Oxygen in water

 What is Dissolved Oxygen? 

Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other liquids. It is an important parameter in assessing water quality because it influenced the organisms living within a body of water. Ex: In limnology (the study of lakes), dissolved oxygen is an essential factor second only to water itself. A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality. Non-compound oxygen, or free oxygen (O₂), is oxygen that is not bonded to any other element. Dissolved oxygen is the presence of these free O_2.

 molecules within the water. The bonded oxygen molecule in water (H₂O) is in a compound and does not count toward dissolved oxygen levels. One can imagine that free oxygen molecules dissolve in water much the way salt or sugar does when it is stirred. 

Dissolved Oxygen Units and Reporting

 Dissolved oxygen is usually reported in milligrams per liter (mg/L) or as a percent of air saturation. However, some studies will report DO in parts per million (ppm) or in micromoles. (1 mg/L is equal to 1 ppm.) The relationship between mg/L and % air saturation varies with temperature, pressure, and, the salinity of the water. One micromole of oxygen is equal to 0.022391 milligrams, and this unit is commonly used in oceanic studies. Thus 100 micro-mole/L O₂ is equal to 2.2 mg/L O₂ Calculating Dissolved oxygen concentration from % Air Saturation To calculate dissolved oxygen concentrations from air saturation, it is necessary to know the temperature and salinity of the sample. Barometric pressure has already been accounted for as the partial pressure of oxygen contributes to the percent air saturation. Salinity and temperature can then be used in Henry’s Law to calculate the DO concentration would be at 100% air saturation. However, it is easier to use an oxygen solubility Chart. These charts show the dissolved oxygen concentration at 100% air saturation at varying temperatures and salinity. This value can then be multiplied by the measured percent air saturation to calculate the dissolved oxygen concentration. 

 Dissolved oxygen and aquatic life 

Dissolved oxygen (DO) is one of the most important indicators of water quality. Oxygen dissolves in surface water due to the aerating action of winds, as a byproduct of aquatic plant photosynthesis. The concentration of oxygen less than 2 mg/L is called hypoxia and no oxygen levels refer to anoxia. Oxygen levels also may be reduced when there are too many bacteria or algae in the water. Oxygen enters water bodies primarily by transfer from the atmosphere across the air-water interface and to a lesser extent by the action of photosynthetic organisms. The rate of transfer of oxygen across the air-water interface is facilitated by increasing the surface area exposed to the atmosphere. The atmospheric transfer is the dominant mechanism for infusing oxygen into an aquatic system, the surface area to volume ratio is very important for establishing the baseline oxygen status for a given water body. Aquatic plants and algae also contribute dissolved oxygen to water bodies during daylight hours through photosynthesis. dissolved oxygen concentrations will typically be highest in the mid-to-late afternoon. When photosynthesis rates are increasing and will reach the lowest concentrations just before the sun rises the next morning due to respiration. Biological Oxygen Demand (BOD) is a measure of the potential for dissolved oxygen within a water body. Oxygen levels are reduced when there are too many bacteria or algae in the water. During the decay process of algae, the bacteria consume the oxygen dissolved in the water. This lead to a decrease in available oxygen in water bodies. Prolonged exposure to low dissolved oxygen levels may not directly kill an organism but may significantly increase its susceptibility to other environmental stresses and diseases. The lethal dissolved oxygen concentrations for fish are between 1 and 3 mg/L. Above 3 mg/L generally seem sufficient for many species. For the detection of the dissolved oxygen in water, there are several methods are used. They are an iodometric method and the membrane electrode method. 

 Dissolved Oxygen Saturation

 In a stable body of water, dissolved oxygen is at equilibrium where water is holding as many dissolved gas molecules as it can in equilibrium. This condition is 100% air saturated. At equilibrium, according to Henry’s law, the percentage of dissolved gas is proportional to the partial pressure of that particular gas. The gas in the air dissolves into the water until equilibrium is reached. This process is hastened by aeration. Shallow water has 100% dissolved oxygen concentration whereas that of deeper water falls below 100% due to the less influence by aeration and consumption of oxygen by microbes in sediments or mud. 

 Factors affect the solubility of oxygen 

 Temperature: The solubility of oxygen rises slightly when the temperature decreases. When the temperature increases, the solubility reduces as the oxygen molecules which have greater kinetic energy, get eliminated from the water surface disrupting its intermolecular forces. 

Pressure: As the atmospheric pressure increases, the solubility of oxygen gets increased because high pressure causes to trap more air in the water surface without escaping. Therefore water sources (sea water) at lower altitudes contain a high concentration of oxygen.

 Salinity: When the salt concentration increases, the solubility of oxygen get reduces. Therefore, seawater has 20% less DO than fresh water that has a similar temperature and pressure.

 How Can Water be More than 100% Saturated? 

 Aquatic respiration and decomposition lower DO concentrations, while rapid aeration and photosynthesis can contribute to supersaturation. During the process of photosynthesis, oxygen is produced and this can add to the dissolved oxygen concentration in the water, potentially bringing it above 100% saturation. Therefore, dissolved oxygen levels can easily be more than 100% air saturation during the day in photosynthetically active bodies of water. Supersaturation is also caused by rapid aeration which can often be seen beside hydro-power dams and large waterfalls. Rapid temperature changes could also make the water supersaturated. 

 Typical Dissolved oxygen levels 

source: https://www.knowyourh2o.com/outdoor-4/dissolved-oxygen-in-water

Dissolved oxygen refers to the level of free oxygen present in water. Water bodies receive oxygen from the atmosphere and from aquatic plants. Levels that are too high or too low can harm aquatic life and affect water quality. Dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration, and decomposition. While water equilibrates toward 100% air saturation, dissolved oxygen levels will also fluctuate with temperature, salinity, and pressure changes. Water at lower temperatures should have higher mg/L of dissolved oxygen and higher dissolve while warmer, polluted waters will have lower mg/L. Healthy water should generally have dissolved oxygen concentrations above 6.5-8 mg/L and between about 80-120 %. Saltwater has a lower capacity (saturation level) to hold   O₂ than freshwater does. The lower the temperature and salinity level, the more oxygen the water can hold. Generally, the concentration of dissolved oxygen in ocean water is 7-8mg/L. Depending on species and stage of life, it is believed that a dissolved oxygen content of 5-6mg/L is sufficient for most marine organisms. While each organism has its own DO tolerance range, generally, DO levels below 3 mg/L are hypoxic, and DO levels below 1 mg/L are considered usually devoid of life. Fish growth and activity usually require 5-6 ppm of DO. Shrimp needs 5-7 mg/L DO to grow healthily and rapidly. Mussels need 0.7-0.8 mg/L and they suffer heavy mortality rates if DO concentrations dipped below this point. Generally dissolved oxygen levels below 3 ppm are stressful to most aquatic organisms. The low level of dissolved oxygen in water is a sign of contamination and is an important factor in determining water quality, pollution control, and treatment process. Just as low dissolved oxygen can cause problems, so too can high concentrations. Supersaturated water can cause gas bubble disease in fish and invertebrates. Significant death rates occur when dissolved oxygen remains above 115%-120% air saturation for some time.

 Consequences of Unusual Dissolved Oxygen Levels

 When the dissolved oxygen concentrations drop below a certain level, fish mortality rates will rise. Sensitive freshwater fish like salmon can’t even reproduce at levels below 6 mg/L. In the ocean, coastal fish begin to avoid areas where DO is below 3.7 mg/L, with specific species abandoning an area completely when levels fall below 3.5 mg/L. Below 2.0 mg/L, invertebrates also leave and below 1 mg/L even benthic organisms show reduced growth and survival rates.  

A fish kill/ winter kill 

     

figure  01: fish kill

figure 02: Winter kill

the above pictures show fish kills occur due to the reduction of oxygen levels in the water. occurs when a large number of fish in an area of water die off. It can be species-based or water-wide mortality. Winterkill is a fish kill caused by a prolonged reduction in dissolved oxygen due to ice or snow cover on a lake or pond. When a body of water is overproductive, the oxygen in the water may get used up faster than it can be replenished. This occurs when a body of water is overstocked with organisms or if there is a large algal bloom die-off. Winterkills occur when respiration from fish, plants, and other organisms is greater than oxygen production by photosynthesis. 

Gas Bubble Disease

                                                              figure 03: gas bubble disease

 is caused by high oxygen concentrations. Supersaturated water can cause gas bubble disease in fish and invertebrates. Significant death rates occur when dissolved oxygen remains above 115%-120% air saturation for some time

 A Dead Zone

                                                  figure04: Algal blooms in the baltic sea

 an area of water with little to no dissolved oxygen present. They are so named because aquatic organisms cannot survive there. These zones are usually a result of a fertilizer-fueled algae and phytoplankton growth boom. When the algae and phytoplankton die, the microbes on the seafloor use A stratification the oxygen decomposing the organic matter. These anoxic conditions are usually stratified, occurring only in the lower layers of the water. While some fish and other organisms can escape, shellfish, young fish and eggs usually die.

 Dissolved Oxygen and Water Column Stratification

 A stratification is an act of sorting data, people, and objects into distinct groups or layers and it can be studied in lakes, oceans, and estuaries. Considering lake stratification; the uppermost layer; epilimnion is warmer than other layers and the depth is dependent on the temperature exchange. In this layer generally algae and phytoplankton engage in photosynthesis and the dissolved oxygen in this layer is nearly 100% saturation. The next level is metalimnion, a transitional layer that fluctuates in thickness and temperature. If the light can penetrate this layer photosynthesis may occur and the dissolved oxygen layer increases. The next layer is hypolimnion. In this layer bacteria and fungi use dissolved oxygen to decompose organic materials which come from sunken dead algae and other organisms. The problem here is that the dissolved oxygen used in decomposition is not replaced because this layer does not contact with the atmosphere for aeration and photosynthesis. The next stratification is oceanic stratification which can occur both horizontally and vertically. In sub littoral zone dissolved oxygen levels may not fluctuate as much as in the littoral layer. In both these zones,  many coral reefs occur and the dissolved oxygen concentration is nearly 100% air saturation. Beyond the damsel, the zone is bathyal, abyssal, and hadal plains, which are in low dissolved oxygen concentration. In vertical strata; the epipelagic zone is known as a photic zone where light penetrates. This is the layer with the highest level of dissolved oxygen due to wave action and photosynthesis. The last stratification is estuary stratification, which is based on salinity distribution because saltwater has less dissolved oxygen concentration than freshwater. The higher river flow increases the dissolved oxygen concentration in the water.

FORMATION OF  ATMOSPHERE 

our atmosphere can be thought of as a complex chemical factory. let's look at how it formed from the beginning of the earth(4.9 billion years ago). the earth is formed by collapsing the huge spinning cloud of hot interstellar gas and dust. it's a rocky volcanic world that has separated its interior into thin low density crust, a thicker silicate mantle, and a high-density partially iron core. this was a plane with an atmosphere of nitrogen and oxygen and also water vapor. 

source: https://www.britannica.com/topic/evolution-of-the-atmosphere-1703862

 3.9 billion years ago the atmosphere is still toxic and the earth's oceans come from both internal and external sources of water. carbon dioxide and ammonia gas dissolved in water and the earth's early oceans quickly become a sink for dissolving enormous quantities of these compounds removing most of them from the atmosphere. due to volcanoes, carbon dioxide is added to the atmosphere continuously. many scientists believe that the result was an archean atmosphere that could have had abundant hydrogen and methane and much less free oxygen. so the scientist refers to this as the reducing atmosphere.

source: https://www.britannica.com/topic/evolution-of-the-atmosphere-1703862

Experiments dating back to 1950 have shown that when liquid water is in contact with such an atmosphere and is exposed to energy sources like lightning or ultraviolet radiation, the result can be the formation of abundant organic molecules including simple amino acids and other essential building blocks of life.

At the beginning of the archean period, the atmosphere had little free oxygen and likely kept the surface quite hot because of the presence of abundant greenhouse gases such as carbon dioxide, methane water vapors, and other gases. the carbon dioxide is still pumping from the volcanoes. the salinity of oceans increases due to the erosion from the continents to the oceans and also it helps to neutralize the acidity of the ocean water.

source: https://earthhow.com/archean-eon/

 only life found on earth is single-celled bacteria (cyanobacteria) in the oceans. those bacteria were used to produce glucose through the processes of photosynthesis. the use of carbon dioxide from the seawater and the sunlight to produce glucose. the side product is oxygen. so that helps to increase the oxygen level in seawater and the atmosphere. for the next two billion years the oxygen level has risen. there the atmosphere cooled and become more oxidizing due to the high oxygen concentration. this high oxygen concentration oxidizes the essential gases for greenhouse effects such as carbon dioxide and methane etc. due to the lack of greenhouse gases the temperature of the atmosphere goes down. 

The cryogenic period has started. the word Cryogenian refers to the cold birth in greek. 

source: https://earthlyuniverse.com/cryogenian-glaciations-birth-animals/

This is 720 million years ago. still, the carbon dioxide released by the volcanoes in huge amounts into the atmosphere. when the continents are apart, many rocks have been exposed and those rocks absorb the carbon dioxide in the atmosphere. the earth began to cover with ice. this glaciation occurred just after the great oxidation event.

 as the earth covers with oxygen, the rocks cannot absorb the carbon dioxide pumps from the volcanoes. therefore carbon dioxide fills in the atmosphere. then the temperature starts to rise and ice starts to melt down. oxygen concentration also starts to rise through the series of reactions. due to the radion of the sun, the ozone has started to form.

source: https://earthlyuniverse.com/cryogenian-glaciations-birth-animals/

This was the beginning of the formation of the ozone layer. the ozone layer protects the earth from the radiation that comes from the sun. For over 120 million years the ozone layer thickening.

 The Permian extinction is the first massive extinction. this nearly leads to the end of all life on earth. so it is called the great dying. 96% of ocean dwellings and 70% of land-dwelling species have died. this extinction is caused by the filling of sulfur dioxide in the atmosphere and the carbon dioxide from the Siberian eruption. Scientists hypothesize that this massive volcanic eruption leads to dramatic changes in the atmosphere. due to the cloudy conditions of the atmosphere sunlight couldn't reach the earth and the acidity level of seawater changed and the oxygen level also changes.

After millions of years from the extinction, the atmosphere becomes normalized and the temperature stabilizes. the acid rains also neutralize. 47 million years ago the temperature is much like today and the duration of a day extends up to 24 hours. plants have evolved to have C4 photosynthesis. then the plants use carbon dioxide and water vapor to produce glucose as fuel. the fore this helps to decrease the carbon dioxide in the atmosphere.

At the end of the 19th century, scientists realized that earth is a habitable oceanic world only because of the influence of two relatively minor but critically important atmospheric gases, water vapor, and carbon dioxide. without those two the oceans would remain as solids due to the freezing of water. with the improvement of meteorological instruments and balloons, researchers can make direct measurements of the temperature and pressure of the atmosphere up to very high altitudes. early they discovered that there are two layers in the atmosphere and they named the lower part the troposphere and the upper part the stratosphere. with the development of science and technology, they were able to find out the five layers of atmosphere instead of the two layers.

source:https://cdn1.byjus.com/wp-content/uploads/2020/10/atmospheric-layers.png 

The five layers of the atmosphere are,

1. Troposphere(0-10 km)
2. Stratosphere(10-30 km)
3. Mesosphere(30-50 km)
4. Thermosphere(50-400 km)
5. Exosphere(>400 km)

The troposphere is the layer responsible for climatic changes. Temperature and oxygen concentration decreases with the altitude. The gas composition of the atmosphere is

• Nitrogen 78%
• Oxygen 21%
• Argon 0.9%
• Carbon dioxide 0.07%

Those are the main gases found in the atmosphere. There are water vapors in the atmosphere which range from 1-4% by volume in the lower atmosphere. The next important and valuable part of the atmosphere is the ozone layer which protects the earth from UV radiation. The temperature of the earth depends on three factors. Those are the amount of sunlight received by the earth, the amount of sunlight the Earth reflects, and the atmospheric retention of the radiated heat.

Carbon dioxide, water vapors, methane, and chlorofluorocarbons absorb the heat energy from the sunlight. Due to these gases, the earth is 40 Celcius degrees warmer than where it could be without these gases. This effect is known as the greenhouse effect. The increase of greenhouse gases such as methane CFC etc causes the depletion of the ozone layer.

 WATER

Water is the most important liquid on the earth for life .water has many leading features.

• Have high heat capacity (4184 J / Kg K)

• Stay in liquid form in a range of temperatures

• Good solvent.

• Polar liquid.

 For living things staying hydrated is very essential. The composition of water is hydrogen and oxygen in a 2:1 ratio. Water can behave in a liquid state for a huge range of temperatures due to its high heat capacity. The triple point of water is 273.16 K. At this temperature it has all three stages liquid, solid, and gas.

75% of the earth's surface is covered by water. Of that percentage, 97% is saline water, and 3% is fresh water. Freshwater is present in many states as glaciers groundwater and surface water. 69%  present as glaciers and 1% as surface water and the rest 30%  as groundwater. Only 1.2%  of total freshwater is drinkable. 
Access to clean fresh water is one of the biggest global problems in this decade. According to the CDC reports 2 billion people don't have access to safely managed drinking water. Much of drinking water is derived from surface water. In developing countries, groundwater is often preferred because it needs fewer treatments and has a better bacteriological quality which helps to minimize water-borne diseases such as cholera. for drinking water, the world health organization  (WHO) has made guidelines for the concentration limits of dissolved items in water. The concentrations of ions can be reported in many units. mg/l, μg/l, ppm,etc.

Groundwater standards depend on eleven variables. they are,

• Temperature
• EC (Electrical Conductivity)
• pH
• The four major cations (Na⁺ ,K⁺ , Mg⁺², Ca⁺² )
• The four major anions ( Cl^- ,HCO₃^- ,SO₄^2- ,NO₃^-  )

Rivers and lakes are surface water sources. They are the most polluted sources because people tend to discard waste. For agriculture, electricity production, and daily uses such as drinking, bathing, etc surface water is used. Access to the groundwater is quite difficult due to the depth of groundwater presence. Their fore people have to dig wells and tube wells depending on the depth of water presence. Rainwater is used by people in the rainy season. Collecting rain waters to small pond tanks and preserving them to dry season is done in most of the dry season areas. Cause the is run out of water more quickly than in other areas.  

 

 Environment and its ecology

The environment includes rock, soil, water, air, atmosphere, and living things. The relation between the components of the environment is called ecology. For the existence of life, ecology is a more important factor. The biosphere is the one where life exists and it occupies 0.00008 percent of the mass of the earth. This includes the lower part of the atmosphere, the hydrosphere, and the lithosphere. In the atmosphere, there is only a small percentage of life exist. The hydrosphere takes the largest portion among these three. The lithosphere is the upper region of soil and rocks. The atmosphere contains all gases that need for the life to exist. The major source of energy is the sun and the plants store that energy using the process called photosynthesis. 

Sunlight broke down into three major components namely visible light, ultraviolet, and infrared radiation. For photosynthesis the visible light range is important. The energy flows through the food chain in the environment. Producers in the environment are plants. primary consumers are herbivores. The predation starts from the next level in the food chain. 

Many cycles control and renew the sources such as the water cycle, nitrogen cycle, carbon cycle, etc. With the human impact, the balance of the environmental processes has been interrupted.