Oxygen is removed from the water by respiration and decomposition of organic matter. Dissolved Oxygen can be measured with an electrode and meter or with field test kits. The electronic meter does not measure oxygen directly; rather, it uses electrodes to measure the partial pressure of oxygen in the water, which is converted to oxygen mass weight concentration. The field test kits such as a drop bottle, a microburet, or a digital titrator involve adding a solution of known strength to a treated sample of water from the stream.
The amount of solution required to change the color of the sample reflects the concentration of DO in the sample. Dissolved oxygen levels are also often reported in percent saturation. Temperature affects DO concentrations, and calculating the percent saturation will factor out the effect of temperature.
The "saturation level" is the maximum concentration of dissolved oxygen that would be present in water at a specific temperature, in the absence of other factors. Scientists have determined the saturation DO level for various temperatures.
Saturation levels also vary with elevation. Percent saturation is calculated by dividing the measured dissolved oxygen concentration by the saturation level and multiplying by In fast-moving streams, rushing water is aerated by bubbles as it churns over rocks and falls down hundreds of tiny waterfalls.
These streams, if unpolluted, are usually saturated with oxygen. In slow, stagnant waters, oxygen only enters the top layer of water, and deeper water is often low in DO concentration due to decomposition of organic matter by bacteria that live on or near the bottom of the reservoir. Dams slow water down, and therefore can affect the DO concentration of water downstream. If water is released from the top of the reservoir, it can be warmer because the dam has slowed the water, giving it more time to warm up and lose oxygen.
If dams release water from the bottom of a reservoir, this water will be cooler, but may be low in DO due to decomposition of organic matter by bacteria.
The colder the water, the more oxygen can be dissolved in the water. Therefore, DO concentrations at one location are usually higher in the winter than in the summer. During dry seasons, water levels decrease and the flow rate of a river slows down. As the water moves slower, it mixes less with the air, and the DO concentration decreases.
During rainy seasons, oxygen concentrations tend to be higher because the rain interacts with oxygen in the air as it falls. More sunlight and warmer temperatures also bring increased activity levels in plant and animal life; depending on what organisms are present, this may increase or decrease the DO concentration.
The type and number of organisms in the water body. During photosynthesis, plants release oxygen into the water. During respiration, plants remove oxygen from the water. Dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration and decomposition.
In freshwater systems such as lakes, rivers and streams, dissolved oxygen concentrations will vary by season, location and water depth. Saltwater holds less oxygen than freshwater, so oceanic DO concentrations tend to be lower than those of freshwater. Coldwater fish like trout and salmon are most affected by low dissolved oxygen levels The mean DO level for adult salmonids is 6.
The mean DO levels should remain near 5. The freshwater fish most tolerant to DO levels include fathead minnows and northern pike. Northern pike can survive at dissolved oxygen concentrations as low as 0.
If all the oxygen at their water level gets used up, bacteria will start using nitrate to decompose organic matter, a process known as denitrification. If organic matter accumulates faster than it decomposes, sediment at the bottom of a lake simply becomes enriched by the organic material.
This does not mean that saltwater fish can live without dissolved oxygen completely. The red hake is also extremely sensitive to dissolved oxygen levels, abandoning its preferred habitat near the seafloor if concentrations fall below 4.
The dissolved oxygen requirements of open-ocean and deep-ocean fish are a bit harder to track, but there have been some studies in the area.
Billfish swim in areas with a minimum of 3. Likewise, white sharks are also limited in dive depths due to dissolved oxygen levels above 1. Tracked swordfish show a preference for shallow water during the day, basking in oxygenated water 7.
Albacore tuna live in mid-ocean levels, and require a minimum of 2. Many tropical saltwater fish, including clown fish, angel fish and groupers require higher levels of DO, such as those surrounding coral reefs.
Coral reefs are found in the euphotic zone where light penetrates the water — usually not deeper than 70 m. Crustaceans such as crabs and lobsters are benthic bottom-dwelling organisms, but still require minimum levels of dissolved oxygen. If dissolved oxygen concentrations drop below a certain level, fish mortality rates will rise.
In the ocean, coastal fish begin to avoid areas where DO is below 3. Below 2. A fishkill occurs when a large number of fish in an area of water die off. It can be species-based or a water-wide mortality. Fish kills can occur for a number of reasons, but low dissolved oxygen is often a factor. 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. High levels of nutrients fuel algae blooms, which can initially boost dissolved oxygen levels.
But more algae means more plant respiration, drawing on DO, and when the algae die, bacterial decomposition spikes, using up most or all of the dissolved oxygen available. This creates an anoxic, or oxygen-depleted, environment where fish and other organisms cannot survive. They occur when the water is covered by ice, and so cannot receive oxygen by diffusion from the atmosphere.
If the ice is then covered by snow, photosynthesis also cannot occur, and the algae will depend entirely on respiration or die off. In these situations, fish, plants and decomposition are all using up the dissolved oxygen, and it cannot be replenished, resulting in a winter fish kill.
Just as low dissolved oxygen can cause problems, so too can high concentrations. Extended periods of supersaturation can occur in highly aerated waters, often near hydropower dams and waterfalls, or due to excessive photosynthetic activity. This is often coupled with higher water temperatures, which also affects saturation. A dead zone is an area of water with little to no dissolved oxygen present. They are so named because aquatic organisms cannot survive there. They can occur in large lakes and rivers as well, but are more well known in the oceanic context.
These zones are usually a result of a fertilizer-fueled algae and phytoplankton growth boom. These anoxic conditions are usually stratified, occurring only in the lower layers of the water. Naturally occurring hypoxic low oxygen conditions are not considered dead zones. Such naturally occurring zones frequently occur in deep lake basins and lower ocean levels due to water column stratification. Bio-available nutrients in the discharge can stimulate algal blooms, which die and are eaten by bacteria, depleting the oxygen in the subsurface water.
The hypoxic zone in the northern Gulf of Mexico is in the center of a productive and valuable fishery. The increased frequency and expansion of hypoxic zones have become an important economic and environmental issue to commercial and recreational users of the fishery. Field and lab meters to measure dissolved oxygen have been around for a long time.
As this picture shows, modern meters are small and highly electronic. They still use a probe, which is located at the end of the cable. Dissolved oxygen is dependent on temperature an inverse relation , so the meter must be calibrated properly before each use.
Do you want to test your local water quality? Water test kits are available from World Water Monitoring Challenge WWMC , an international education and outreach program that builds public awareness and involvement in protecting water resources around the world. Teachers and water-science enthusiasts: Do you want to be able to perform basic water-quality tests on local waters? WWMC offers inexpensive test kits so you can perform your own tests for temperature , pH , turbidity , and dissolved oxygen.
Do you think you know a lot about water properties? Want to know more about dissolved oxygen and water? Follow me to the Nutrients and Eutrophication website! Looking at water, you might think that it's the most simple thing around. Pure water is practically colorless, odorless, and tasteless. But it's not at all simple and plain and it is vital for all life on Earth.
Where there is water there is life, and where water is scarce, life has to struggle or just "throw in the towel. The range goes from 0 to 14, with 7 being neutral. The pH of water is a very important measurement concerning water quality. Yes, water below your feet is moving all the time, but, no, if you have heard there are rivers flowing below ground, that is not true. Water moves underground downward and sideways, in great quantities, due to gravity and pressure.
Eventually it emerges back to the land surface, into rivers, and into the oceans to keep the water cycle going. Water temperature plays an important role in almost all USGS water science. Water temperature exerts a major influence on biological activity and growth, has an effect on water chemistry, can influence water quantity measurements, and governs the kinds of organisms that live in water bodies.
Water and electricity don't mix, right? Well actually, pure water is an excellent insulator and does not conduct electricity. The thing is, you won't find any pure water in nature, so don't mix electricity and water.
Our Water Science School page will give you all the details. Lucky for us all, our drinking water is almost always clear very low turbidity. Other water, such as the creek behind your house after a rainstorm, is likely to be highly turbid—brown with floating sediment. Turbidity is the clarity of water and it is an important factor in water quality. The USGS collaborates with local, state, federal, tribal, university, and industry partners to conduct the science necessary to understand the causes and effects of toxic HABs and inform water management and public health decisions.
USGS is characterizing the life cycle of HABs, their asociated toxins, and the genes responsible for cyanotoxin production. This work is enhancing the ability of Cyanobacterial harmful algal blooms HABs are increasingly a global concern because HABs pose a threat to human and aquatic ecosystem health and cause economic damages.
Toxins produced by some species of cyanobacteria called cyanotoxins can cause acute and chronic illnesses in humans and pets. Eutrophication, or excess nutrients in streams, is typically one of the top reasons that a stream is listed as impaired on the d list as part of the Clean Water Act. How nutrients, primarily nitrogen and phosphorus, are transported to streams and groundwater greatly affects the best management plan to keep them on fields and out of streams and groundwater.
Likewise, environmental managers Accurate data for the concentration of dissolved oxygen in surface and ground waters are essential for documenting changes in environmental water resources that result from natural phenomena and human activities.
Dissolved oxygen is necessary in aquatic systems for the survival and growth of many aquatic organisms and is used as an indicator of The Lees Ferry site pictured here is one of six sites on the Colorado River being continuously monitored for dissolved oxygen concentrations.
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