How does salinity affect dissolved oxygen

However, there are a few variables that can prevent you from acquiring accurate dissolved oxygen data. DO meter accuracy must take these 4 variables into account.

Drum roll please Therefore, it is important to ensure the temperature sensor on the instrument is measuring correctly because temperature affects DO measurements in two ways. First, due to the increase or decrease in molecular activity, diffusion of oxygen through the membrane of an electrochemical probe or sensing element of an optical probe changes with temperature.

Therefore, the sensor signal must be compensated for changes in temperature. This is done by adding a thermistor to the circuit of older, analog instruments. In addition to this effect, temperature also affects the ability of water to dissolve oxygen.

It is a scientific fact that the solubility of oxygen in water is directly proportional to temperature; see the Oxygen Solubility Table.

Warmer water cannot dissolve as much oxygen as colder water. For newer, digital instruments such as the optical ProSolo and the traditional Pro20 , the software compensates for both of these temperature-related factors after instrument calibration and during readings.

In order to perform this conversion, the temperature and salinity of the sample must be known. The second variable that affects DO concentration is the salinity of the water sample. As the salinity of water increases, its ability to dissolve oxygen decreases. For YSI dissolved oxygen instruments that do not have a conductivity sensor, the salinity value of the sample must be manually entered by the end-user.

See the salinity guide below for a list of typical salinity values for various types of water. Today, ppt is commonly replaced by PSU Practical Salinity Units as the preferred unit to describe salinity calculated by the Practical Salinity Scale; however, these values are equivalent since they are determined by the same method. The ability of water to dissolve gases decreases with the addition of electrolytes. Salt ions attract water molecules leaving fewer hydrogen and oxygen ions available to capture and disassociate gas molecules.

The carbon dioxide content of a carbonated drink will fizz out if salt is added to it. Oxygen comprises Under normal circumstances, about 12 parts of oxygen can dissolve in one million parts of water. The sources of this oxygen are the atmosphere and plant photosynthesis that produces oxygen as an end-product. A high concentration of plant life in water can push dissolved oxygen levels to 20 ppm. Higher temperatures decrease the ability of water to dissolve oxygen. Bubbles of air emerging from boiling water demonstrate this effect.

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. 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. Instead of chlorinity [Cl - ] - the amount of chloride in parts per thousand - which was used as a measure of the amount of salt in water, the term salinity is often used.

To obtain oxygen solubility from the Bunsen absorption coefficient, the same procedure as described previously is used. Seawater has a typical salinity of 0.