sampling program, sampling variability (spatial and temporal) available from either published work or from the result of a prior survey should be used to estimate the replication and frequency of sampling required to have the power necessary to detect the desired change. As a result of these considerations, the researcher may want to test the hypothesis that
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the mortality (up to 24 hr following passage) of the three main commercially important species does not exceed 10%. For Area (ii) The issues associated with the organisms that pass through the plant (i.e., that are entrained) are similar to those associated with the organisms that are transferred, with the exception that the temperature changes are much greater (over 20 ~ and they may be also exposed to pressure changes. The organisms of concern are planktonic or semi-planktonic. A variety of studies have investigated the effect of entrainment on phytoplankton and zooplankton in the Great Lakes, but no major effects have been detected (Dunstall, 1978, 1981). This is probably due to the rapid turn-over rates of the organisms and the high day-to-day variability. The ichthyoplankton, both the fish eggs and larvae, are vulnerable to entrainment. The issues of concern are then: (i) how many are entrained? (ii) what is the viability of the entrained individuals? (iii) what is the effect of passage? (iv) what is the effect of relocation? The necessity of addressing these questions ultimately rests on the ability of assessing the importance of the consequences of the changes. Questions (i) and (ii) can be addressed by a simple collection of intake and discharge samples and measurements of viability. If reduced viability is encountered, then it may be desirable to investigate questions (iii) and (iv) by carrying out laboratory experiments simulating the plant passage. In this case a hypothesis with two components can be formulated. Is a significant proportion (for example a 10%) of the viable available larvae and eggs passing through the near shore zone in the vicinity of the plant being entrained? And, if a significant proportion is entrained, then does a significant proportion of those entrained die within a predetermined period (3 hr) after returning to ambient temperature? As above, the power necessary to test the hypothesis has to be a crucial consideration. For Area (iii) Several effects can be determined a priori. One of these is that warm water fish (gizzard shad) will move to the discharge area during the witner, while cold water species (trout, perch) will leave it during the summer. In general, it can be assumed that the fish have preferred temperatures and distribute themselves along temperature gradients. It is then possible to formulate hypotheses to test the association between fish population and thermal gradient generated by the discharge. For example, the hypothesis that there is a dose-response relationship of the available fish with respect to the spatiotemporal temperature gradient may be formulated. To test this hypothesis it will be necessary to gather data over a grid covering the expected plume of the discharge. Before concluding this section, it is important to note that the overall impact assessment was reduced to a series of small studies designed to address specific hypotheses, which
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dealt with issues that are recognizable in terms of their impact, and are bounded by acceptable and unacceptable changes. The hypotheses must be testable, both ecologically and quantitatively. What is still missing is an evaluation of the actual significance of these impacts in terms of the values that the society places on its resources. Although this is important, it is outside of the scope of the present discussion. 5. C o n c l u s i o n s
A way of approaching environmental studies in an effective and efficient manner has been presented. The method consists of two main components: (i) an identification of the type of study that is to be undertaken for obtaining desired results; and (ii) an adequate design which centers on the clear identification of the desired objectives. With clearly identified objectives, it becomes easier to prioritize and to obtain valuable results. Also, it avoids the need for mining the data in search for possible effects. As illustrated by the example, there are advantages in breaking down large-scale environmental studies into small parts. - The goals of the study are better focused through the formulation of concise objectives, many of these stated as testable hypotheses. - With small objectives, it is possible to design adequate sampling plans that will provide enough data to obtain the required precisions, and in particular, enough power to test the related hypotheses. - The results of the study are easy to interpret. - The researcher does not have to rely on the data to generate the hypotheses to be tested, giving more control over the type of results that can be obtained. - The studies do not have to rely on non-testable assumptions that could limit the usefulness of the results. - The cost of the study can often be reduced because wasted efforts are eliminated. As a result of the ideas presented in the paper, several things have become obvious: (i) There are many common elements among studies which may offer opportunities for better apportioning of the research efforts. (ii) There is a critical need to improve the emphasis placed on the power of the studies carried out, on the formulation of meaningful and testable hypotheses and on the validity of the types of controls that may be used. And, (iii) There is a need to put into perspective the ecological and economic implications of the changes detected and at the same time recognizing that all industrial activities will impose an associated environmental cost whose acceptance or rejection will depend on society's will. References
Bealands, G. E. and Duinker, P. N., 1984: 'An Ecological Framework for Environmental Impact Assessment', Journal of Environmental Management 18, 267-277.
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Beanlands, G. E. and Duinker, P. N.: 1983, 'An Ecological Framework for Environmental Impact Assessment in Canada', Institute for Resource and Environmental Studies. Dalhousie University in cooperation with the Federal Environmental Assessment Review Office. Dunstall, T. G.: 1981, 'Effect of Entrainment on Phytoplankton Primary Production - A Summary of Studies Conducted at Four Ontario Hydro Generating Stations, 1975-1977', Ontario Hydro Research Division Report No. 81-139-K. Dunstall, T. G.: 1978, 'Use of a Sample Grid to Determine the Effect of Once-through Cooling on the Distribution of Zooplankton and Phytoplankton', Ontario Hydro Research Division Report No. 78-257-K. Esterby, S. R.: 1986, 'Spatial Heterogeneity of Water Quality Parameters', in E1 Shaarawai, A. H. and Kwiatkowski, R. E. (eds.), Developments in Water Science, Elsevier. Green, R. H.: 1979, Sampling Design and Statistical Methods for Environmental Biologists, John Wiley & Sons, Toronto. Greig, A. L., Cunningham, G., Everitt, R. R., and Jones, M. L.: 1984, 'Final Report of two Workshops to Consider the Environmental Effects and Monitoring Options for the Darlington NGS', ESSA Environmental and Social Systems Analysts Ltd. Report prepared for Ontario Hydro. Kwiatkowski, R. E.: 1986, 'The Importance of Design Quality Control to a National Monitoring Program', in El Shaarawai, A. H. and Kwiatkowski, R. E. (eds.), in Developments in Water Science, Elsevier. Maher, J. F. B.: 1984, 'Outline of Environmental Pre-operational and Post-operational Studies for Darlington GS', Ontario Hydro, Environmental Studies & Assessment Department, Report No. 84252. Rosenberg, D. M., Resh, V. H. et al.: 1981, 'Recent Trends in Environmental Impact Assessment', Canadian Journal of Fisheries and Aquatic Sciences 38, 591-624.
