Results and Discussion

The sustained inflow of untreated sewage into the Ulsoor Lake has enriched the aquatic ecosystem with nutrients. Water temperature at the time of sampling ranged from 25.7° – 29.7°C. All metabolic and physiological activities and life processes such as feeding, reproduction, movement and distribution of aquatic organisms are greatly influenced by temperature (Trivedy and Goel, 1986). An increase in the temperature, enhances the metabolic activities and reduces the solubility of gases like dissolved oxygen and carbon dioxide in the water. pH of the water samples ranges from 7.88 – 9.29 indicating alkaline conditions. The pH of aquatic ecosystems fluctuates during photosynthesis, respiration and nitrogen assimilation. pH is also governed by the equilibrium between carbon dioxide or bicarbonate or carbonate ions. pH increase due to increased photosynthetic processes (uptake of carbon dioxide with the release of oxygen) and decreases during high rate of respiration/ decomposition (consumption of oxygen with the release of carbon dioxide) (Ramachandra and Ahalya, 2001).

Sustained inflow of untreated sewage at the inlet has impacted the integrity of water evident from higher amount of TDS, EC, turbidity, BOD, COD, alkalinity, hardness, nutrients, sodium and potassium. The increase or decrease in TDS and EC are attributed to the ions concentration - calcium, magnesium, sodium, and potassium (cations) and carbonate, bicarbonate, chloride, sulfate, and nitrate (anions). High values of TDS are well correlated with the EC of lake waters and can have negative influence on the biological production efficiencies of lake ecosystems (Kiran and Ramachandra, 1999). Higher turbidity in lake water indicates the presence of higher concentrations of organic and also non-biodegradable components. Turbidity of the lake water, along with its warm temperature, alkaline pH and low oxygen levels, could lead to prolonged survival of pathogenic bacteria for up to several days. Since, there is a high demand of oxygen (as indicated by higher values of BOD and COD) in sewage as well as lake water. Lower values of dissolved oxygen indicate of organic contamination. Higher values of chloride and total hardness (also calcium and magnesium hardness) in lakes are attributed to the regular addition of sewage and detergents. Nitrogen and phosphate are limiting factors for primary production. Phosphorus occurs mainly as orthophosphate. Nitrogen occurs mainly as nitrate, nitrite, ammonia and as ammonium ions. The major sources of ortho-phosphate and nitrate are domestic sewage, detergents, agricultural runoff and industrial wastewater. The high nutrient contents (nitrate and ortho-phosphate) had supported algal growth in the Ulsoor Lake. The presence of higher amount of different physico-chemical parameters like total dissolved solids (217 – 454 mg/l); electrical conductivity (436 - 815 µS); chemical oxygen demand (14 - 278 mg/l); biochemical oxygen demand (8.13 – 93.5 mg/l); alkalinity (189.33 – 510.67 mg/l); chloride (73.37 – 109.81 mg/l); total hardness (95.33 – 214  mg/l); calcium hardness (18.7 – 50.5 mg/l); magnesium hardness (18.3 – 39.73 mg/l); nitrate (0.302 – 1.664 mg/l); phosphate (0.27 – 2.24 mg/l) sodium (89.6 – 125.2 mg/l) and potassium (24.4 -29.6 mg/l) and lower values of dissolved oxygen (0 – 5.69 mg/l), indicate pollution due to untreated sewage entry (table 4) and Table 5 lists CPCB standards and Ulsoor lake falls under Class E of Inland Surface Water.

Acidic waters with pH <6.5 and alkaline waters with pH >9.5 retards reproduction and growth of fish and could lead to diseases. Low DO retards intake of food and growth in fishes. The most suitable value of total hardness for fish culture is said to be in the range of 75 – 150 mg/L. Optimum total hardness prevents the outbreak of common diseases in fish. High accumulation of humic substances through the decomposition of profuse organic matter and macro-vegetation allows the harboring of a higher number of disease producing organisms than is possible in clean waters, having transparency >20cm (Kar, 2016).

Excess quantity of ammonia in surface waters impair aquatic biota, is an indication of eutrophication. Some key environmental factors that control nitrification include dissolved oxygen (DO), temperature, substrate concentration and pH (Allen et al., 2010). Ammonia release under anoxic conditions is mediated by 2 predominant mechanisms namely (i)  lack of biological nitrification activity (biological oxidation of ammonia to nitrate in sediments becomes impossible) and (ii) low rates of ammonia assimilation (by slow-growing anaerobic bacteria in sediments). Some of the nitrate may be lost from the aquatic ecosystem via subsequent de-nitrification in anoxic subsurface sediments. The toxicity of un-ionized ammonia is a function of pH, temperature, alkalinity and total ammonia concentration measured at the gill surface. Ammonia at elevated pH and temperature, which shifts the ionization equilibrium to unionized gaseous form, which is toxic to fish. Ammonia toxicity is apparent by hyperactivity, convulsions, loss of equilibrium, lethargy and coma. The ammonia toxicity in aquaculture ponds results in the sub-lethal reduction of fish growth or suppression of immuno-competence, rather than as acute toxicity leading to mortality (Hargreaves, 1998).
The sewage water containing organic components, human waste (urine and faeces), phosphate, nitrate and detergent phosphate when discharged into the water bodies causes eutrophication of lakes. Detergents can be easily absorbed from the surrounding water either through by gills or intestinal epithelium of fishes and due to their potential toxicity induces histological and biochemical alteration in the organs of fishes. The synthetic detergents can also alter pH and salinity of receiving freshwater body, which affect oxygen consumption by aquatic organisms including fishes (Pattusamy et al., 2013). The predominant mechanism responsible for phosphorus release from sediments under anoxic conditions is the microbial reduction of phosphate-containing iron-oxide complexes (utilizing ferric iron in sediment as an electron acceptor) resulting in a release of ferrous iron and phosphate into overlaying water (Beutel, 2006).

Figure 4: Phyoplankton in Ulsoor lake: a) Pediastrum sp.; b) Actinastrum sp.; c) Microcystis sp.; d) Trachelomonas  sp. and e) Cyclotella sp.

Zooplanktons are the primary consumers occupying the second trophic level in the food chain after phytoplankton. Zooplankton diversity responds rapidly to changes in the aquatic environment, serves as bioindicators and are being used in the investigation of water pollution. Four groups of zooplanktons found In Ulsoor lake are Rotifera, Copepoda, Ostracoda and Protozoa. The distribution of zooplankton population varied among sites and were in the order: Copepoda > Rotifera  > Protozoa > Ostracoda (figure 5 and 6).

Predominant fish species that were impacted due to water contamination with the sustained flow of nutrient rich sewage and industrial effluents are Tilapia nilotica, Puntius ticto, Labeo rohita, Catla catla and Gambusia affinis (figure 7 and table 6). Among these, Puntius ticto has higher mortality.

Table 6: Impacted fish species of Ulsoor lake

 P. ticto a small fish commonly known as “ticto” or “two-spot” barb is found in fresh and brackish-water subtropical species. It is the most popular aquarium fish among barb species in Asian countries. Ticto barb inhabits still shallow, marginal waters of tanks and rivers, mostly with muddy bottoms. It feeds on crustaceans, insects, plankton, plants and other benthic invertebrates (Hossain et al., 2015).

Mortality of fish species was mainly due to lowered dissolved oxygen levels, which needs to be improved on priority in Ulsoor Lake through the introduction of aerators (water fountains or introduction of ducks). Aeration increases DO levels causing fish to be less stressed. It also removes hydrogen sulphide, methane and various volatile organic compounds responsible for bad taste and odour in lakes. Aeration improves the quality of water and decreases the treatment costs. Aeration also provides an aerobic environment for the degradation of organic matter by microorganisms (Ramachandra et al., 2015).

Conservation programmes help in maintaining diversity while enhancing the scope for sustainable production. Conserving diversity also improves the likelihood by maintaining minimal viable populations of rare and late-successional species. Diversity reduces instances of pathogens attack and allows recovery from disturbance. Some measures such as de-siltation, letting only treated sewage to the lake would lead to appreciable improvement in the water quality (parameters like pH, dissolved oxygen, etc.), and quantum jump in fish yield within a short period (Kar et al., 2006). Sustainable wetland management mainly include strategies to control of invasive species, encroachment, drastic land cover changes in the catchment and identification of buffer zone, providing aquatic resources with adequate water quality and limiting the spread of exotic biota (Ramachandra et al., 2011).