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Chapter one : Aquatic ecosystem Learning outcomes: , : 1. Recognize the hydrosphere and its relationship with other atmospheres on Earth. 2. Explain the role of the water cycle in nature in causing various environmental...

Chapter one : Aquatic ecosystem Learning outcomes: , : 1. Recognize the hydrosphere and its relationship with other atmospheres on Earth. 2. Explain the role of the water cycle in nature in causing various environmental changes. 3. Explain the chemical reactions in the aquatic ecosystem and their effect on water quality and the sustainability of marine life. 4. Explain the effect of the physical properties of water such as specific heat, and other physical factors such as temperature and pressure on the distribution of organisms and the sustainability of the aquatic ecosystem. 5. Evaluate the biological adaptations of organisms in the aquatic environment and their role in the sustainability of the ecosystem. Issues involved 1. Water pollution 2. Climate change 3. Sustainability of water resources 4. Biological diversity conservation 5. Water resources management 6. Sustainability challenges in the face of population growth. Chapter 1 Aquatic ecosystem 1-1 Chemical reactions and their impact on water quality Get ready Have you ever thought when you drink a glass of water, about the chemical reactions that may occur within this vital liquid? Water is not just a transparent liquid; it is a medium in which many chemical compounds may react, affecting the quality of water and the health of living organisms that depend on it. In this chapter, we will learn about the hydrosphere and the water cycle in nature, as well as some of the basic physical properties and chemical reactions that occur in water, and how these properties and reactions can affect the components of the environment. Learn Water has unique properties that support life. Water can dissolve many chemicals and can exist in all three states of matter - solid, liquid, and gaseous states - within the range of known temperatures on the Earth's surface. Water is essential to the continuation of life on Earth. All living cells have a membrane that separates the organism from its environment. Water passes from the environment to the inside of the living cell through this membrane, carrying the substances needed to produce energy, as well as eliminating waste products to the outside. ⯁ The hydrosphere on Earth: The hydrosphere distinguishes Earth from other planets in the solar system. About 70% of the Earth's surface is covered by water (Figure 1). About 97% of this water is found in the oceans, seas, and salt lakes as salt water. The fresh water and is found in rivers, freshwater lakes and groundwater represents approximately 1%. And the reminder part represents the frozen water in polar regions, mountain peaks and glaciers. Egypt is characterized by its diverse aquatic environments, which include the Nile River, the Gulf of Suez, the Gulf of Aqaba, the Red Sea, the Mediterranean Sea, and many salt and freshwater lakes. The water envelope 8 Chemical reactions and their impact on water quality 1−1 Lesson ⯁ Water Cycle in Nature Water exists on or near the Earth's surface in a state of continuous change between its three states. Water is constantly moving from one place to another in many different paths that form a nearly closed system called the water cycle in nature or the hydrologic cycle. The water cycle as a system is capable of changing the Earth's surface physically, chemically, and biologically. The water cycle in nature mainly includes the process of evaporation, which contributes to the formation of clouds and the process of rain or snowfall. In addition to other biological processes such as transpiration in plants, respiration in plants and animals, and water leakage through the pores of soil and sedimentary rocks to form groundwater. Water vapor in clouds may react chemically with compounds in the air, forming some acids that fall as acid rain, which decomposes rocks. Research activity Using various sources, research about : 1- What are the different tools that meteorologists use to measure the amount of annual rainfall that falls on a particular area of the Earth's surface ? 2- Can scientists predict future changes in the Earth's water cycle? 9 Chapter 1 Aquatic ecosystem ⯁ Chemical structure of water: Water is composed of the two elements hydrogen and oxygen, in the ratio of 2: 1 by volume, respectively. Oxygen represents 88.89% of the mass of the water molecule and hydrogen represents 11.11%. The two hydrogen atoms are connected to the oxygen atom by two covalent bonds with an angle of about 104.5 between them. ⯁ Chemical properties of water: Water does not exist on Earth in a pure form as it contains many ions and chemicals that interact with it in different ways. Here are three of the main properties of water: 1- Water polarity: The oxygen atom is characterized by its higher electronegativity than the hydrogen atom, so the bonding electrons are attracted towards the oxygen atom, forming a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atom, which is known as the polarity of the water molecule. The polarity of water molecules causes them to bond with other water molecules or polar molecules of other substances to form hydrogen bonds, which gives water the ability to dissolve many salts and break them down into hydrated ions. Example Dissolving sodium chloride salt in water The ability of water molecules to form hydrogen bonds with each other is also a key reason pure water has a higher boiling point of 100°Cat normal atmospheric pressure than compounds of similar structure, such as hydrogen sulphide, which boils at -61°C. 10 Chemical reactions and their impact on water quality 1−1 Lesson 2-Hydrolysis (hydration): A small percentage of water molecules exist as hydrogen ions (H⁺) and hydroxide ions (OH-). As a result of chemical reactions of water with different compounds, hydrolysis of some salts, present in natural water, may occur. This hydrolysis affects the balance of these ions in water, leading to acidity or alkalinity of the water. A practical example: When table salt (NaCl) is added to water, it dissociates into sodium ion (Na⁺) and chloride ion (Cl-) and the salt ions remain in solution without binding to water ions, making the solution neutral because the concentration of hydrogen ions (H+) is equal to the concentration of hydroxide ions (OH-).In the case of sodium bicarbonate salt (NaHCO3), hydrolysis of the salt leads to a decrease in the concentration of hydrogen ions (H⁺) and an increase in the concentration of hydroxide ions (OH-), making the salt solution basic. The opposite happens when ammonium chloride salt (NH4Cl) dissolves in water; it hydrolyses and causes a decrease in the concentration of hydroxide ions and an increase in the concentration of hydrogen ions, making the salt solution acidic. 3- Acid-base balance (equilibrium( : The acid-base balance in water depends on the relationship between the concentrations of hydrogen ions (H⁺) and hydroxide ions (OH-). This relationship can be recognized by the pH value of the solution. It is a scale that ranges from 0 to 14. If the concentration of H⁺ increases, the water becomes acidic and the pH value is less than 7, if the concentration of OH- increases, the water becomes basic and the pH value is greater than 7, while if the concentration of the two ions is equal, the water is neutral and the pH value is equal to 7. pH value: It is the measure of the acidity or basicity of liquids or solutions. The pH value of pure water is about 7, which is considered neutral. However, this value may vary in natural environments, affecting the organisms that live in them. The pH value of water from different sources: 1- Seawater: The pH value of seawater generally ranges from 7.5 to 8.4, depending on the region in which the sea is located and the environmental factors surrounding it. 2- Fresh water (rivers and lakes): The pH value varies and normally ranges from 6.5 to 8.5 3- Distilled water: The pH value is around 7, because it is free of most of the impurities and ions that contribute to the acidity or alkalinity of natural water sources. 4- Groundwater: The pH of groundwater varies from one region to another depending on several factors, the most important factor is the rock structure of the area. Groundwater is either neutral or alkaline, and its pH value varies due to exposure to salts of certain rocks such as calcium carbonate or magnesium carbonate. 5- The pH of the clouds is generally slightly acidic, with values ranging from 4.5 to 5, due to the presence of carbon dioxide and other acidic gases dissolved in the rain droplets. These values can vary depending on different environmental factors, and human activities in that area which can affect the pH level when forming clouds or rainwater 11 Chapter 1 Aquatic ecosystem Practical activity Measuring the pH values in different water samples: To measure the pH value of different water samples (sea water, river water, and spring water), you can perform the following experiment: Required materials: 1- Water samples (seawater, river water, and spring water) 2- A pH meter or pH test strips. 3-Cups for the samples. 4- Distilled water (for calibration) 5-Stirring rod The procedures for the experiment: 1- Calibration: Calibrate the pH meter according to the manufacturer's instructions using distilled water. 2- Sample preparation: Number the beakers according to the type of water and place a small amount of this type in each beaker. 3- Testing: Immerse the electrode of the calibrated pH meter in each sample and record the reading when it stabilizes. 4- Measuring by using test strips: When using test strips, dip the strip into each sample for few seconds, then compare its colour against the attached chart to determine the approximate pH value Research activity With a group of your colleagues, do research using data that show the different pH values of clouds and rainfall in different regions and the reasons for this. Examples for these regions are, A. Industrial cities b. Agricultural areas c. Coastal cities To minimize the negative impacts on water quality and on the health of living organisms because of hydrolysis of salts and its effects on water, it is important to closely monitor salinity levels as well as the changes in ionic structure of natural water bodies. Proper waste disposal minimizes the addition of harmful salts to water bodies and maintains water quality for wildlife habitats and human consumption. Check your understanding (1) Choose the correct answer : Which of the following represents the proportion of fresh water on the Earth's surface? Ⓐ ○ 1% Ⓑ ○ 3% Ⓒ ○ 70% Ⓓ ○ 97% ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ (2) Explain how a change in the pH value of a river's water could affect the surrounding ecosystem. Propose suggestions for improving the water quality of this river. ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ (3) Design an experiment that examines the effect of different chemicals on water quality and identify how the results of this experiment can be used to preserve aquatic environments ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ 12 Chemical reactions and their impact on water quality 1−2 Lesson 1-2 physical properties of water and their role in the distribution of living organisms Water has unique physical properties that distinguish it from other fluids (liquids and gases), such as the decrease in its density when it reaches the freezing point and the high value of its specific heat, which affect many natural phenomena, and the distribution of living organisms in different environments. Density It is the mass of a unit volume of matter at a given temperature. Because matter is made up of molecules, the density of matter depends on the mass of the molecules and the distances between them. In case of pure water, the mass of 1 cm3 of it at a temperature of 4oC equals 1 g, that is, the density of water at 4oC equals 1 g/cm3, which is equivalent to 1000 kg / m3 in the international unit of density, and as the temperature of water decreases from 4oC to its freezing point, its density decreases as shown in the opposite graph. The ratio between the density of a given substance and the density of pure water at the same temperature is known as the relative density of the substance The density or the relative density of liquids is measured by hydrometer, which is a sealed hollow glass reservoir with a wider bottom for buoyancy, containing lead (or mercury) balls for vertical stabilization and connected to a long, small-diameter glass stem that is graduated in units of density so that the lower scale indicates the highest density measured by the hydrometer and the higher scale indicates the lowest density measured by the hydrometer. Practical activity: Measure the density of different samples of water Use a hydrometer to determine the density of water from different sources: (sea, river, canal, pond, lake, underground). Discuss how the hydrometer can be used to predict the presence of soluble pollutants in a sample of water. 13 Chapter 1 Aquatic ecosystem Water density and water currents in the oceans: The density of water in the oceans is affected by the pressure inside the oceans, the amount of salt dissolved in it, and its temperature. As the pressure increases with increasing depth, the water molecules get closer together, and therefore the density of the water increases. Density is also affected by the amount of dissolved salt (salinity) in the water. The higher the salinity of water, the higher its density. The normal salinity of ocean water is 35 grams per liter of water (or the equivalent of two teaspoons per cup of water). Finally, the temperature of the water affects its density. The lower the temperature of water (down to 4°C), the closer the molecules are to each other, the lower the volume they occupy and the higher the density of water. The differences in water density are one of the causes of water currents in oceans. These water currents carry heat and salt from the tropics to the poles, nutrients from the deep ocean to the surface, and fresh water from rivers or melting snow to different places when these currents travel around the globe. Density of water in Polar Regions The density of water changes as its temperature changes, generally the volume of a liquid increases as the temperature increases and the volume of a liquid decreases as the temperature decreases. Water is an exception to this rule. As the temperature of pure water increases from 0°C to 4°C, the water shrinks and as a result its density increases, and the density of water reaches its highest value (1000 kg/ m3) at 4°C. Water expands as the temperature rises above 4°C, so its density decreases. This helps to understand why a lake in polar regions starts to freeze at the surface rather than at the bottom. When the air temperature is between 4°C and 0°C, the surface water of the lake expands, becoming less dense than the water below it. Finally, the surface water freezes, and the ice remains on the surface as the density of the ice is less than the density of the water while the water remains near the bottom at 4 oC. If not, fish and other marine life would not survive. 14 Chemical reactions and their impact on water quality 1−2 Lesson Practical experiment The effect of the difference in density on the movement of water Prepare ice cubes and add food dyes to the water before it freezes, so that it is easy to observe the melting process of the ice cubes and the direction of water movement after it melts. Put one ice cube in a quantity of fresh water, and another ice cube in an equal quantity of salt water with the salt concentration equal to the salt concentration in ocean water at room temperature. -In which case does the ice cube dissolve at a faster rate? -What are your observations about the movement of water resulting from the melting of each cube? This is already happening in the ocean! If fresh water from melting icebergs enters the ocean, that fresh water spreads out on the surface of the ocean and does not sink. If the freshwater freezes, it forms an insulator between the deeper parts of the ocean and the cold atmospheric air above. Check your understanding (1) Analyze the opposite graph and conclude what happens to the density of water as the temperature changes. ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ (2) Give an example of how a change in temperature and density of water affects organisms in an aquatic environment. ‫ـــــــــــــــــــــــــــــــــــــــــــــــ‬ 15