How does dissolved oxygen affect fish

In limnology the study of lakes , dissolved oxygen is an essential factor second only to water itself. A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality. 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.

One can imagine that free oxygen molecules dissolve in water in much the same way that salt or sugar does when it is stirred. 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. The amount of dissolved oxygen needed varies from creature to creature. Microbes such as bacteria and fungi also require dissolved oxygen.

These organisms use dissolved oxygen to decompose organic material at the bottom of a body of water. Microbial decomposition is an important contributor to nutrient recycling.

However, if there is an excess of decaying organic material from dying algae and other organisms , in a body of water with infrequent or no turnover also known as stratification , the oxygen at lower water levels will get used up quicker. 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.

Dissolved oxygen is also produced as a waste product of photosynthesis from phytoplankton, algae, seaweed and other aquatic plants. 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 metres below the surface. In clear water there is no longer enough light for photosynthesis to occur beyond metres 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:. As aquatic photosynthesis is light-dependent, the dissolved oxygen produced will peak during daylight hours and decline at night.

At equilibrium, the percentage of each gas in the water would be equivalent to the percentage of that gas in the atmosphere. The water will slowly absorb oxygen and other gasses from the atmosphere until it reaches equilibrium at complete saturation. This process is sped up by wind-driven waves and other sources of aeration. This water is below an invisible boundary called the thermocline the depth at which water temperature begins to decline.

First, the solubility of oxygen in water decreases as temperature increases. Second dissolved oxygen decreases exponentially as salt levels increase. This is shown in the graph above. Third, dissolved oxygen will increase as pressure increases.

This is true of both atmospheric and hydrostatic pressures. Water at lower altitudes can hold more dissolved oxygen than water at higher altitudes. Aquatic respiration and decomposition lower dissolved oxygen 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. Supersaturation caused by rapid aeration is often seen beside hydro-power dams and large waterfalls. 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. The graph above, where dissolved oxygen is the black line, shows how dissolved oxygen levels vary with the seasons.

While groundwater usually has low dissolved oxygen levels, groundwater-fed streams can hold more oxygen due to the influx of colder water and the mixing it causes. Saltwater holds less oxygen than freshwater, so oceanic dissolved oxygen concentrations tend to be lower than those of freshwater. There are lower dissolved oxygen concentrations near the equator because salinity is higher.

Coldwater fish like trout and salmon are most affected by low dissolved oxygen levels. The mean dissolved oxygen level for adult salmonids is 6. The mean dissolved oxygen levels should remain near 5. The freshwater fish most tolerant to dissolved oxygen levels include fathead minnows and northern pike. Northern pike can survive at dissolved oxygen concentrations as low as 0. 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. 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. DO sensor method , and strict adherence to analytical methodology.

The USF Water Institute is committed to ensuring that our websites conform with Accessibility Support guidelines for people who need to use assistive technologies. We are continually improving the user experience for everyone, and applying the relevant accessibility standards. View our Accessibility Statement for more information. The higher temperatures may also cause other chemical or biological reactions to occur that "consume" or use up the existing oxygen content of the water faster than it can be reintroduced to the solution through diffusion or mixing.

Decreased DO levels may also be indicative of too many bacteria and an excess amount of biological oxygen demand - BOD untreated sewage, partially treated sewage, organic discharges, anoxic discharges which use up DO. A third reason for decreased DO may be fertilizer runoff from farm fields and lawns. The same fertilizer which was meant to make land plants grow better, now makes the aquatic plants do the same. If the weather becomes cloudy for several days, respiring plants will use much of the DO while failing to photosynthesize.

When the increased numbers of aquatic plants eventually die, their organic matter supports greater amounts of bacteria which use up large amounts of the dissolved oxygen. For the surface water quality index calculator, the chart for Dissolved Oxygen is actually based on the percent saturation of Dissolved Oxygen in water.

For example, the field-measured Dissolved Oxygen concentration is 5. Therefore, using the graph below, the Q-value would be approximately The major benefit of the web-based Water Quality Index Calculator is that it uses a more uniform and accurate way to interpret the Q-value from the historic graph below.

This has consequences for both marine organisms and humans. The increased levels of percentage of dissolved oxygen have improved the possibilities of aquatic live. Source: The Skeptical environmentalist; measuring the real state of the world. Author: Bjorn Lomborg. A high DO level in a community water supply is good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes. For this reason, industries use water with the least possible amount of dissolved oxygen.

Water used in very low pressure boilers have no more than 2. Toggle navigation. Why oxygen dissolved in water is important Why Dissolved Oxygen is Important Dissolved oxygen The dissolved oxygen DO is oxygen that is dissolved in water.

The temperature effect If water is too warm, there may not be enough oxygen in it. In the graph below you can see the effect of the temperature in the DO Environmental Impact Total dissolved gas concentrations in water should not exceed percent.

River pollution In the graph below you can see the percentage levels of Oxygen dissolved in the river "The Thames" in the period , The New York harbor in the period , and the river "The Rhine" in the period