For example, more oxygen can be held in water of equal temperature and salinity in Miami than at the much higher altitudes of Denver. DO may be supplemented by use of an aeration system.
In an aquarium, this aeration system often consists of only a simple air pump, air tubing and airstone. While this type of system does not greatly increase the oxygen level by pumping air and, therefore, oxygen into the tank, it can help circulate the water and break the surface tension of the water to increase the rate of diffusion.
Very small air bubbles, like those from a fine pore stone, that travel slowly from the bottom to the top of the tank are much more efficient in adding oxygen to the water than large air bubbles which boil the water or airstones which sit near the water surface. Another excellent source of oxygen in an aquarium, and in many natural bodies of water, is plants. Plants produce oxygen as a by-product of photosynthesis, a process by which plants use light energy to produce food from carbon dioxide and water.
In the presence of light, the plants consume carbon dioxide and produce oxygen during photosynthesis. In the absence of light, the fish, plants and other organisms in an aquarium continue consuming oxygen and producing carbon dioxide, but no oxygen is produced. Therefore, oxygen levels are lowest early in the morning after a night of respiration and no photosynthesis. DO is highest in the late afternoon after a full day of oxygen-producing photosynthesis.
See Figure 1. Aquarists should attempt to keep oxygen as close to saturation as possible. 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.
The hypolimnion is separated from the upper layers by the chemocline or halocline. These clines mark the boundary between oxic and anoxic water and salinity gradients, respectively. While lab conditions would conclude that at colder temperatures and higher pressures water can hold more dissolved oxygen, this is not always the result.
This organic material comes from dead algae and other organisms that sink to the bottom. This turnover redistributes dissolved oxygen throughout all the layers and the process begins again.
Stratification in the ocean is both horizontal and vertical. The littoral, or coastal area is most affected by estuaries and other inflow sources. The sublittoral, also known as the neritic or demersal zone, is considered a coastal zone as well.
In this zone, dissolved oxygen concentrations may vary but they do not fluctuate as much as they do in the littoral zone. This zone is also where most oceanic benthic bottom-dwelling organisms exist. Oceanic benthic fish do not live at the greatest depths of the ocean.
They dwell at the seafloor near to coasts and oceanic shelves while remaining in the upper levels of the ocean. Beyond the demersal zone are the bathyal, abyssal and hadal plains, which are fairly similar in terms of consistently low DO.
The exact definitions and depths are subjective, but the following information is generally agreed upon. The epipelagic is also known as the surface layer or photic zone where light penetrates.
This is the layer with the highest levels of dissolved oxygen due to wave action and photosynthesis. The epipelagic generally reaches to m and is bordered by a collection of clines. These clines can overlap or exist at separate depths. Much like in a lake, the thermocline divides oceanic strata by temperature. Each of these clines can affect the amount of dissolved oxygen the ocean strata can hold. Within this strata, the oxygen minimum zone OMZ can occur. 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.
Water flowing from the ocean is generally salty and cooler, while river water is fresh and warmer. Because of these differences, river water weighs less than ocean water and floats on top of it—although wind and other strong mixing forces may change this pattern. The boundary where the fresh water layer meets the salt water layer below is called the pycnocline.
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