ENVIS Technical Report: 25 |
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ECOLOGICAL ASSESSMENT OF LENTIC WATER BODIES OF BANGALORE T. V. Ramachandra and Malvikaa Solanki
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ECOLOGICAL ASSESSMENT OF WETLANDS The overall condition or health of aquatic ecosystems is determined by the interaction of all its physical, chemical and biological components, which make up its ecosystem. Information on and understanding of environmental change is necessary to allow for the protection and remediation of ecosystems. Ecological assessment by way of analyses of all the components of the ecosystem helps in arriving at restoration methods towards the conservation, management and sustainable use of natural resources. Assessment of water quality Water pollution may be defined as the presence of impurities in such quantity and of such nature as to impair the use of water for a stated purpose. Thus, the definition of water quality is predicted on the intended use of the water, and the gross determination of the quantity of suspended solids and dissolved impurities, while useful in some cases, it is not sufficient to completely define water quality (Howard S. Peavy., Donald R. Rowe., and George Tchobanoglous, 1985 33). Many parameters have evolved that qualitatively reflect the impact that various impurities have on selected water uses. Analytical procedures have been developed that quantitatively measure these parameters. APHA’s Standard methods for the Examination of Water and Wastewater [2-15] have been one of the standard test procedures. Many methods and criteria are available to assess aquatic ecosystems. A physico-chemical approach to monitor water pollution is most common and plenty of information is available on these aspects. Such data is valuable and necessary but does not provide all the information required in the assessment of water quality of the waterbody. It is not the pollution or the contamination alone that is the concern in water quality, rather the effects on the organisms. It is only by documenting these effects that the true impacts are defined. One of the most striking features of the past water assessment procedures has been the reliance placed upon physical and chemical techniques with relative neglect of biological parameters. Since water pollution in many instances is a biological phenomenon, it would appear logical that it ought to be measured biologically (Ramachandra T.V. Ahalya N., and Rajasekara Murthy C, 2005 34). Traditionally, water quality monitoring actions have focused on physical and chemical measurements. It is widely recognised that the use of other indicators, in addition and complimentary to traditional chemical and physical water quality monitoring techniques, can greatly enhance the assessment and management of aquatic ecosystems. In this regard, biological monitoring or biomonitoring has proved to be an important tool in assessing the condition of aquatic ecosystems. Biological methods used for assessing the water quality include the collection, counting, and identification of the aquatic organisms (APHA, 1985). Biomonitoring in conjunction with physical and chemical observation of water quality is potentially useful in characterising waterbodies. Chemical data measure concentration of pollutants, etc. in the waterbody, and the ecosystem imbalances are measured by biological information. Biological and chemical data are essential in understanding the ecosystem integrity. Physico Chemical Assessment 35, 36, 37, 38 Physical character of lakes such as size, depth, number and the size of inflowing and out flowing streams and shoreline configuration influence the character of the lake. They also influence decisions about sampling locations, water quality parameters and how to interpret data collected. Shallow lakes are more likely homogenous, i.e. they are the same from top to the bottom and water is well mixed by wind. Physical characters like the temperature and oxygen vary little with depth. Sunlight reaches all the way to the lake bottom, photosynthesis and growth occurs throughout the water column and thus the growth rate or productivity is higher. Physical parameters define those characteristics of water that respond to the sense of sight, touch, taste or smell. Suspended solids, turbidity, colour, taste, odour and temperature fall under this category. Chemical parameters are related to the solvent capabilities of water. Total dissolved solids, alkalinity, hardness, fluorides, metals, organics, and nutrients are chemical parameters of concern in water-quality management. Some of the important physical and chemical parameters, few of which are estimated in this study are discussed below. Temperature: Temperature exerts a major influence on the biological activities and growth. To a certain point the increase in temperature leads to greater biological productivity, above and below which it falls and it also governs the kind of organisms (species composition). At elevated temperatures metabolic activity of the organisms’ increases, requiring more oxygen but at the same time the solubility of oxygen decreases, thus accentuating the stress. Temperature influences water chemistry, e.g. DO, solubility, density, pH, conductivity etc. Water holds lesser oxygen at higher temperatures. Some compounds are more toxic to aquatic organisms at higher temperatures. Additionally temperature of drinking water has an influence on its taste. Temperature is expressed in Celsius and a thermometer- 0.1o C division is used to measure temperature. Total solids, Total suspended solids & Total dissolved solids: Solids refer to matter suspended or dissolved in water or wastewater. Solids may affect water or effluent quality adversely in a number of ways. Waters with high dissolved solids generally are of inferior palatability. Waters high in suspended solids may be aesthetically unsatisfactory especially for purposes such as bathing. Solid analyses are important in the control of biological and physical wastewater treatment processes. Total solids are the term applied to the material left in the vessel after evaporation of a sample and its subsequent drying in the oven at a defined temperature. Total solids include total suspended solids, the portion of total solids retained by a filter, and total dissolved solids, the portion that passes through the filter. Total solids are expressed as mg/L of water. A limit of 500 mg/L TDS is desirable for drinking waters. Total dissolved solids can be measured using a meter and the total solids and total suspended solids can be weighed in the lab by evaporation. Turbidity: Suspension of particles in water interfering with passage of light is called turbidity. Turbidity of water is responsible for the light to be scattered or absorbed rather than its straight transmission through the sample, it is the size, shape, and refractive index of suspended particulates rather than the total concentration of the latter present in the water samples that are responsible for turbidity. The turbidity and transparency values are inter related, just like that of total dissolved solids and conductivity values. Turbidity in natural waters restricts light penetration thus limiting photosynthesis, which consequently leads to depletion of oxygen content. Turbid waters are highly undesirable from aesthetic point of view in drinking water supplies and may also affect products in industries. Turbidity can be removed by filtration, coagulation, etc. in drinking water treatment plants. Turbidity in water is caused by a wide variety of suspended matter, which range in size from colloidal to coarse dispersions and also ranges from pure organic substances to those that are highly organic in nature.Clay, silt, organic matter, phytoplankton and other microscopic organisms cause turbidity in natural waters. Turbidity is expressed in Nephlometric turbidity units (NTU) and is measured using a nephelometer. The term Nephelometric refers to the way the instrument estimates how light is scattered by suspended particulate material in the water. Turbidity can also be expressed in ppm.Permissible limit in drinking water is <10 NTU. Transparency: Transparency is a characteristic of water that varies with the combined effect of colour and turbidity. It measures the light penetrating through the waterbody. Clear water lets light penetrate more deeply into the lake than does murky water. This light allows photosynthesis to occur and oxygen to be produced. Pollution tends to reduce water clarity. Transparency values are expressed in cm or mm. Secchi disc, a metallic disc of 20 cm diameter with four quadrats of alternate black and white on the upper surface is used to measure transparency. pH : pH – potential of hydrogen, is the measure of the concentration of hydrogen ions. It provides the measure of the acidity or alkalinity of a solution and is measured on a scale of 0 – 14. The pH of water is 7, which is neutral, and lower than 7 is acidic, while higher than 7 is termed as alkaline. The pH of water determines the solubility and biological availability of certain chemical nutrients such as phosphorus, nitrogen, carbon and heavy metals like lead, copper, cadmium, etc. pH determines how much and what form of phosphorus is most abundant in water. It also determines whether aquatic life can use the form available. In the case of heavy metals the degree to which they are soluble determines their toxicity. Metals tend to be more toxic at lower pH because they are more soluble in acidic waters. pH is measured on a scale of 0 – 14. pH of natural waters would be around 7, but mostly basic. pH of seawater is around 8.5. pH of natural water usually lies in the range of 4.4 to 8.5. BDH Indicator (Universal Indicator) and test tubes or a pH meter can be used to measure pH. Specific conductivity: Conductivity is the capacity of water to conduct electric current which varies both with the number and types of ions the solution contains. Most dissolved inorganic substances in water are in the ionised form and hence contribute to conductance. Thus, measurement of conductivity also gives a rapid and practical estimate of the dissolved mineral contents of water. Conductivity is highly dependant on temperature and therefore is reported normally at 25°C to maintain comparability of data from various sources. Conductivity is reported in mmho or µ mhos/cm, though the recent unit of conductivity has been named Siemens/cm (S) instead of mho. Conductivity meter is used to measure conductivity. Dissolved oxygen: Sources of oxygen in water are bydiffusion of oxygen from the air into the water, photosynthetic activity of aquatic autotrophs and inflowing streams. DO is a very important parameter for the survival of fishes and other aquatic organisms. Diffusion of oxygen or transfer of oxygen in these organisms is efficient only above certain concentrations of oxygen. Too low concentrations of oxygen may not be enough to sustain life. Oxygen is also needed for many chemical reactions that are important to lake functioning (oxidation of metals, decomposition of dead and decaying matter, etc.). Measurement of DO can be used to indicate the degree of pollution by organic matter. DO is expressed as mg/L. DO concentrations of below 5 mg/L may adversely affect the functioning and survival of biological communities. Below 2 mg/L may lead to fish mortality. DO can be determined in the laboratory using various methods, which are explained in detail in the forth-coming chapters. Table 1.7: Dissolved oxygen level and water quality
Alkalinity: The alkalinity of water is a measure of its capacity to neutralise acids. The alkalinity of natural waters is due to the salts of carbonates, bicarbonates, borates, silicates and phosphates along with the hydroxyl ions in free state. However, the major portion of the alkalinity in natural waters is caused by hydroxide, carbonate and bicarbonate, which may be ranked in order of their association with high pH values. Alkalinity values provide guidance in applying proper doses of chemicals in wastewater treatment processes, particularly in coagulation, softening and operational control of anaerobic digestion. Alkalinity is expressed as mg/L. Total hardness, Calcium hardness & Magnesium hardness: Water hardness is the traditional measure of the capacity of water to react with soap, hard water requiring a considerable amount of soap to lather. Hardness is generally caused by the calcium and magnesium ions (bivalent cations) present in water. Polyvalent ions of some other metals like strontium, iron, aluminium, zinc and manganese, etc. are also capable of precipitating the soap thus contributing to hardness. However, the concentration of these ions is very low in natural waters, therefore hardness is generally measured as concentration of only calcium and magnesium, which are far higher in quantities over other hardness producing ions. Hardness is measured in terms of mg/L using standard methods involving reagents. Nitrates: Freshly polluted systems, especially by sewage contamination, show higher concentration of ammonia nitrogen, which in an aerobic environment is converted into nitrites and then to nitrates. Nitrate nitrogen is an indicator of past pollution in the process if stabilisation. The increasing application of fertilisers in agricultural land has resulted in water pollution due to leaching of nutrients like nitrogen and phosphorus. Total oxidised nitrogen is the sum of nitrate and nitrite nitrogen. Nitrates, in excessive amounts in drinking water leads to an illness known as methemoglobinemia in infants. Nitrates are measured in terms of mg/L and are measured in the laboratory using reagents. WHO has imposed a limit of 10 mg/L nitrate as nitrogen on drinking water to prevent the disorder of methemoglobinemia. Nitrates may be found in concentrations of up to 30 mg nitrates as nitrogen /L in some effluent of nitrifying biological treatment plants. Phosphates: Phosphorus occurs in natural waters and in wastewaters almost solely as phosphates. These are classified as orthophosphates, condensed phosphates and organically bound phosphates. They occur in solution, in particles or detritus or in the bodies of aquatic organisms. Orthophosphate is the phosphorus form that is directly taken up by algae, and the concentration of this fraction constitutes an index of the amount of phosphorus immediately available for algal growth. Phosphorus is essential to the growth of organisms and can be the nutrient that limits the primary productivity in water. In instances where phosphate is a growth limiting nutrient, the discharge of raw or treated wastewater, agricultural drainage, or certain industrial waste to that water may stimulate the growth of photosynthetic aquatic micro and macro organisms in consequential quantities. Phosphates are measured in terms of mg/L in the laboratory using standard methods. Sodium: The increased pollution of surface and groundwater during the past has resulted in a substantial increase in the sodium content of drinking water in many regions. Sewage, and industrial effluents and the use of sodium compounds for corrosion control and water softening processes all contribute to sodium concentration in water because of high solubility of sodium salts and minerals. It is expressed as mg/L. In surface waters the concentrations may be less than 1 mg/L or exceed 300 mg/L depending on the geographical area. The recommended guideline value is 200 mg/L. Sodium in water is measured using a flame photometer in the laboratory. COD: COD is the oxygen required by the organic substances in water to oxidise them by a strong chemical oxidant. The determination of COD values are of great importance where the BOD values cannot be determined accurately due to the presence of toxins and other such unfavourable conditions for growth of microorganisms. COD usually refers to the laboratory dichromate oxidation procedure. COD test has an advantage over BOD determination in that the result can be obtained in about 5 hours as compared to 5 days required for BOD test. Impact of pollution on DO – BOD and COD: Pollution like sewage contributes oxygen demanding organic matter or nutrients that stimulate growth of organic matter, which causes a decrease in the average DO concentrations. The decomposition process takes up the DO and results in the decrease in average DO. If the organic matter is formed in the lake by algal growth, at least some oxygen is produced during growth to offset the loss of oxygen during decomposition. It is expressed as mg/L and the analysis is done in the laboratory. Biological Monitoring Biological monitoring or bio-monitoring is the use of biological response to assess changes in the environment. Bio-monitoring is a valuable assessment tool that is receiving increased use in water quality monitoring programs of all types (Kennish, 1992 39). In the operational context, the term aquatic biomonitoring is used to refer to the gathering of biological data in both the laboratory and the field for the purposes of making some sort of assessment, or in determining whether regulatory standards and criteria are being met in aquatic ecosystems. Biomonitoring involves the use of biotic components of an ecosystem to assess periodic changes in the environmental quality of the ecosystem. Biomonitoring of aquatic communities can be subdivided into a number of categories, as follows: (Roux et al, 1993): (RHP-South African River Health Programme 40)
Biomonitoring involves the use of indicators, indicator species or indicator communities. An indicator signals messages, potentially from numerous sources, in a simplified and useful manner. An indicator may reflect biological, chemical or physical attributes of ecological condition. The primary uses of an indicator are to characterise current status and to track or predict significant changes. An ecological indicator has been used to identify major ecosystem stress through their presence, condition, and numbers of the types of fish, insects, algae, amphibians, and plants, etc. These types of plants and animals are called biological indicators or bioindicators (EPA). The fundamental principle behind biological indicator theory is that organisms provide information about their habitats. Bioindicators are evaluated through presence/absence, condition, relative abundance, reproductive success, community structure (i.e. composition and diversity), community function (i.e. trophic structure), or any combination thereof (Hellawell, 1986 41, Landres et al. 1988 42). The presence or absence of the indicator or of an indicator species or indicator community reflects the environmental conditions of the waterbody under study. Absence of a species is not as meaningful as it might seem, as there may be reasons, other than pollution, that result in its absence (e.g. Predation, competition or geographic barriers which prevented it from ever being at the site). Absence of multiple species of different orders with similar tolerance levels that were present previously at the same site is more indicative of pollution than absence of a single species. It is necessary to know which species should be found at the site or in the system. Biological indicators integrate, in themselves, the effects of various stressors, aquatic organisms and their communities reflecting current conditions, as well as changes over time and cumulative effects. Biodiversity of microbes, invertebrates and fishes can be used to indicate chronic pollution problems. Biological indicator species are unique environmental indicators as they offer a signal of the biological condition in a watershed. Using bioindicators as an early warning of pollution or degradation in an ecosystem can help sustain critical resources. Assessing the status of aquatic systems through biological analyses is an old endeavour. European studies of pollution dating back to the 1800’s identified aquatic organisms indicating environmental degradation. Since then, investigations and monitoring of aquatic environment health has relied heavily on indicator species as the primary diagnostic and monitoring tool (Mark B. Bain et al. 43). Generally, benthic macro invertebrates, fish / or algae are used. Certain aquatic plants have also been used as indicator species for pollutants including nutrient enrichment (Philips and Rainbow, 1993; Batiuk et al., 1992 44). A great deal of work has been done on using algae as bio-indicators of pollution (Mohanty, 1983 45; Reddy and Venkateswarlu, 1986 46; Tripathy, 1989 47; Mohapatra & Mohanty, 1992 48). Apart from information derived from monitoring of in-stream biotic communities, the evaluation of the health aquatic ecosystems must also include other system descriptors. The assessment of the available habitat is crucial when comparing biomonitoring results from different sites. The characterisation of geomorphological characteristics, hydrological and hydraulic regimes, chemical and physical water quality and riparian vegetation form essential components in aquatic ecosystem health assessment. Different approaches have been used to prepare and analyse biological indicator data. But they all start out with a list of aquatic life that was collected and identified. This list is often called a "species" list, but because many benthos can not be identified to species, it is also called a "taxon" list. In the past, the presence or absence of a few indicator species, such as game fish, was used to assess watershed health. Eventually, length and weight measurements of fish were also used, and numeric indices for benthos were developed. Such indices were first called biotic indices because they assigned number scores to the pollution tolerance of many different biological indicator species. While biotic indices were expanding in use, other indices, such as diversity indices, grew in popularity and were used for many years. Recently, multiple metric indices, such as the Index of Biotic Integrity by Dr. James Karr (1981), have become the standard in the United States for accurately assessing watershed health. Biological indicators are currently used and promoted by numerous conservation agencies as a means to monitoring and assess human impacts on environments, including the World Conservation Union (IUCN), World conservation Monitoring Centre (UNEP), U.S. Environmental Protection Agency (US EPA), as well as the Nature Conservancy, World Wide Fund for Nature (WWF), Friends of the Earth (FOE), and Greenpeace (IUCN 1989 49, US EPA 2002a 50, UNEP 2002 51). Plankton as indicators of water quality Phytoplankton (microscopic algae) usually occurs 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 sometimes creating offensive tastes and odours or toxic conditions. Because of their short life cycles planktons respond quickly to environmental changes, and hence the standing crop and species composition indicate the quality of the water mass in which they are found. They strongly influence certain non-biological aspects of water quality such as pH, colour, taste, odour and in a very practical sense they are a part of the water quality. Phytoplankton growth is dependent on sunlight and nutrient concentrations. An abundance of phytoplankton / algae is indicative of nutrient pollution (De Lange, 1994 52). Moreover algae are sensitive to some pollutants at levels, which may not visibly affect other organisms in the short term or may affect other communities at higher concentrations. Algae is used as indicator organisms because of the following advantages:
For a number of years there has been a series of proposals indicating that one or more algae could be used as organisms indicative of water quality (Palmer, 1959 54). It also demonstrated that algal assemblages could be used as indicators of clean water or polluted water. Clean water would support a great diversity of organisms, whereas polluted water would yield just a few organisms, with one or few dominant forms (Trainor, 1984 55). In this context, the micro algae have great potential for monitoring and evolving the water quality of the waterbodies. (Venkataraman et al,1994 56). Zooplankton in freshwaters principally comprises of microscopic protozoan, rotifers, cladocerons and copepods, a much greater variety of organisms are encountered in marine waters. They range in size from microscopic protozoan to larger jellyfish of over 10 m long, wherein in freshwater they generally are small and microscopic in size: in salt water, larger forms are observed more frequently. The species assemblage of zooplankton is also used in the assessment of water quality. Most zooplankton occupy an intermediate second or the third trophic level of aquatic food webs feeding on algae and bacteria and in turn being eaten by numerous invertebrates and fish. Therefore, any adverse effect to them will be indicated in the health of the fish populations. Protozoan plankton, though important requires specific and more elaborate study techniques. Zooplankton as indicators for the assessment of water quality has the following advantages:
Macroinvertebrates as indicator species They make good indicators of watershed health because they:
The use of aquatic organisms, particularly benthic invertebrates, as biomonitors of the local availabilities of potentially toxic trace metals has become increasingly widespread (Cain 58; Hare 59 and Phillips 60). Bioaccumulation of heavy metals by aquatic insect larvae including mayflies is comparatively well studied (Hare 61; Hare and Goodyear 62), and larvae have been employed in biomonitoring studies of freshwaters. Fish as indicator species
However, sampling fish requires high level of resources (time labour, and cost of equipment) and this increases with the size of the habitat. |
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