The aquatic environment can be affected by both natural and anthropogenic events. These may include the addition of synthetic substances to the water or some physical or chemical change that alters the hydrological regime.
Most organisms that live in a water-body are sensitive to environmental change, whether it is increased turbidity, chemical contamination or decreased dissolved oxygen caused by the introduction of sewage. These organisms may die, migrate, or they may reproduce or metabolize differently. What is important is to be able to identify these changes, through measuring and monitoring, in order to relate them water quality.
Note that in most cases, indices only work well for the water bodies in the regions in which they were developed, and may give anomalous results in other types of water body.
The basis of any aquatic eco-system is water and the quality of the ecosystem may be determined by the quality of the water. Biotic indices can give an indication of the quality of the water.
Biotic Indices, which are based on the presence or absence of particular indicator groups or species at the sampling point, are best suited for waters polluted with organic matter such as sewage. However, they have been recently developed for acidification of streams and lakes. Indices may be based upon the number of invertebrate fauna that are collected, identified and then counted.
Community Structure Indices
Each group is given a score, according to abundance, with the total score representing the index, and with the higher the score the cleaner the water.
Diversity indices attempt to combine the data on species abundance in a community into a single number. Stable eco-systems are characterized by a great diversity of species, most of which are represented by relatively few individuals. However, high numbers of individuals of only a few species may occur in instances where physical or chemical factors restrict habitat range.
Differences in species diversity may indicate change in water quality, as long as all natural factors are comparable from one sampling point to another. This method is quite suitable for benthic organisms which are relatively resistant to river currents.
Similarity Indices are obtained by comparing two samples, one of which is a control sample from a clean-water sampling station. This method is quite suitable for point sources of pollution in a river where samples can be taken upstream and downstream of the input. Organisms used as indicators.
Biological indices are usually specific for certain types of pollution. This is because organisms are not all equally sensitive to all types of pollution. Typical organisms are:
Macroinvertebrate populations remain quite popular though as they can be more easily and reliably collected, handled and identified. (See Fig. 1)
Typical Changes in the Aquatic Environment.
Fig. 2 depicts different situations of change in the aquatic environment, as follows:
A & B : Physical and chemical changes,
C : Changes in micro-organism population,
D: Changes in invertebrate populations.
Aquatic life depends upon oxygen. Where there is a change in dissolved oxygen, for example at the bottom of nutrient rich lakes, or at night in slow moving rivers, some species have adapted to low concentrations of oxygen. While they may be rare under normal conditions, their presence, especially if widespread, indicates pollution or nutrient enrichment. The basis of some biotic indices is formed by the ability of organisms to survive different levels of oxygen depletion in water.
Some water bodies may be located in catchment areas where the rocks are acidic and some organisms are sensitive to the acidity / alkalinity of water. Additional acid input from ‘acid rain’ may produce a drop in pH. This generally results in a reduction of fish...