Abstract

Water is one of the wonders of nature and the main constituent to create life on earth. Water that is useful for human use should have an optimum physicochemical constitutional regime. This study was done by critical analysis of different results obtained from recent studies on different physicochemical parameters of freshwater bodies. This study is also related to plankton and human interferences on different physicochemical parameters. However, due to recent urbanization, and an explosion in population together with industrial growth, these values are often not at the acceptable range. Human activities are affecting water quality both qualitatively and quantitatively. Industrial and domestic wastes discharge into the river; water from crop fields laden with pesticides, vitiate the pond and lake waters. Prevention of diseases of human is also dependent on the quality of the aquatic ecosystem. Therefore, it is essential to have in depth information about the water quality parameters and regular monitoring of important water bodies is also a necessity. Major factors like temperature, hardness, pH, nitrate, chloride, DO, BOD, TDS, alkalinity, etc. have a profound impact on aquatic biota. Abiotic factors influence the biotic conditions including the life of plankton in the aquatic ecosystem. This article provides a brief knowledge of the current scenario of the water quality of the freshwater ecosystems.

1. Introduction

“Water, water everywhere and not a drop to drink”, the famous quote from Samuel Taylor Coleridge’s poem, ‘The Rime of the Ancient Mariner’ correctly indicates the actual scenario even of a polluted freshwater environment. Among aquatic environments, there are marine, brackish, and inland waters like rivers, lakes, ponds, impoundments, etc. Since times immemorial ponds and other lentic waters have been serving as a traditional water source for different domestic uses, including drinking, in India, particularly in rural and suburban areas. The water quality of these lentic water bodies has been degraded by several pollutants like detergents, agricultural pesticides, urban run-off, industries, farmlands, etc. ¹. However, chemically pure water is very rare in nature ². Indian agriculture mainly depends upon different rivers, reservoirs, dams, etc. for irrigation water. But industrial and other wastes, being discharged into rivers deteriorate the condition of the river and thus cause water pollution and shift in the normal values of different physicochemical parameters. However, rivers can self-purify itself to a certain extent but in recent times and most cases the amount of waste and discharged effluents are too much to purify. Different municipal and industrial activities in different seasons on the river watersheds have myriad effects on river pollution. The waste and effluents containing heavy metals poison aquatic animals. Climatic scenario, river basin lithographic, and atmospheric conditions also have an effect on the quality and nature of the river. Irrigation is carried out with such polluted river water without suitability assessment resulting in adverse effects on the production of crops and soil quality ³.

Plastic wastes now become a global issue and threat to mankind. Inland sources contribute to approximately 80% of the plastic wastes in the sea ⁴. Microplastics can form in the sea by the process of weathering, abrasion, mechanical disintegration, and photolysis, ultimately damaging the aquatic ecosystem ⁵. Consequently, nano plastics are added to the freshwater situations through precipitation; land-fill with plastic wastes is also vitiating the groundwater with nano plastics causing a negative impact on biota including humans.

Pure water, as the name implies not only has a very little amount of suspended solids or obnoxious gases but is also low in presence of biological life ⁶. Properly purified water is used for drinking purposes but the types of water that are used in agriculture or industries are of moderate quality and this quality may vary. The components of the aquatic ecosystem have a direct impact on the biological diversity of the water body ^{6, 7}. Therefore, the monitoring of different physicochemical parameters of freshwater environments should be carried out to get current pertinent data on the water bodies so that a healthy freshwater ecosystem can be maintained. These factors and their seasonal variations could be indicative of freshwater pollution and help to identify the sources of pollutants ^{8, 9}. Thus, this review will also, assess the seasonal variations of physicochemical factors of the freshwater bodies. Water quality is dependent on several parameters. So, water quality varies as these physical, chemical, and biological parameter changes.

2. Water Quality Parameters

Water quality assessment is necessary on a regular basis to check the pollution, regulate the quality of water, and maintain a healthy aquatic life. To maintain the quality and restore the water bodies, water quality assessments have been done by several agencies ¹⁰.

Studies that are conducted by different agencies followed some common procedures to analyze water samples by selecting different sampling sites (Table 1). These sampling sites were selected for different seasons like pre-monsoon, monsoon, and post-monsoon periods.

Different instruments like pH meter, conductivity meter, and TDS meter were used to determine different parameters. Other main parameters are dissolved oxygen (DO), alkalinity, total hardness, biochemical oxygen demand(BOD), chemical oxygen demand( COD ), sodium( Na), potassium(K), sulphate(SO₄), Nitrate(NO₃), Magnesium(Mg), etc. Usual standard APHA methods are used to estimate all of these data. Commonly used methods for water parameter determinations are presented in Table 2.

The acidity of water is determined by pH which ranges from 0 to 14 and the scale of the pH is logarithmic. A solution with a pH below 7 is called acidic and above 7 is called basic ¹¹. Generally in water bodies pH value fluctuates between 6.5 and 8.5 as proposed by WHO, ICMR, and BIS ¹². Moreover, to mention that water having a pH of greater than 8.5 tastes salty and if in some rare cases pH goes beyond 11 then it will cause eye irritation and skin disorder ¹³. Aquatic life can be greatly hampered if the pH range is between 3.5 and 4.5 ¹⁴. The pH values ranging from 6 to 9 are also considered as the quality level standards for aquatic living ¹⁵.

The aquatic environments and organisms living within them are very much dependent on the temperature of the water bodies. Water has properties to stabilize the temperature change. The climatic changes also make a big impact on water temperature. The drastic temperature change can have an adverse effect on fish. Metabolism of the aquatic environment has been maintained by temperature. High temperature in water reduces the amount of dissolved gases ¹⁶.

Water samples contain different types of ions and their mobility, relative concentrations, valence, temperature, and total concentration vary too. The electric current carrying capacity of an aqueous solution is expressed by the conductivity of the sample. The conductivity is proportional to the salinity and temperature of the water. Inorganic chemicals have a role to conduct electrical current which is why distilled water shows a conductivity of less than 1 μS/cm. Generally, the conductivity of drinking water should have a value of 200µS/cm) ¹⁷. Electrical conductivity has a linear relation to the total dissolved solids and increases with the depth of the water body ¹⁸.

There are some chemical constituents in the water bodies that can raise the pH of the water to above 7 i.e. they make the water alkaline. Alkalinity value is expressed in mg/L as calcium carbonate. Determination of alkalinity is very much essential for assessing an aquatic ecosystem as pH has a direct link not only to the aquatic organisms but to the toxicity of some other pollutants. Among these chemicals, there are bicarbonates (a component of both alkalinity and acidity), phosphates, carbonates, hydroxides, etc. Buffering capacity of water can be measured by alkalinity. Alkalinity protects or buffers against fast pH changes. Acid rain and different acid wastes in surface waters are buffered by required alkalinity levels ¹⁹. The study on river Hooghly, a tributary of the Ganges revealed that the alkalinity was high in winter and diminished gradually with the arrival of spring ²⁰.

Aquatic plant growth is controlled by the amount of phosphate-phosphorus, an essential nutrient of aquatic plants. Among freshwater aquatic plants, algae and macrophytes are noteworthy.

Therefore, during the determination of parameters in water bodies, it is very much important to assess the value of phosphate-phosphorus seasonally like other parameters ¹¹. DO level is inversely related to the amount of phosphorus present in the aquatic ecosystems ²¹. The tolerable range of phosphorus for aquatic life viability is 0.1 mg/L. If the value of phosphorus is greater than 0.03 mg/L then the lakes and the reservoirs can be filled by algae (algal bloom) ^{21, 22}.

Total hardness is an important physicochemical parameter in determining the quality of the water. The cumulative value of dissolved calcium and magnesium hardness is known as total hardness. Based on the value of total hardness, water can be classified as soft water (0 to 60 mg/L), moderately hard (61 to 120 mg/L), and hard (121 to 180 mg/L) ²³. Leaching from the soil can be a major factor contributing to more hardness or increasing the hardness concentration in the water. Drinking hard water regularly may cause diabetes, neural diseases in the human body, or the onset of congenital heart diseases ²⁴. Soft water contains relatively more sodium. According to the WHO permissible limit value of total hardness is 150 mg/L. Alkalinity and hardness are both estimated by calcium carbonate. Thus have a relationship with natural waters. However, there can be a difference between the two in some water, like low alkalinity may accompany high hardness. Hardness can exceed alkalinity where the geology of an area dominates by gypsum (CaSO₄) as opposed to limestone (CaCO₃) ²².

Another important physicochemical parameter for aquatic bodies is nitrogen-nitrate. A fluctuation in the nitrogen-nitrate level in the aquatic ecosystem causes harm to the aquatic organisms and can change the environment of the aquatic water bodies ²⁵. Blue-green algae generally break down the dissolved nitrogen, present in the water bodies into ammonia and inorganic nitrogen. In these ways, the amount of dissolved nitrogen has been maintained ¹¹. Deviations from this can give rise to several types of diseases in the human body (if the water is consumed). Blue baby syndrome is a type of disease that is caused by a reaction of nitrate and iron ¹¹. It is seen in the children who are generally under the age of 1 year. ICMR, WHO, and BIS have proposed an acceptable of Nitrate-Nitrogen for the water bodies which is 50 mg/L ²⁵. Excess nitrogen causes the overgrowth of aquatic plants and algae. Such excess growth of these organisms, in turn, can clog water intake facilities, dissolved oxygen use up due to their decomposition, and block light to deeper waters ²⁶.

For human nutrition and metabolism, iron plays a vital role in the human body. It also acts as a constituent of blood. But still, excessiveness of iron in the aquatic environment can damage human body tissues through the ingestion of water. The permissible limit of iron in drinking water is 0.3 ppm in maximum ²⁷. Iron toxicity studies revealed that, even though Iron is an essential nutrient for growth, an excess amount can negatively affect aquatic organisms. Iron is a natural capping agent and increases sediment phosphorus binding capacity. This reduces phosphorus availability, resulting in a shift from algal to a macrophyte-dominated lake ²⁸.

Mineral weathering can be a major source of calcium for the water bodies. A decrease in calcium value in the water bodies can have a derogatory effect on the health of the particular aquatic ecosystem ²⁹.

The growth and population dynamics of freshwater flora and fauna depend on calcium. Calcium controls cell to cell signaling, neuron activity, muscle contraction, and enzyme activities ^{29, 30}. Ca is important as a key structural component of calcified exoskeleton for invertebrates, bony structures in vertebrates, and egg-shell formation in birds. Several studies reveal that Ca concentration has a significant role in community structure in freshwaters ^{31, 32, 33}. A study in Canadian lakes showed the replacement of Ca demanding cladoceran Daphnia spp. by the Ca-poor cladoceran Holopedium sp. A process is known as jellification ²⁴.

Chloride value for an aquatic ecosystem should be in an optimum condition (1 to 10 ppm) in normal freshwater ¹⁰. It is the most abundant inorganic ion which is found in natural water ³⁴. Chloride is continuously deposited in freshwater ponds and lakes from agriculture fields during the rainy season. Chloride water mix in the natural freshwater sources. Such water also percolates through the soil and adds to the underground water and increases the chloride level ³⁵. Chloride levels also increase in the ponds and lakes during summer due to high temperature and evaporation.

Sun provides light and energy to the aquatic bodies and that is essential for the metabolic activities of aquatic organisms ³⁶. Chlorophyll-bearing aquatic plants absorb this visible wavelength for the purpose of photosynthesis to maintain the food chain. In aquatic bodies, photosynthesis takes place in ponds and lakes (autochthonous) or within the terrestrial drainage systems (allochthonous) and the energy transports through trophic levels following Lindemann’s law. ³⁷. That's why the measurement of light transmission is very much essential for aquatic bodies to get an idea about the condition of water bodies. 1% light level (water level or depth at which 1% surface light penetrates) should be measured and moreover to mention this level denotes the euphotic zone or lower limit of growth of algae ¹⁰.

The dissolved form of oxygen present in the water bodies is very much needed for the fishes and the other aquatic organisms to respire and this is known as dissolved oxygen. So measurement of DO for the aquatic bodies indicates the health of the water bodies. It is estimated that a minimum 4-6 mg/L value of dissolved oxygen is necessary for the fishes to survive. If the value of the dissolved oxygen decreases then aquatic lives become disturbed. This dissolved oxygen value also depends on several other factors like temperature, salinity, altitude, turbulence, etc ³⁸. Scarcity of dissolved oxygen leads to hypolimnetic conditions in lakes. This condition indicates a high rate of respiration and an increase in the rate of decomposition ³⁹.

There are several types of bacteria and other microorganisms in the water bodies and they can grow at a specific temperature. These organisms consume the oxygen in the water bodies and their rate of respiration is represented as BOD or biochemical oxygen demand. If the BOD level increases very much from its optimum value in an aquatic environment then the aquatic organisms will get less oxygen and this leads to a deteriorating effect on all the organisms ⁴⁰.

All the solid materials present in the surface water and groundwater may change the odours, and colour and can cause offensive taste of water (Table 3). That's why TDS should be measured ⁴¹. Deviations in the value of TDS can cause several derogatory effects on human health like damage to the central nervous system, problems in the lips, and tongue, or in some serious cases it can even lead to paralysis in the human body ⁴². For a healthy aquatic body, as prescribed by the USEPA and WHO ^{43, 44}, the range of TDS should be between 50 and 150 mg/L ⁴⁵.

Water can be opaque due to the introduction of several soil particles or dead leaves or due to the increase in the number of algae, and construction site erosions. Secchi disk transparency refers to the depth up to which this disk is prominent in the lake water or aquatic environment. Silt, plankton density, turbid water coming from catchment sites, industrial wastes, agriculture runoff, etc., are the causes to reduce the light penetration capacity of water. Activities of humans and cattle also affect to reduce the transparency of water ²⁰. This transparency value also indicates the health of that particular aquatic ecosystem ¹¹.

The plant pigments of algae consist of chlorophylls, carotenoids, etc. Chlorophyll-a is the most dominant chlorophyll pigment in the green algae (Chlorophyta) but is only one of several pigments in the blue-green algae (Cyanophyta), golden-brown algae (Chrysophyta), and others ¹¹. Chlorophyll-a can be used to measure the algal biomass of the lake ecosystem ¹⁰.

Water quality can vary from season to season due to fluctuation in the different parameters of water for the urban, rural, and industrial areas. A clear picture of water quality is available by using an index, known as the water quality index or WQI ⁴⁷. It cumulatively gives a single value of all the water quality parameters (pH, BOD, COD, temperature, nutrients, dissolved oxygen, etc.) by converting all these individual values into a single number. The WQI estimation is an attempt to represent a single value instead of a big quantity of parameters ⁴⁸.

WQI ranges between 0 and 25 and is considered excellent for drinking, irrigation, and industrial use. Water with a WQI higher than 150 is unfit for consumption and needs proper treatment for this water before use ⁸.

Phytoplankton are the free-floating uni-cell producers of the aquatic ecosystems ^{49, 50}. Global nutrient cycles of the earth are controlled by them. They are present in the water column to the extent the light penetrates. Phytoplankton populations are controlled by nitrogen, phosphorus, silica, etc. ⁵¹. Deviations from the optimum values of the water physicochemical parameters have a direct impact on the plankton population. Such deviation may affect the quantitative and qualitative composition of phytoplankton, their distribution, periodicity, etc ⁵². Phytoplankton biomass has a relation with the number of live fishes in aquatic bodies ⁵³. The quality of water and different environmental factors can be monitored by the phytoplankton ^{54, 55}. Pearson’s correlation analysis is generally used to find correlations between different water parameters and planktons ⁵⁶. A positive correlation has been found between nitrate, phosphate, salinity, and DO with zooplankton, whereas a negative correlation exists between temperature, clarity, and pH with zooplankton abundance ^{57, 58}. An increase in the sediment load will decrease the plankton populations in a stream ⁵². Physicochemical factors have their impact both on individual species of zooplankton and entire zooplankton clusters ⁵⁹.

3. Discussion

Due to the irresponsible activities of humans, several aquatic ecosystems like reservoirs, lakes, etc. are destroying. For example, waste disposal is a common phenomenon in rivers and lakes. These harmful anthropogenic activities gradually deteriorate surface waters worldwide as surface waters are most vulnerable to pollution ^{60, 61}. Different beautiful freshwater ecosystems are being deteriorated ¹². Water bodies nearer to the crop fields are polluted rapidly in the season of maximum agricultural or horticultural activities. Sampling was done from different sites based on the topography, structure, and land covers. The study reveals that rivers and lakes adjacent to tourist spots are the most vulnerable area to water pollution. Summer in the Kashmir Valley of India is the most vulnerable period when parameters of water fluctuate mostly. Increase in anthropogenic activities in the monsoon, water quality falls because of erosion of soil, turbulent flow, poor level of sanitation, etc. ³². Nambul river is the most polluted among all rivers that discharge into the Loktak lake ^{62, 63}. In the river Narmada, most water quality deterioration is observed in the monsoon season whereas, water is found moderate to good in summer. The aquatic environment is rejuvenated to some extent during lockdown during the Covid-19 outbreak. A clear difference has been observed among different parameters of water between pre and post-lockdown ^{52, 64}. Nowadays, satellite imageries help to determine the water quality parameters to a certain extent without field measurements ^{53, 65}.

4. Conclusion

Systematic and comparative evaluation of water quality is done with the help of water sampling from different aquatic sources. Planktons like Melosira, and Microcystis, which are resistant to water pollution can give us an idea about the conditions of the aquatic ecosystems. Anthropogenic interferences cause variations in water quality which may lead to hindrance in aquatic life. We can control water pollution by proper management like adding fertilizers in proper amounts during agricultural activities and purifying the industrial wastes which have toxic agents before discharging them into rivers. Fishing, boating, and other aquatic recreational activities should be restricted and minimized in ponds and lakes to stop the acute degradation of water. This will not only have serious consequences on fish or birds but also on other animals and mankind.

Acknowledgements

The authors are thankful to the HOD, Zoology, University of Burdwan, and the Principal Government General Degree College, Singur for their encouragement during the preparation of this article. The authors are gratefully acknowledging the help of Dr. Biplob Kumar Modak, Professor of Zoology, Sidhu Kanhu Birsa Univerity, Purulia, and Dr. Subhro Mukhopadhyay, Emeritus Scientist in writing this manuscript.

Statement of Competing Interests

The authors declare that they have no competing interests.

References