Why dissolved oxygen may exceed 100

Dissolved oxygen is the presence of these free O2 molecules within water. The bonded oxygen molecule in water H2O is in a compound and does not count toward dissolved oxygen levels. Dissolved oxygen is necessary to many forms of life including fish, invertebrates, bacteria and plants.

These organisms use oxygen in respiration, similar to organisms on land. Fish and crustaceans obtain oxygen for respiration through their gills, while plant life and phytoplankton require dissolved oxygen for respiration when there is no light for photosynthesis 4. The amount of dissolved oxygen needed varies from creature to creature.

Microbes such as bacteria and fungi also require dissolved oxygen. These organisms use DO to decompose organic material at the bottom of a body of water. Microbial decomposition is an important contributor to nutrient recycling. Dissolved oxygen enters water through the air or as a plant byproduct. The aeration of water can be caused by wind creating waves , rapids, waterfalls, ground water discharge or other forms of running water.

Man-made causes of aeration vary from an aquarium air pump to a hand-turned waterwheel to a large dam. Dissolved oxygen is also produced as a waste product of photosynthesis from phytoplankton, algae, seaweed and other aquatic plants 8.

While most photosynthesis takes place at the surface by shallow water plants and algae , a large portion of the process takes place underwater by seaweed, sub-surface algae and phytoplankton. Light can penetrate water, though the depth that it can reach varies due to dissolved solids and other light-scattering elements present in the water.

Depth also affects the wavelengths available to plants, with red being absorbed quickly and blue light being visible past m. In clear water, there is no longer enough light for photosynthesis to occur beyond m, and aquatic plants no longer grow.

In turbid water, this photic light-penetrating zone is often much shallower. The basic reaction of aquatic photosynthesis remains:. At equilibrium, the percentage of each gas in the water would be equivalent to the percentage of that gas in the atmosphere — i.

The water will slowly absorb oxygen and other gasses from the atmosphere until it reaches equilibrium at complete saturation This is true of both atmospheric and hydrostatic pressures. Water at lower altitudes can hold more dissolved oxygen than water at higher altitudes. As oxygen in the atmosphere is about However, there are several factors that can affect this. Aquatic respiration and decomposition lower DO concentrations, while rapid aeration and photosynthesis can contribute to supersaturation.

During the process of photosynthesis, oxygen is produced as a waste product. In addition, the equalization of water is a slow process except in highly agitated or aerated situations. Unlike small rapids and waves, the water flowing over a dam or waterfall traps and carries air with it, which is then plunged into the water.

As water temperature rises, oxygen solubility decreases. But if there is no wind to move the equilibration along, the lake will still contain that initial 9. 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. If you are using a dissolved oxygen meter, be sure that it is calibrated immediately prior to use.

Check the cable connection between the probe and the meter. Make sure that the probe is filled with electrolyte solution, that the membrane has no wrinkles, and that there are no bubbles trapped on the face of the membrane.

You can do a field check of the meter's accuracy by calibrating it in saturated air according to th e manufacturer's instructions. Or, you can measure a water sample that is saturated with oxygen, as follows. Once the meter is turned on, allow 15 minute equilibration before calibrating. After calibration, do not turn the meter off until the sample is analyzed.

Once you have verified that the meter is working properly, you are ready to measure the DO levels at the sampling site. You might need an extension pole this can be as simple as a piece of wood to get the probe to the proper sampling point. Simply secure the probe to the end of the extension pole. A golfer's ball retriever works well because it is collapsible and easy to transport. To use the probe, proceed as follows:. Three types of titration apparatus can be used with the Winkler method: droppers, digital titrators, and burets.

The dropper and digital titrator are suited for field use. The buret is more conveniently used in the lab Fig. For titration with a dropper or syringe, which is relatively simple, follow the manufacturer's instructions. The following procedure is for using a digital titrator to determine the quantity of dissolved oxygen in a fixed sample:.

Some water quality standards are expressed in terms of percent saturation. To calculate percent saturation of the sample:. If you are using the Winkler method and delivering the samples to a lab for titration, double-check to make sure that you have recorded the necessary information for each site on the field data sheet, especially the bottle number and corresponding site nu mber and the times the samples were collected.

Deliver your samples and field data sheets to the lab. If you have already obtained the dissolved oxygen results in the field, send the data sheets to your sampling coordinator.

Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water. BOD also measures the chemical oxidation of inorganic matter i.

A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time usually 5 days at 20 C. The rate of oxygen consumption in a stream is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water. BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream.

This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die. Sources of BOD include leaves and woody debris; dead plants and animals; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.

BOD is affected by the same factors that affect dissolved oxygen see above. Aeration of stream water by rapids and waterfalls, for example will accelerate the decomposition of organic and inorganic material. Therefore, BOD levels at a sampling site with slower, deeper waters might be higher for a given volume of organic and inorganic material than the levels for a similar site in highly aerated waters.

Chlorine can also affect BOD measurement by inhibiting or killing the microorganisms that decompose the organic and inorganic matter in a sample. If you are sampling in chlorinated waters, such as those below the effluent from a sewage treatment plant, it is necessary to neutralize the chlorine with sodium thiosulfate. See APHA, BOD measurement requires taking two samples at each site.

One is tested immediately for dissolved oxygen, and the second is incubated in the dark at 20 C for 5 days and then tested for the amount of dissolved oxygen remaining.

This represents the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample bottle during the incubation period. Because of the 5-day incubation, the tests should be conducted in a laboratory.

Sometimes by the end of the 5-day incubation period the dissolved oxygen level is zero. This is especially true for rivers and streams with a lot of organic pollution. Since it is not known when the zero point was reached, it is not possible to tell what the BOD level is. Seasonal changes also affect dissolved oxygen concentrations.

Warmer temperatures during summer speed up the rates of photosynthesis and decomposition. When all the plants die at the end of the growing season, their decomposition results in heavy oxygen consumption. Other seasonal events, such as changes in lake water levels, volume of inflow: Water flowing into a lake.

To the degree that pollution contributes oxygen-demanding organic matter like sewage, lawn clippings, soils from streambank and lakeshore erosion, and from agricultural runoff or nutrients that stimulate growth of organic matter, pollution causes a decrease in average DO concentrations.

If the organic matter is formed in the lake, for example by algal growth, at least some oxygen is produced during growth to offset the eventual loss of oxygen during decomposition. However, in lakes where a large portion of the organic matter is brought in from outside the lake, oxygen production and oxygen consumption are not balanced and low DO may become even more of a problem. The development of anoxia: Condition of being without dissolved oxygen O 2.

Besides the direct effects on aerobic organisms, anoxia can lead to increased release of phosphorus: Key nutrient influencing plant growth in lakes. Soluble reactive phosphorus PO 4 -3 is the amount of phosphorus in solution that is available to plants. Total phosphorus includes the amount of phosphorus in solution reactive and in particulate form. It also leads to the buildup of chemically reduced compounds such as ammonium and hydrogen sulfide H 2 S, rotten egg gas which can be toxic to bottom dwelling organisms.

In extreme cases, sudden mixing of H 2 S into the upper water column: A conceptual column of water from lake surface to bottom sediments. Oxygen saturation is calculated as the percentage of dissolved O 2 concentration relative to that when completely saturated at the temperature of the measurement depth.

The elevation of the lake, the barometric pressure p : The force exerted per unit area. In most lakes, the effect of dissolved solute: A substance which can be dissolved into another substance. Use the chart below for nomagrams for calculating saturation. For a quick and easy determination of the percent saturation value for dissolved oxygen at a given temperature, use the saturation chart above. The percent saturation is the value where the line intercepts the saturation scale.

Read the rest of this section and the Lake Ecology Primer for more about dissolved oxygen in lakes. Note that this nomogram assumes that the lakes are at sea level whereas the Minnesota WOW lakes vary from to feet elevation. The saturation value can also vary slightly depending on barometric pressure with lower values expected when a storm front moves through as compared to bright and sunny "high pressure" days.

There is also a series of equations you can use to calculate percent saturation. You begin by determining the equilibrium oxygen at nonstandard pressure, C p , using the equation shown below.

But even before you can do that you first need to determine the atmospheric pressure at your lake's altitude h in kilometers using equation Now you can dive into equation 2 below. You can use this Javascript utility to perform the calculations.

It uses the equations from above, which were obtained from the Mortimer, C. The following formula is an Excel version of Equation 2 -- you can use it to calculate C p in a spreadsheet:. Enter P atm at your altitude in spreadsheet cell "C3", and enter the water temperature degrees C in cell "B7".

Copy the above formula in a single line to a spreadsheet cell -- it will calculate C p. Standard methods for the examination of water and wastewater.