2.1 General review of assessment procedures and definitions

2.1.1 Physical and chemical water analysis
2.1.2 Biological evaluation of water pollution
2.1.2.1 Evaluation of water pollution with the aid of bioindicators
2.1.2.2 Evaluation of water pollution with the aid of biomonitors
2.1.3 Remote sensing

It would go beyond the scope of this paper to present here an exhaustive discussion of all possibilities which are available for the use of bioindicators and biomonitors in aquatic ecosystems. Instead, in the following I will attempt to provide a number of positive and negative examples in order to call attention to methods which are used in the Federal Republic of Germany. I will therefore devote a considerable amount of space to the so-called "saprobity system" for classification of water pollution. To begin with, however, a brief review of the different types of evaluative procedures is in order. These are:

1. Physical and chemical water analysis
2. Biological procedures based on the use of

a) Bioindicators
b) Biomonitors

3. Remote sensing

2.1.1 Physical and chemical water analysis

With the aid of physical and chemical analysis techniques, it is possible to obtain information on the condition of water at the place and time that samples are taken. Depending on the quality of the investigatory methods used and the number of parameters studied, more or less useful data are yielded on the quality of the water at that point in time. How accurate and detailed the results must be naturally depends on the purposes for which analyses are performed (see also Chapter 3). In order to obtain more precise data, repeated analyses are necessary; these can even be aimed at identifying changes during the course of a single day.

A program conducted by the state of Baden Wuerttemberg is a good example: along certain flowing waters, above all in the navigable portion of the Neckar River, measurement stations were installed with devices for continuous collection of water samples and regular monitoring of their chemical composition (Figure 1) (5). A number of parameters are measured, including the amounts present of organic carbon, ammonium, nitrite, nitrate, orthophosphate and oxygen, as well as the biochemical oxygen demand (BOD). By means of additional tests, it is of course also possible to obtain additional information, for example on the concentrations of toxic substances. As is to be expected, continuous taking of samples and performance of analyses are associated with high technical, financial and labor inputs. The costs incurred grow with the number of samples taken and the range of substances, which are tested for. Even the industrialized countries can only afford to do this within certain limits, as can be seen in Figure 1. Nevertheless, within the scope of major technical projects in developing countries it could be useful to carry out similar sample-taking and analysis programs here and there for monitoring purposes. This would have to be decided upon in each individual case.

For physical and chemical water analysis, the methods described in the following sections can be used. They are based on many years of experience, have been modified in accordance with the most recent scientific insights and discoveries, and continue to be developed further on an ongoing basis (713).

Figure 1: Location of measurement stations for continuous taking of water samples and regular chemical analysis along the rivers of Baden-Wuerttemberg (from 5).

2.1.2 Biological evaluation of water pollution

Living organisms make certain demands on their environment, i.e. they are dependent on certain factors or groups of factors and their qualitative and quantitative configurations. Based on this observation, in Central Europe it is possible, for example, to classify the different regions of flowing waters (streams and rivers) on the basis of their dominant fish species; here, the primary factors of relevance are temperature, oxygen content, and flow. The uppermost region, for example, is characterized by cold, oxygenrich water populated by trout (Salmo trutta); this is referred to as the trout zone. At the opposite end of the spectrum is the bream zone in the lower courses of major rivers where the current velocity is greatly reduced, the water temperature is high in the summer, and the oxygen content is low. This zone is characterized by bream (Ahramis brama).

Over 140 years ago the first observations were published according to which domestic sewage is capable of changing the natural population patterns of flowing waters. The mechanisms of this change were not immediately understood, however. The important thing is that even then, as today, the ability of living organisms to react positively or negatively to changes in their environment was observed and made use of. Today the fact is also taken advantage of that many organisms are able to assimilate and sometimes accumulate substances present in the water.

2.1.2.1 Evaluation of water pollution with the aid of bioindicators

A short, precise and comprehensive definition of the concept of "bioindicators" is given by ELLENBERG, sent (14) with which I fully agree: "A bioindicator is a related group or community of organisms whose occurrence or an easily observed behavioral trait can be so closely correlated with certain environmental conditions that it can be utilized as a pointer or quantitative test." (see 15)

In other words, when evaluating water pollution with the aid of bioindicators the circumstance is taken advantage of that organisms react to changes in their environment; these reactions can take the form of growth and/or increased population density, modified activity, reduced growth, a decline in population. or even death. Depending on their degree of complexity, size, generation time and other factors, organisms or different species react faster or slower. Most bacteria adapt very quickly to environmental changes. Protozoa and algae take longer, and insects - many of which live for a year or longer as larvae in the same aquatic environment - require correspondingly longer to react to positive changes in their surroundings. As a rule, organisms with longer generation times respond also more quickly to negative changes - if they exceed the limits of what is tolerable - for instance by migrating to zones with satisfactory living conditions or by dying. Depending on the time which bioindicators or indicator organisms spend in a body of water, they are subjected to the prevailing environmental conditions and any changes in them. That means that, as members of a related group of organisms or biological community, in contrast to physical or chemical analysis they integrate and reflect environmental conditions (and possible changes) over an extended period of time. Consequently, critical evaluation of the species compositions of a biocoenosis can yield sufficient data on the situation of a body of water and the range of fluctuations which the environmental conditions experience over a lengthy period of time. It is upon this recognition that the systems are based which are used in Germany for evaluation of aquatic ecosystems with the aid of bioindicators, foremost among them the saprobity system.

2.1.2.2 Evaluation of water pollution with the aid of biomonitors

As will be discussed in greater depth below (Chapter 3), relatively few organisms occur in aquatic habitats that are capable of indicating specific substances contained in the water. Because of this, in recent years more extensive use has been made of the ability of organisms to assimilate certain contaminants, such as heavy metals or pesticides, etc., through their gills or skin or in the food they eat and to accumulate it in their bodies. It has been possible to identify the above mentioned substances in such organisms just like in terrestrial organisms (cf. 2, 16). Such species are generally referred to as cumulative indicators (17), but will be called biomonitors or monitor organisms here. Biomonitors are also used for assessing brackish and marine water, in other words for waters for which it has not yet been possible to develop a "saprobity system". In addition, there are other organisms that can provide information on the kind and/or concentration of contaminants; these are the so-called reactive indicators, in this paper referred to as bioindicators. To name a few examples: Metopus es, a ciliate, is a useful indicator species for hydrogen sulfide. Gnathonemus petersi, a fish belonging to the family of the Mormyridae, can be utilized for indication of certain toxic substances; in their presence it changes the electric discharges it emits (18). Such organisms can also be used for evaluation of drinking water ( 19).

2.1.3 Remote sensing

Aerial photography can yield qualitative and quantitative information on changes in environmental conditions. The studies of this type which have been performed on the expansion of dry regions (examples: Sahara, Sahel) are well-known. Changes in the condition of large bodies of water can be detected in the same way.