Algal blooms can occur under such conditions. Rapidly moving water, such as in a mountain stream or large river, tends to contain a lot of dissolved oxygen, whereas stagnant water contains less. Bacteria in water can consume oxygen as organic matter decays.
Thus, excess organic material in lakes and rivers can cause eutrophic conditions, which is an oxygen-deficient situation that can cause a water body to "die. Water near the surface of the lake— the epilimnion— is too warm for them, while water near the bottom—the hypolimnion— has too little oxygen. Conditions may become especially serious during a period of hot, calm weather, resulting in the loss of many fish.
You may have heard about summertime fish kills in local lakes that likely result from this problem. Water temperture affects dissolved-oxygen concentrations in a river or water body. As the chart shows, the concentration of dissolved oxygen in surface water is affected by temperature and has both a seasonal and a daily cycle.
Cold water can hold more dissolved oxygen than warm water. In winter and early spring, when the water temperature is low, the dissolved oxygen concentration is high. In summer and fall, when the water temperature is high, the dissolved-oxygen concentration is often lower.
Dissolved oxygen in surface water is used by all forms of aquatic life; therefore, this constituent typically is measured to assess the "health" of lakes and streams. Oxygen enters a stream from the atmosphere and from groundwater discharge. The contribution of oxygen from groundwater discharge is significant, however, only in areas where groundwater is a large component of streamflow, such as in areas of glacial deposits.
You may have heard about a Gulf of Mexico "dead zone" in areas of the Gulf south of Louisiana, where the Mississippi and Atchafalaya Rivers discharge. A dead zone forms seasonally in the northern Gulf of Mexico when subsurface waters become depleted in dissolved oxygen and cannot support most life. The zone forms west of the Mississippi Delta over the continental shelf off Louisiana and sometimes extends off Texas. The oxygen depletion begins in late spring, increases in summer, and ends in the fall.
Orange and red colors indicate lower dissolved oxygen concentrations. The formation of oxygen-depleted subsurface waters has been associated with nutrient-rich nitrogen and phosphorus discharge from the Mississippi and Atchafalaya Rivers.
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.
After the algae complete their life cycle and die, they are consumed by bacteria. During this decay process the bacteria also consume the oxygen dissolved in the water. This can lead to decreased levels of biologically available oxygen, in some cases leading to fish kills and death to other aquatic organisms. Florida's surface water quality standards include minimum values for dissolved oxygen saturation.
The standards for freshwater vary in different bioregions of the state:. Dissolved oxygen standards for marine water bodies are expressed as minimums for daily, weekly, and monthly averages.
Detailed water quality standards for dissolved oxygen can be found in Chapter DO can be expressed as a concentration per unit volume, or as a percentage. In aquatic environments, oxygen saturation is a ratio of the concentration of dissolved oxygen O 2 , to the maximum amount of oxygen that will dissolve in that water body, at the temperature and pressure which constitute stable equilibrium conditions.
Oxygen enters water through several methods, including diffusion from the atmosphere, rapid movement of water waves, e. Dissolved oxygen is routinely recorded as part of basic water quality sampling in most surface waters and near-shore coastal systems.
There are three common methods for measuring DO. The most practical and consistently accurate method for field measurements employs the polarographic DO sensor. If calibrated correctly, this method provides accurate measurements that can be performed in-situ i. However, since this method requires a relatively high degree of titration skill, the handling of hazardous reagents chemicals , and great care with sample collection technique and preservation, it is used in the laboratory only by skilled analysts.
Dissolved oxygen saturation is expressed as a percentage. The accuracy of DO measurement is completely dependent upon proper calibration and maintenance re. Over the years, the technology has evolved into three common electrochemical sensors technologies: galvanic, polarographic and pulsed polarographic. Galvanic and polarographic DO sensors both use a thin semi-permeable membrane that wraps over an electrolyte solution and two metal electrodes.
Oxygen in the water diffuses through the membrane at a rate proportional to its partial pressure. These probes measure the current as oxygen is reduced at the cathode and more oxygen diffuses through the membrane. Membrane-based DO sensors, however, have a number of deficiencies making them difficult to use in extended deployments. The membrane is fragile, requires frequent replacement, and these sensors require frequent calibration and stirring or flowing water.
Manufacturers have now also developed dissolved oxygen sensors with optical sensing technology. These units have more durable sensing elements than membrane-based DO sensors.
0コメント