6. RESULTS AND DISCUSSION

In diversity studies the sampling is done so as to represent entire population and to avoid the sampling errors. Plankton population estimate depends on choosing optimal quantity of water for filtering. Earlier studies concerned with zooplankton diversity have filtered water less than 100 L. In order to standardise and to arrive at an optimal volume, an experiment was conducted at different volumes namely 25, 50, 75, 100, 125 and 150L. The results show that 100 L is the appropriate sampling volume.

Standardization of volume of water to be filtered for zooplankton analysis

The experiment to standardize volume of water to be filtered for zooplankton analysis was performed. The results presented in tables 6 to 9 and figures 10 to 13 depict the standardization of volume of water to be filtered for zooplankton analysis. The species-volume curve reveals that there is no further increase in the number of species even after the filtration volume was increased to 125 L and 150 L. This suggests that the optimum volume of water to be filtered for zooplankton analysis is 100 L were all the species in the area where the samples are collected have been sampled. Thus, with respect to this result the volume of water filtered for zooplankton analysis in the four lakes is 100 L. However, the organisms per liter in the 25 L sample is found to be highest in both the experiments since the 25 L sample is the first sample taken in the whole procedure.

Figure 10: Experiment 1: Relation between species richness and volume of water filtered.

Figure 11: Experiment 1: Number of organisms obtained for different volumes of water filtered.

Figure 12: Experiment 2: Relation between species richness and volume of water filtered.

Figure 13: Experiment 2: Number of organisms obtained for different volumes of water filtered

In order to know the water quality of the selected lakes a biomonitoring approach has been undertaken, which includes both physicochemical and biological analysis. The physicochemical and biological analysis of Sankey lake, Ulsoor lake, Rachenahalli lake, and Tippagondanahalli lake was carried out as discussed in the earlier section (Chapter 5). The next section discusses the results for physicochemical analysis.

PHYSICOCHEMICAL ANALYSIS

The results for the physicochemical analysis of water samples from different sampling points in Sankey lake are given table 10. The lake is alkaline nature with pH ranging from 8.45 to 8.61. The dissolved oxygen levels at sampling points 2 (5.42 mg/L) and 3 (5.32 mg/L) are slightly lower than the desirable limits cited in table 5, because of the entry of organic matter through runoff. The dissolved oxygen level in the sampling point 1 (6.53 mg/L) is higher compared to the other sampling points because of the turbulence created by the flow from inlet. The calcium and magnesium hardness of water are high ranging from 144.9 mg/L to 149.4 mg/L indicating the hard water. The water is also contaminated by faecal matter in all the sampling points, thus an accidental intake may lead to water borne diseases.

The results for the physicochemical investigations of water samples from different sampling points in Rachenahalli lake are presented in table 11. The pH of the lake is alkaline in all sampling points ranging from 8.53 to 9.02. The dissolved oxygen levels in the lake was low particularly in sampling point 3 (4.1 mg/L), where most of the physicochemical parameters were higher when compared to other sampling points. The level of sodium in the lake ranged between 157.74 mg/L to 196.04 mg/L and that of potassium was found to be in the range of 90.84 mg/L to 114.17 mg/L, which are very high and may be attributed to the runoff from agricultural fields. This may also be due to the concentration of chemical constituents present in the lake water, which is drying up. The high potassium, sodium and chloride (114.71 to 128.07 mg/L) contents have thus made the lake water not preferable for irrigational purpose, as it would cause damage for the crops (water intake by the roots is blocked). All the samples in the lake indicate the presence of Coliforms suggesting faecal contamination.

The results for physicochemical analysis of water samples from different sampling points in Ulsoor lake are presented in table 12. The pH of the water samples from all the sampling points are alkaline in nature with sampling point 3 (8.96) and 4 (8.62) exceeding the tolerance limits given in Table 5. The dissolved oxygen level at sampling point 1 (2.05 mg/L) which is near the Madras Engineering Group (MEG) camp is well below the tolerance limits due the organic loading from the sewage entering the lake. The sampling point 2 (5.14 mg/L), which is the center of the lake, is also low in dissolved oxygen levels. The dissolved oxygen level in sampling point 3 is higher because of the aerated sewage entering the lake. The heavy metals level, particularly that of chromium and mercury, are higher than the tolerance limit, which reflect the heavy metal pollution in the lake due to the possible entry of industrial effluents. The lake water is also faecal contaminated except in Sampling point 4.

The table 13 represents the physicochemical characteristics of the water samples in Tippagondanahalli lake. The pH of the lake is alkaline in nature for all the sampling points ranging from 8.7 to 9.18. This may due to the nature of the soil in the catchment area. The dissolved oxygen level in the sampling points 1, 2, 3 and 5 ranging from 3.07mg/L to 3.72 mg/L is below the desirable limits shown in table 5 and that of the sampling point 4 (8.23 mg/L) is relatively higher than the desirable limits. This may be due to the less turbulence in the sampling points 1, 2, 3 and 5, which has resulted in the less contact between the atmosphere and the water surface resulting in low dissolved oxygen content owing to the less solubility of oxygen in water. Relatively higher concentration of oxygen in the sampling point 4 is due to enough turbulence created in the region. All of the sampling points indicate the presence of coliforms because of the faecal droppings from the high bird population.

Thus the physicochemical analysis of all the four lakes reveals that pH of the lakes is alkaline. The total hardness of the lake waters indicates that Sankey lake and Rachenahalli lake are hard. The dissolved oxygen content in all the lakes are low but the factors that contribute to low dissolved oxygen content varied from lake to lake. Thus from the physicochemical analysis it is understood that Ulsoor and Rachenahalli lakes are polluted. The Rachenahalli lake is polluted due to the entry of agricultural runoff and shoreline degradation, whereas the Ulsoor lake is polluted due to entry of sewage and industrial effluents.

ZOOPLANKTON ANALYSIS

The zooplankton analysis was carried out after standardization of volume of water to be filtered and by using standard methods as given in chapter 5. The identification of zooplankton was done to the taxonomic precision of species level in rotifera and genus level in both cladocera and copepoda using self-made keys and standard keys given Appendix 2.

The results shown in tables 14 & 15 reveal that the zooplankton community in surface waters of Sankey lake is comprised of Rotifera, Cladocera and Copepoda. A total of 23 species of zooplankton is obtained in the study. All the groups are equally distributed in terms of number of species though nauplii and copepodids, the developmental stages of Copepoda, were found to dominate among the Copepoda. Thus, the zooplankton analysis for Sankey tank suggests that the health of the community is undisturbed.

The zooplankton analysis of Rachenahalli lake is given in tables 16 & 17. The phytoplankton analysis has also been carried out for this lake. Some members of Chlorophyceae have been recorded. The zooplankton community in Rachenahalli lake is comprised of Rotifera, Cladocera and Copepoda, though pollution sensitive group Cladocera is found to be less species rich with only two species representing the whole group. The rotifers were found to be most species rich group among the zooplankton recorded. The abundance of Keratella tropica, which is a pollution tolerant species contributed to the increase in the total number of organisms per liter in sampling point 3 (figure 14). This supplements the physicochemical data obtained and highlights the influence of water quality in determining the structure of the zooplankton community. This suggests that the sampling point 3 in particular and the lake in general is polluted and thus affecting the diversity of the zooplankton community.

Figure 14: Abundance of Keratella tropica in Sampling point 3 of Rachenahalli lake.

The results for the qualitative and quantitative analysis of zooplankton of Ulsoor lake given in table 18 & 19 respectively shows that the zooplankton community in the lake is represented by only two groups which include Rotifera and Cladocera. The absence of Copepoda, which is the major food for planktivorous fishes and absence of zooplankton community in the inlet and outlet of the lake reveals that the entry of sewage has an adverse effect on the trophic structure of the lake and contributed to the death of fishes in the lake, which was reported on January 26 th in leading news papers.

The results for the qualitative and quantitative analysis of zooplankton of Tippagondanahalli lake given in tables 20 & 21 shows that the zooplankton community in the lake is comprised of Rotifera, Cladocera and Copepoda. The zooplankton population in the lake is very less in numbers and species richness. This may be due to the lesser inflow of nutrients into the lake leading to lesser primary production, which has resulted in the less food for zooplankton consumption. Hence, the lake is in oligotrophic condition.

Table 22 reflects the Shannon-Weiner index for zooplankton diversity in the lakes studied. The Shannon-Weiner index reveals that Sankey tank is more diverse where as Rachenahalli is the least diverse among the lakes followed by Ulsoor.

Table 23 shows the species composition of study lakes and their pollution status. Each lake has been classified according to the physicochemical and biological data obtained during the study period. This indicates that among the lakes surveyed Ulsoor lake is heavily polluted and Rachenahalli lake is also polluted. Sankey lake was found to be in a mesotrophic condition supporting a diverse fauna of zooplankton whereas Tippagondanahalli lake was oligotrophic in nature with less primary production and hence sparse zooplankton population.

The results for the biomonitoring of selected lakes shown in table 24 shows that only two of the lakes among the four studied are suitable for the intended usage. Ulsoor lake and Rachenahalli lake were polluted due to different sources. The industrial effluents and sewage contributed to the pollution level in Ulsoor and agricultural runoff affected the Rachenahalli lake. Physicochemical analysis has revealed the causes and levels of pollution whereas the biological analysis has revealed the extent of damage done to the biota due to pollution.