The chemical and physical environment obviously has a profound effect on plant and animal communities. Thus, the plants found on a chalk- or limestone-based soil with its high pH are noticeably different to the plants found growing on a peat moorland, where pH's are much lower. Similarly the fauna associated with the two types of plant community will be completely different. Of course the biota can themselves influence the chemical and physical environments. This especially true of plants which can affect such factors as pH and shading. For example, heathers and sphagnum moss can actively reduce the pH of their surroundings. In aquatic systems large populations of phytoplankton, or surface-dwelling macrophytes such as Lemna (duckweed),can shade out the rooted macrophytes. Thus, the interaction between the physical, chemical and biotic environment is extremely intricate and is made infinitely more complex by the presence of pollutants, be they natural or anthropogenic. Therefore, it is of some interest and concern that these relationships be investigated in order that any actual or potential damage can be recognised and the stage that it has reached identified.
Any investigation of the health of ecosystems and the environment generally must involve quantitative measurements at some point and herein lies the problem. It is not so much the actual taking of measurements, although there are times when this can be difficult enough, but rather interpreting the measurements. Thus, if we consider some of the chemical and physical parameters commonly associated with the health of aquatic ecosystems we can see that these are factors which can change quite considerably with time. For example, temperature and light penetration are functions of climatic/meteorological conditions. Nitrate levels and pH values show diurnal and seasonal variations. Nitrate levels rise in winter and drop in summer, reflecting the activity of living organisms, while pH values rise during the hours of sunlight and in summer, but drop during the night and in winter, as photosynthesis waxes and wanes.
Even if it can be established that pollution is present, it is still not clear to what extent an ecosystem has been affected. A lot will depend on
how much pollutant is present,
which, if any, of the organism/communities it affects,
over what period of time it has been entering the environment
and how long it remains within a particular environment.
Given this situation it is not surprising that, with respect to aquatic environments, simple measurements of physical and chemical parameters by themselves are not particularly informative. Some investigation of the aquatic communities is necessary and, since populations are subject to natural variations (eg on a seasonal basis), it follows that such investigations should be extended over periods of time.
It must be stressed that the important considerations in this type of investigation are biological diversity and species abundance. In most healthy systems there is a wide range of species (high diversity) and a population pyramid (ie comparatively large numbers of producers and herbivores and increasingly fewer carnivores as food chains are ascended. In damaged systems diversity tends to be lower as certain significant communities are missing. The dominant species are often detritivores, scavengers and/or opportunists and their populations, especially those of micro-organisms can be huge.
2. Choice of Biological Parameters
As mentioned above, to know such chemical parameters as dissolved oxygen, BOD and pH of a water body is not, by itself, sufficient to conclude that the water body is damaged. Similarly, to determine these parameters for an effluent does not enable us always to predict the effects of that effluent on the flora and fauna of a river or lake. Only direct examination of the biota can show what those effects may be.
But what form should this examination take? Looking at every single type of organism is obviously not practicable. It would take an inordinate amount of time to catch and count every animal in a particular stretch of water. Besides which some of the more mobile species can wander far from their ‘home’ ground and so their absence or presence can be the source of ambiguity. Therefore, it is necessary to identify so-called indicator species the presence or absence of which is a direct reflection of water quality and not due simply to the vagaries of chance. Also, such species must be easily observed and counted. Possible groups of organism which might be examined are
Vertebrates, including fish, amphibians and mammals.
Vertebrates: In general these do not make satisfactory indicators of pollution and ecosystem damage. They are obviously affected by pollutants and disturbance but most are difficult to observe, difficult to catch and they are less abundant than smaller organisms. In addition, their very mobility, especially in the case of fish and mammals, can mean that even if such an animal is observed in a particular location, it may just be passing through, being some distance from its usual habitat.
Micro-organisms: The inhabitants of severely polluted water are almost exclusively micro-organisms. However, they are not easy to sample quantitatively since the procedure is time-consuming, it requires certain level of expertise and specialised equipment. Furthermore, actually identifying micro-organisms is not easy and is a very specialised skill.
Invertebrates: This group provides the most useful indicator species. Many aquatic invertebrates are relatively slow moving or sedentary. They are easy to collect and they are fairly easy to identify, at least down to family level which is usually sufficient. They are readily preserved so that the process of identification can be undertaken in relative comfort off-site.
Now, pollution affects the abundance and distribution of animals and plants in aquatic systems. Consequently waters can be classified according to the distribution and abundance of the macro-fauna within them. In general, pollution appears to restrict the variety of organisms present. Thus it tends to suppress certain key species but this, in turn, leads to a large increase in numbers of pollution-tolerant species; primarily the opportunists, scavengers and detritivores as their more sensitive predators either die or move away. As the degree of pollution in an aquatic system increases so the key organisms disappear in the following order
Plecoptera (stoneflies): Ephemeroptera (mayflies, damsel flies etc): Trichoptera (caddis flies): Gammarus (freshwater shrimp): Asellus (water hog louse): Chironomidae (‘blood worms’): Oligochaeta (tubificid worms).
Therefore, investigations into the possible effects of pollution on aquatic communities tend to concentrate on identifying and counting the members of these groups of animals.
3. Biotic Indices
Having identified and counted the appropriate organisms the data so obtained has to be processed in some way. Various procedures are available for this, most leading to the construction of biotic indices. An index so calculated is then usually compared to a scale of values relating to differing degrees of damage. Most biotic indices are quick and easy to construct, but they are only worthwhile if they form part of a wider survey or monitoring programme. Also, they must take into account the following points.
The presence or absence of an organism must be a function of water quality and not some other ecological factor.
The system so devised must assess water quality in a reliable manner, be expressible in a simple form yet be sufficiently quantifiable to allow for comparisons to be made between measurements.
The assessment should relate to water quality conditions over an extended period of time rather than applying only to the time of sampling.