However, the amount of oxygen an animal needs varies depending on how large or complex the animal is and where it lives. Anoxic, or no-oxygen, areas are regions with less than 0. Areas in the Bay that have low dissolved oxygen levels are the result of a complex interaction of several natural and man-made factors, including temperature, nutrient pollution , water flows and the shape of the Bay's bottom.
Temperature limits the amount of oxygen that can dissolve in water: water can hold more oxygen during winter than during the hot summer months. So, although high temperatures can influence dissolved oxygen levels, temperature is not the only cause of low-oxygen areas found in the Bay each summer.
Excess nutrients in the water known as eutrophication can fuel the growth of algae blooms. Oysters, menhaden and other filter feeders eat a portion of the excess algae, but much of it does not end up being consumed. During this process, bacteria consume oxygen until there is little or none left in these bottom waters. 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. Stratification separates a body of water into layers. This layering can be based on temperature or dissolved substances namely salt and oxygen with both factors often playing a role. The stratification of water has been commonly studied in lakes, though it also occurs in the ocean.
It can also occur in rivers if pools are deep enough and in estuaries where there is a significant division between freshwater and saltwater sources. The uppermost layer of a lake, known as the epilimnion, is exposed to solar radiation and contact with the atmosphere, keeping it warmer.
Within this upper layer, algae and phytoplankton engage in photosynthesis. The exact levels of DO vary depending on the temperature of the water, the amount of photosynthesis occurring and the quantity of dissolved oxygen used for respiration by aquatic life.
Below the epilimnion is the metalimnion, a transitional layer that fluctuates in thickness and temperature. Here, two different outcomes can occur. This means that the dissolved oxygen level will be higher in the metalimnion than in the epilimnion. The next layer is the hypolimnion. If the hypolimnion is deep enough to never mix with the upper layers, it is known as the monimolimnion. 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. DO sensor method , and strict adherence to analytical methodology.
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