Dissolved Oxygen Percent Saturation



Effects of oxygen on water quality

Dissolved oxygen is the term for atmospheric oxygen that becomes mixed in water and occurs between the water molecules. The presence of oxygen in water is good. Dissolved oxygen is necessary for healthy lakes and rivers. Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration. Fish will drown in water when the dissolved oxygen levels get too low. The absence of dissolved oxygen in water is a sign of possible pollution.

Most dissolved oxygen gets into the water from contact with the atmosphere. Waves on lakes and slow-moving rivers, water tumbling over riffles or waterfalls on fast- moving rivers mixes oxygen into the water. Anything that increase the surface contact of water and the atmosphere will increase oxygen in the water. Plants and algae also add oxygen to the water through photosynthesis. Because plants need light for photosynthesis, dissolved oxygen levels tend to be highest in the late afternoon and lowest at dawn.

Climate can affect oxygen levels in other ways. During dry seasons water levels decrease and the flow rate or discharge of a river is lower. As the water moves slower, it mixes less with the air; and the dissolved oxygen level goes down. During rainy seasons oxygen levels tend to be higher.

The main man-made factor causing dissolved oxygen levels to change in a negative way involves the build-up of organic wastes. Organic wastes are the remains of any living or once-living thing. Leaves, grass clippings, dead plants or animals, and sewage are examples of organic wastes that can lower oxygen levels in water. Organic wastes are decomposed by bacteria which take oxygen out of the water. When people dump organic wastes into lakes and streams it causes dissolved oxygen levels to decrease which can harm the aquatic life.

When dissolved oxygen levels get lower, they can cause major changes in the types and amounts of aquatic organisms found living in the water. Species that need high levels of dissolved oxygen such as mayfly nymphs, stonefly nymphs, caddisfly larvae, pike, trout, and bass will move out or die. They will be replaced by organisms such as sludge worms, blackfly larvae, and leeches which can tolerate lower dissolved oxygen levels.

Dissolved Oxygen Percent Saturation

Percent Saturation is the amount of oxygen dissolved in the water sample compared to the maximum amount that could be present at the same temperature.

When water holds all the dissolved oxygen [DO] it can hold at a given temperature, it is said to be 100% saturated. If water holds half-as-much oxygen as it can hold at a given temperature, it is 50% saturated.

Sometimes water can become supersaturated with oxygen because of rapidly tumbling water. This usually only lasts for a short period of time because as tiny air bubbles rise to the surface of the water and pop, the percent of saturation soon drops to 100% percent or less. Nevertheless supersaturation can be harmful to fish and other aquatic organisms. Capillaries in the fishes gills can rupture or tear from too much oxygen. Fish with this condition usually die. It is referred to as gas bubble disease.

Streams that have a dissolved oxygen saturation percent value of 80- 120% are considered to be excellent. If the DO saturation percent is 60-79%, it is okay, but not great. If the DO percent saturation value is over 125% or below 60%, the water quality is poor.

Reasons for Natural Variation

Oxygen is produced during photosynthesis and consumed during respiration and decomposition.

Because it requires light, photosynthesis occurs only during daylight hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in DO concentrations. During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, DO concentration may steadily decline. It is lowest just before dawn, when photosynthesis resumes.

Other sources of oxygen include the air and inflowing streams. Oxygen concentrations are much higher in air, which is about 21% oxygen, than in water, which is a tiny fraction of 1% oxygen.

Where the air and water meet, this tremendous difference in concentration causes oxygen molecules in the air to dissolve into the water. More oxygen dissolves into water when wind stirs the water; as the waves create more surface area, more diffusion can occur. A similar process happens when you add sugar to a cup of coffee - the sugar dissolves. It dissolves more quickly, however, when you stir the coffee.

Another physical process that affects DO concentrations is the relationship between water temperature and gas saturation. Cold water can hold more of any gas, in this case oxygen, than warmer water. Warmer water becomes "saturated" more easily with oxygen. As water becomes warmer it can hold less and less DO. So, during the summer months in the warmer top portion of a lake, the total amount of oxygen present may be limited by temperature. If the water becomes too warm, even if 100% saturated, O2 levels may be suboptimal for many species of trout.

Effects of pH on water quality

A pH test measures the alkalinity or acidity concentration in water. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic or alkaline. Acid rain, from vehicle exhausts or coal-fired power plants, causes a drop in the pH of water. Pollution from accidental spills and sewer overflows can also change the pH. Buffering capacity is water's ability to resist changes in pH, and is critical to the survival of aquatic life. Limestone soils act to neutralise these acids and often result in a more basic pH.

Other external factors that can cause fluctuations in the river pH include agricultural runoff and fossil fuel emissions such as carbon dioxide, which creates a weak acid when dissolved in river water.

Typical rainwater has a pH of about 5 to 6. This means that it is naturally a neutral, slightly acidic liquid. During precipitation rainwater dissolves gasses such as carbon dioxide and oxygen. The industry now emits great amounts of acidifying gasses, such as sulphuric oxides and carbon monoxide. These gasses also dissolve in rainwater. This causes a change in pH of the precipitation – the pH of rain will fall to a value of or below 4. When a substance has a pH of below 6.5, it is

acid. The lower the pH, the more acid the substance is. That is why rain with a lower pH, due to dissolved industrial emissions, is called, “acid rain.”

The pH values of natural surface waters (including rivers) usually range from 5.5 to 8.5. Extremely high (9.6) or low (4.5) values are unsuitable for most aquatic organisms. Young fish and immature stages of aquatic insects are extremely sensitive to pH levels below 5. Low pH levels cause fish kill by stressing animal systems and causing physical damage, which in turn makes them more vulnerable to disease. Very acidic waters can also cause the leeching of heavy metals, such as copper and aluminum, that then become soluble and mobile in the water. Heavy metals can collect on fish gills or cause deformities in young fish, making it harder for them to survive.

The pH of natural water depends on several factors, which include the bicarbonate buffering system, types of rock, types of soil and nature of discharged pollutants. The concentration of carbonates and carbon dioxide is the main influence on the pH of clean water. High concentrations of bicarbonate produce alkaline waters (high pH), while low concentrations usually produce acidic waters (low pH).

Acidic and alkaline compounds can be weathered into the stream from the different types of rock present. When limestone (CaCO3) is present, carbonates can be released, affecting the alkalinity of the water. The types of soil in the drainage area also affect the pH. Drainage water from forests and marshes is often slightly acidic, due to the presence of humic acids produced by decaying vegetation.

Nitrogen oxides (NO, NO2) and sulphur dioxide (SO2) from vehicle and power plant emissions are converted into nitric acid (HNO3) and sulphuric acid (H2SO4) in the atmosphere. These acids can affect the pH of streams by combining with moisture in the air and falling to the earth as acid rain or snow.

When pollution results in higher algal and plant growth (e.g., from increased temperature or excess nutrients), pH levels may increase, as allowed by the buffering capacity of the lake.

Although these small changes in pH are not likely to have a direct impact on aquatic life, they greatly influence the availability and solubility of all chemical forms in the lake and may aggravate nutrient problems. For example, a change in pH may increase the solubility of phosphorus, making it more available for plant growth and resulting in a greater long-term demand for dissolved oxygen.

Effects of temperature on water quality

Temperature impacts the rates of metabolism and growth of aquatic organisms, rate of plants' photosynthesis, solubility of oxygen in river water, and organisms' sensitivity to disease, parasites, and toxic materials. At a higher temperature, plants grow and die faster, leaving behind matter that requires oxygen for decomposition.

Water temperature also affects how much oxygen the water can hold. The warmer the water the lower the dissolved oxygen, and vice versa. It is no accident that fish requiring a lot of oxygen are found in cold water. If dissolved oxygen levels are too low, fish and other aquatic animals will move away or die.

In the winter, dissolved oxygen levels are usually higher than in summer. That is why fish kills usually occur in late summer just before dawn.

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