Detailed information of the status of pollution of any water body is of much importance because it ultimately helps in the proper management of these waters. A physico-chemical approach to monitor water pollution is most common and plenty of information is available on these aspects. Such data usually provides a mosaic picture of the whole scenario. Chemical analysis is valuable and necessary, but does not provide all the information required in pollution assessment. It is not the contamination that are concerned, but rather the effects of their concentrations on organisms. It is only by documenting these effects that the true effects are defined. Analysis of a particular chemical is directed towards a specific contaminant and its expression will be a union of other contaminants that is not in the suspected list. It is thus possible that a key toxic contaminant may be overlooked and so a supplementary pollution study is most essential.

One of the most striking features of the past water assessment procedures has been reliance placed upon physical and chemical techniques; with relative neglect of biological parameters. Since water pollution is in many instances a biological phenomenon, it would appear logical that it ought to be measured biologically. Biological indicators show the degree of ecological imbalance that has been caused and chemical methods measure the concentration of pollutants responsible. The majority of lake systems of biological assessment have been devised mainly to deal with conditions arising out of organic pollution. Chemically the effects to measure all the organic pollution are rather difficult to monitor. It is not possible to measure all the organic compounds directly and even though the BOD test provides an insight into their rates of oxygen consumption, this only measures one component of the complex oxygen balance. Assessment of solids pollution in lakes that receive continuous sewage can be very damaging mainly due to siltation, which may be considerably exacerbated by deoxygenation when the solids are organic. One of the difficulties in making the exact assessment of organic pollution is the lack of any absolute scale of measurement.

Algae consume a number of elements from water during their growth, some in sufficient quantity to be readily absorbed. Of these, nitrate, phosphate and bicarbonate are significant. Algae utilise bicarbonate directly or indirectly, by its disassociation into carbon dioxide and in so doing cause an increase in pH of the water and precipitation of calcium carbonate (Langliers index). Some waters are supersaturated with bicarbonate but is stable until a threshold pH is achieved, and a massive precipitation of calcium carbonate is initiated. Possibly this is the ability of bluegreen algae to do this, that within a dense surface bloom an intense carbondioxide demand occurs. This leads to a very local, but high pH, which is sufficient to trigger the precipitation of calcium carbonate. Once initiated this can occur throughout the lake. Therefore biological studies not only help in a realistic assessment of pollution but it also assists in other ways.

During recent times, lakes are becoming the victim of cultural eutrophication which in turn is due to increased anthropogenic pressure in their catchment area, increased utilisation of domestic articles, agricultural wastes and fertilisers thus affecting the quality of raw water. Silt deposits in lakes are increasing and are choked due to excessive growth of algae. A fresh water body plays a   significant role in ecology and exhibits relationship with the organisms that produce, transform and use it. Therefore an integrated approach towards indicators of organic pollution in freshwaters is discussed. The advantage of each method is also indicated.

Biological indicators of water quality are

•  Saprobic index

•  Nyggard's index

•  Palmer's algal pollution index

•  Biological index of pollution

•  Inhibition threshold for dehydrogenase activity

•  Colilert defined substrate test

•  H 2 S strip test: Detection of fecal pollution in water

•  Saprobic Index (Kolkurtz and Marrson): The saprobic index was introduced by Pantle and Buck (1955) and has the following calculation: S= is S (h Si)/ £ where Si is the individual Saprobic index for each species and h is the abundance according to sale of abundance. In the calculation, each species is given a number according to the group to which it belongs.

Range of average values of the extended saprobic index “S”

Comparing the results with some data given by the chemical and bacteriological analysis, individual saprobic levels can be calculated.

•  Nygaard's algal index

•  Palmers algal Pollution Index: Pollution index of algal species

Calculation: If a sample is having the species Chlorella, Synedra, Navicula, Oscillatoria, Euglena; the score according to the above table is 4+3+1+1+6 = 15. This confirms high organic pollution of the sample.

•  Biological Index of Pollution

BIP = A/A+B (x) 100

Where A is the number of chlorophyll bearing organisms

            B is the number of non-chlorophyll bearing organisms.

•  Determination of the inhibition threshold for Dehydrogenase activity (Bursteeg and Thiele, 1935)

   Inhibition of toxic substances caused by toxicants in water:

•  The presence of the dehydrogenase activity indicates living cells

•  The absence of dehydrogenase activity indicates dying cells (cells refer to Bacteria)

•  Colilert defined substrate test (Coliform and Fecal coliforms)

A specialised medium containing ONGP (O-nitrophenyl- b -D galactopyranoside) and MUG (4-methylumbelliferyl- b -Dgluceronide) as only nutrients is used. 100 ml sample is added to this medium in small screwcap bottles, closed tightly and incubated at 35 ° C for 24 hours. If coliforms are present the medium turns yellow within 24 hours. If these bottles are observed under long UV light and if they emit fluorescence, the E.coli that cause faecal contamination is present.

•  H 2 S strip test detection of Fecal pollution in water

(Medium contains: Peptone, Dipotassium hydrogen phosphate, Ferric ammonium citrate, sodium thiosulphate, 1 ml Teepol and 50 ml distilled water)

1ml concentrated medium is absorbed on folded tissue paper; the tissue paper is kept in dry sterilised bottles; 20 ml of water sample to be tested is poured into this bottle; it is incubated at 30 - 37 ° C for 18 hours; if the water is contaminated by sewage the contents of the bottle turns black within 24 hours.

The reason is that Enterobacter and E.Coli produce H 2 S gas that reacts with Ferrous ammonium citrate to turn black.

•  General indicators

The presence of large number of desmids is an indication of potable water. The presence of blooms of Microcystis aeruginosa is an indication of phosphate pollution

The suitability of an index for a particular situation and its usefulness in the interpretation of results depends on the discretion of a trained biologist.

Department of Studies in Botany,
University of Mysore,
Manasagangotri,Mysore – 570 006,
Karnataka. India.