An indicator is something which makes visible, audible or perceptible which in itself is not visible, audible or perceptible. Many times indicators concern rather concrete matters, that might be experienced also in a more direct way. Acidity can be tasted, but a pH-meter is a better instrument. If such an instrument is not available the presence of Sphagnum cuspidatum tells, that the pH cannot be higher than 6.5. And when the accumulation of mercury in special tissues offish constitutes a good reflexion of mercury content of water, the observation will be far easier.
But speaking of indicators we mostly have other things in mind. In assessing the influence of the sea on inland waters chlorine concentrations can be measured by means of simple titrations. Nevertheless an impressive typology of brackish water exists, in which organisms function as an indicator for the 'degree of brackishness'. Many times we even experience that the assessment with the help of these organisms does not agree with that of our titrations (den Hartog, 1963). In such cases we tend to believe more in the values indicated by our organisms, rather than in the actual chlorine contents. We are not interested in chlorine as such, but in the response of the structure of the ecosystem as a whole, a response to the dynamics characteristic for a water which in some way is influenced by the sea. Brackish water shows its own character, and hence we attribute to it of its own quality.
With this description the concept of 'quality' is introduced. Quality is something which is not visible, audible or perceptible, but which can be made it by our indicators. We experience how behind the perceived reality another reality is hidden, an abstract reality, not of matters but of principles. Is 'quality' such a principle? Are we able, by looking to the things around us -plants, animals, communities, tissues, oxygen concentrations -to tell something about this difficult phenemenon 'quality'?
2. Quality, a Source of Concern. An Example
Recently the hydrobiologists of the Provincial Department for the maintenance of dikes, roads, bridges and the navigability of canals of Utrecht found a series of interesting diatoms in a ditch near the city of Utrecht. These diatom species belong to a community, normally found in a special type of shallow waters on pleistocene soils, called 'fens'. This * Paper presented at a Symposium held on 14 and 15 October 1982, in Utrecht, The Netherlands.