ENVIS Technical Report: 25

The physical and chemical characteristics of water affect the abundance, species composition, stability and productivity of the indigenous populations of aquatic organisms. The biological methods used for assessing water quality include collection, counting and identification of aquatic organisms; and processing and interpretation of biological data. The work involving plankton analysis would help in:

Explaining the cause of colour and turbidity and the presence of objectionable odour, tastes and visible particles in waters.
The interpretation of chemical analyses.
Identifying the nature, extent and biological effects of pollution.
Providing data on the status of an aquatic system on a regular basis.

Plankton: A microscopic community of plants (phytoplankton) and animals (zooplankton), found usually free floating, swimming with little or no resistance to water currents, suspended in water, non-motile or insufficiently motile to overcome transport by currents, are called "Plankton". Phytoplankton (microscopic algae) usually occur as unicellular, colonial or filamentous forms and is mostly photosynthetic and is grazed upon by the zooplankton and other organisms occurring in the same environment. Zooplankton principally comprise of microscopic protozoans, rotifers, cladocerans and copepods. The species assemblage of zooplankton also may be useful in assessing water quality.

Plankton, particularly phytoplankton, has long been used as indicators of water quality. Because of their short life spans, planktons respond quickly to environmental changes. They flourish both in highly eutrophic waters while a few others are very sensitive to organic and/or chemical wastes. Some species have also been associated with noxious blooms causing toxic conditions apart from the tastes and odour problems.

Plankton net: The plankton net is a field-equipment used to trap plankton. It has a polyethylene filter of a defined mesh size and a graduated measuring jar attached to the other end. A handle holds the net. The mesh size of the net determines the size range of the plankton trapped

Sampling Procedure: Plankton net number 25 of mesh size 60 µm was used for collecting samples. 100 litres of water was measured in a graduated bucket and filtered through the net and concentrated in a 100 ml bottle. Samples were collected as close to the water surface as possible in the morning hours and preserved for further analysis.

Labelling: The samples are labelled with the date, time of sampling, study area-lake name, sampling site name and the volume measured and pasted on the containers.

Preservation of the sample: Between the time that a sample is collected in the field and until its analysis in the laboratory, physical, chemical and biochemical changes may take place altering the intrinsic quality of the sample. It is therefore necessary to preserve the samples before shipping, to prevent or minimise changes. This is done by various procedures such as keeping the samples in the dark, adding chemical preservatives, lowering the temperature to retard reactions by freezing or by a combination of these methods. For a phytoplankton sample to be analysed for an extended period, commonly two preservatives are used: Lugol’s iodine using acetic acid which will stain cells brownish yellow and will maintain cell morphology and of 4% formaldehyde. The samples collected for this study were preserved by adding suitable amounts of 1 ml chloroform to act as the narcotising agent and 2ml of 4% formalin for preservation and analyses.

Concentration technique: The plankton nets were used to collect samples for the qualitative and quantitative estimation of the plankton, by filtering a known volume of water (100 litres) through the net. The sample was allowed to settle for 24-48 hours and was further concentrated to approximately 30 ml by decanting. The concentration factor is used during the calculations.

Qualitative and quantitative evaluation of plankton: Detailed analyses of phytoplanktonic populations are done by estimating the numbers in each species. The phytoplankton consisting of individual cells, filaments and colonies are counted as individual cells. When colonies of species are counted, the average number of cells per colony is counted, and in filamentous algae, the average length of the filament has to be determined.

Mounting the slides: Preserved samples in bottles are mixed uniformly by gentle inversion and then one drop of the sample is pipetted out from a calibrated pipette onto the glass slide for analysis. A cover slip is carefully placed ensuring no air bubbles remain and the cover slip is ringed with a transparent nail enamel to prevent evaporation during the counting process.

Microscope: A binocular compound microscope is used in the counting of plankton with different eyepieces such as 10X and 40X. The microscope is calibrated using an ocular micrometer.

Counting method: Drop count method – In this method one drop of the sample is pipetted out from a calibrated pipette onto a glass slide and the planktonic organisms are counted in strips. The total area under the cover slip represents the number of organisms present per given volume of the sample. This volume expanded to an appropriate factor yields the organisms per litre of water for the lake.

Phytoplankton Counting Units: Some plankton are unicellular while others are multicellular (colonial), posing a problem for enumeration. For analysis, a colony of plankton is accounted as a single count.
Formula for calculating organisms per litre:
                                     Total plankton count per litre = A * (1/L) * (n/v)
Where, A = number of organisms per drop
            L = volume of original sample (l)
            n = total volume of concentrated sample (ml)
            v = volume of one drop (ml)