Importance of water for aquatic flora and fauna and water quality

Importance of water for aquatic fauna: Different types of aquatic plants like water hyacinth, water lily, algae, bindweed, duckweed (lemna), ori pana, water nut, water lettuce, lotus, hydrilla, water cress, Jussiacea repens etc. are known to all of you. Where do they grow? Most of them grow in water. Some of them like bindweed grow both in water and land. Most of the aquatic flora could not grow without water or even if a few could grow, they would neither survive nor grow up. What would happen in that case? In that case, the aquatic ecosystem could be hampered because the aquatic flora produce oxygen by photosynthesis and maintain the level of dissolved oxygen in water in one hand, on the other hand, they also work as food reserves for aquatic animals like fish. So if there was no aquatic flora, aquatic fauna could not survive which would be disastrous for the environment.

You know that the plants basically take water and other necessary elements through their roots, however, aquatic flora collect water and necessary elements especially minerals through the whole parts of their body. So if all the parts of the aquatic plants do not come in contact with water, their growth might be hampered. It is also to be noted that, stems and other parts of the aquatic plants are soft which is suitable to adapt with water current and movement of aquatic fauna.

If they grow in land instead of water, they would breakdown and could not grow up and even they could not survive. Do you know how the aquatic plants reproduce? Aquatic plants usually reproduce asexually and reproduction would have been hampered in absence of water. So we can say that water is essential for reproduction and growth of aquatic plants which are very important for ecological balance. If there was no water, aquatic plants could not grow, even though they grow, they could not survive and as a result environmental disaster could occur.

Importance of water for aquatic fauna:

Among thousands of aquatic animals, fish is the most well-known to us. What happens when we catch a fish and put it out of water? It dies. Why? Because as we can?t survive without air or oxygen, we die of breathing problem, in case of fish, it happens in the same way. Fish takes oxygen by gill which is suitable to take up oxygen only from water not from air. If there were no water, no fish could sustain. Not only fish but also other animals respiring through gill by taking up oxygen would not live. As a result, environment would degrade and it would be very difficult for us to survive. You know that protein is an essential element for our growth. Approximately 80% of protein needed for us comes from fish. Therefore, if there were no water, we might not have necessary protein. Therefore, all physiological activities including physical growth could be disturbed.

Water quality parameter:

Water is a very valuable natural resource. It is a habitat for aquatic flora and fauna which are very important components of our environment. In addition, water is also being utilized for irrigation. Moreover, mariners, boatmen and people who are in the similar profession depend on water for drinking and other purposes. So, if the quality of water is not maintained, it will be threatening for the environment in one hand, on the other hand, the use of water for other purposes will be limited. Now, let us know about the water quality parameters. The quality of water depends on the purposes where water to be used. At first, let us know about the required quality of water of river, canal, beal, and seas.

 Color and taste: You all know that pure water is colorless and odorless. Natural surface water in rivers, lakes, seas or oceans should be colorless and tasteless to sustain aquatic lives.

Turbidity: Turbid water could be harmful for aquatic flora and fauna because turbid water hinders the penetration of sunlight resulting in reduced photosynthesis in aquatic plants. As a result, growth of the aquatic plants is hampered and oxygen production by photosynthesis decreases as well. Moreover, in turbid water fish and other aquatic animals suffer from problems related to food collection. Now the question is how water becomes turbid? Water becomes turbid due to the presence of insoluble substances like soil, sand, oil, grease etc. If these type of substances especially soil and sand increase significantly, at a stage, they settle down as sediment at the bottom of the river. As a result, navigability decreases resulting in huge problem for the water vehicles like ships, launches, boats etc. In Bangladesh, it is common problem of getting stuck of launch or steamer during their movement from one place to another. Why does it happen? It happens due to reduced navigability.

Presence of Radioactive Substance: The presence of radioactive substances in surface water can cause dangerous diseases like cancer in aquatic biota. Therefore, surface water, a habitat for aquatic biota should be free of radioactive substances.

Presence of waste: Surface water must be free of waste materials because waste materials may produce infectious microorganisms and that may cause disaster for aquatic biota.

Dissolved Oxygen: As we need oxygen for respiration, aquatic fauna also need oxygen for their respiration. How do they get oxygen? They get oxygen from water where oxygen is present as a dissolved gas. If the level of dissolved oxygen in water is reduced, it results in respiration problem. In fact, aquatic fauna can?t survive without dissolved oxygen. It is to be noted that the minimum level of dissolved oxygen required for sustaining life in water bodies is 5mg/Liter.

Temperature: Temperature is an important water quality parameter. The increase in temperature in water results in reduced amount of dissolved oxygen in one hand, on the other hand, starting from hatching, many physiological processes of aquatic animals fall into trouble.

pH: Do you know what is pH? pH is a parameter with which we can easily evaluate whether a water sample is neutral, acidic or alkaline. For neutral water, pH is 7. If water is acidic pH becomes lower than 7 and if it is alkaline, pH becomes more than 7. Higher the amount of acid in water, the lower the pH, while higher the amount of alkali, the higher the pH.  pH is a very important water quality parameter. Usually surface waters are alkaline. It is found from research that a pH range of 6-8 is suitable for sustaining aquatic biota. If pH value alters significantly, it results in huge damage to aquatic lives. Fish eggs and newly hatched fish are very sensitive to pH. Highly acidic condition of water i.e. very low pH of water extracts important elements like calcium leading to deformation to aquatic biota.

Salinity: Do you know why our national fish Hilsha comes to fresh water during spawning? Although Hilsha is a sea fish, it comes in fresh water for spawning because sea water is saline it contains huge amount of salt which sterilizes fish eggs which cannot produce young fish. But there are some aquatic fauna including certain species of fish which can spawn in water of high salinity.

Recycling of water and role of water in conservation of water:

It is known to us that approximately 75% of the earth surface is covered with water, most of the water (about 97%) is saline and that?s why they can?t be utilized directly for different purposes. Only 1% of water resources we have is fresh and the major portion of it is available in rivers, lakes etc. and it is being polluted (the pollution of fresh water will be discussed later in details in this chapter). Even the underground water being used everywhere including drinking purpose, is getting contaminated by toxic chemical substances like arsenic, and becoming unsuitable for drinking and cooking. Therefore, it clearly indicates that although we have a lot of water resources, amount of potable and usable water is very limited. Hence, we need to be very careful in using water and we have to think about reuse of water, indeed.

Do you think that water is being recycled naturally? Yes, it is. You have already learnt previously from water cycle that water from surfaces evaporates at day time with the help of sunlight and enter into the atmosphere as vapor which goes upwards and at a stage condenses to cloud and finally comes back as rain water. A major part of the rain water flows as surface runoff and falls into water bodies from where it is converted into vapor, condensed to cloud and precipitated as rain water. This recycling of water is very important because disruption of this recycling of water could lead to droughts or floods which in turn decrease the food production, and eventually long term droughts might turn the whole earth into a desert. Precipitation is a kind of natural recycling of water. Is it possible to recycle water that is used once? Of course we can. Water obtained after being used i.e. wastewater can be collected, treated and used again. The whole process can be considered as recycling of water.

Role of water in conservation of nature:

As all the components and process of the environment depend on water directly or indirectly, so water is essential for a sustainable environment. If there were no water, plant would not grow, there will be no food production and our existence i.e. the whole environment will be destroyed.

Necessity of quality water: What do we do at first after getting up from the bed in the morning? We wash our hands and face. Can we do these works without water? No, it is impossible. Whatever we do in our daily life, we need to use water. Starting from taking shower to cooking food we need water. Finally, we have to drink water. If such an important thing in our life is not of good quality, we have to face lots of troubles. For example, if the water is saline or has bad odor, we cannot drink it. Surface water in several districts in south western part of Bangladesh has become saline and that is why residents in those areas have been suffering from severe lack of fresh water. They can?t utilize surface water for drinking and other purposes. So they had to collect rainwater, purify it and use. Moreover, if the drinking water is not of good quality, particularly if it contains disease causing germs, it may lead to severe public health problem. Do you think that we can use sea water in industry or agriculture? No, we can?t because high salinity of sea water corrodes the equipment such as boiler used in industry. Similarly, most of our crops cannot grow in saline water i.e., saline water is not suitable for irrigation in agriculture. After all, we can say that starting from industrial use to agriculture and in our daily works, supply of good quality water is very essential, otherwise it may cause health problem in one hand, and it may hamper us economically on the other.

Purification of water

Surface water may contain harmful substances including toxic chemicals and pathogenic microorganisms. Contamination of groundwater by toxic chemicals is also well known. So whatever the sources, water must be purified before use. Methods of purification of water depend on the purpose of use of water. As usual, although very pure water is required for drinking, such pure water is not required for irrigation. The methods that are involved in water purification usually are filtration, chlorination, boiling, distillation etc. They are discussed below:

Filtration: You have learnt about filtration in class VI. What is filtration? Filtration is a process to separate solid substances from a mixture of solid and liquid substances. Usually water contains insoluble dust or soil particles or waste materials which are removed by filtration. For that water is passed through a layer of sand which traps the solid particles present in water. In addition to sand, finely woven cloths can be used for this purpose. The modern filters used in residences are made of quality materials.

Chlorination: Disease causing microorganisms in water are killed by disinfectants. Different types of disinfectants are used for purification of water. One of them is chlorine gas (Cl2). Bleaching powder [Ca(OCl)Cl] and other chlorine containing compounds are also used for this. Do you know what makes the tablets or kits used for water purification during flood in our country? It is basically sodium hypochlorite (NaOCl).Chlorine present in it kills the germs present in water. Other than chlorine, germs can also be destroyed by ozone gas (O3) or ultraviolet radiation. In the bottled water factories, water is disinfected by these methods.

Boiling: Boiling of water is known to all of us. Is it possible to kill germs in water by boiling? Yes, it is possible. Boiling of water can kill germs present in water. Do you know how long the water should be boiled to kill germs in water completely? After starting the boiling, heating for additional 15-20 minutes can disinfect water completely. It is a simple and economical process for purifying drinking water at home.

Distillation: You have learnt about distillation. When very pure water is needed, water is purified by distillation method. For example, to manufacture medicine or to carry out chemical reactions, 100% pure water is needed. In this method, basically water taken in a container is heated to vapor which is condensed and collected in another container. The possibility of having other substances in water purified by this method is very low.