Section Overview
`1. What is Dissolved Oxygen
`2. Dissolved Oxygen and Aquatic Life
`3. Where Does Dissolved Oxygen Come From
`Dissolved Oxygen From Photosynthesis
`4. Dissolved Oxygen Saturation
`What Aects Oxygen Solubility
`How Water Can be More Than 100% Saturated
`5. Typical Dissolved Oxygen Levels
`Freshwater Organisms and DO Requirements
`Saltwater Organisms and DO Requirements
`6. Consequences of Unusual Dissolved Oxygen Levels
`Fish Kills
`Gas Bubble Disease
`Dead Zones
`7. Dissolved Oxygen and Water Column Stratication
`Lake Stratication
`Oceanic Stratication
`Estuary Stratication
`8. Dissolved Oxygen Units of Measurement
`Calculating DO from % Air Saturation
`9. Cite This Work
`10. Additional Information
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`Dissolved Oxygen
`monitoring instruments
`
`find it at fondriest.com
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`Reena
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`Dissolved Oxygen
`What is Dissolved Oxygen?
`Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other liquids. It is an
`important parameter in assessing water quality because of its inuence on the organisms living within a body of
`water. In limnology (the study of lakes), dissolved oxygen is an essential factor second only to water itself ¹.  A
`dissolved oxygen level that is too high or too low can harm aquatic life and aect water quality.
`Non-compound oxygen, or free oxygen (O2), is oxygen that is not bonded to any other element. 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 much the way salt or sugar does when it is stirred ².
`Non-bonded oxygen molecules in water
`Dissolved Oxygen and Aquatic Life
`Dissolved oxygen is necessary to many forms of life including sh, 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
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`Dissolved oxygen is important to
`many forms of aquatic life.
`How dissolved oxygen enters water
`there is no light for photosynthesis . The amount of dissolved oxygen
`needed varies from creature to creature. Bottom feeders, crabs, oysters and
`worms need minimal amounts of oxygen (1-6 mg/L), while shallow water sh
`need higher levels (4-15 mg/L)
`.
`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. 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 stratication), the oxygen at lower water levels will
`get used up quicker
`.
`Where Does DO Come From?
`Dissolved oxygen enters water through the air or as a
`plant byproduct. From the air, oxygen can slowly
`diuse across the water’s surface from the
`surrounding atmosphere, or be mixed in quickly
`through aeration, whether natural or man-made .
`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 .
`Dissolved Oxygen from Photosynthesis
`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 aects the wavelengths available to plants, with red being absorbed quickly and blue light
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`Dissolved oxygen can enter the water as a byproduct of
`photosynthesis.
`Not all water depths reach 100% air saturation
`being visible past 100 m. In clear water, there is no
`longer enough light for photosynthesis to occur
`beyond 200 m, and aquatic plants no longer grow. In
`turbid water, this photic (light-penetrating) zone is
`often much shallower.
`Regardless of wavelengths available, the cycle doesn’t
`change
`. In addition to the needed light, CO2 is
`readily absorbed by water (it’s about 200 times more
`soluble than oxygen) and the oxygen produced as a
`byproduct remains dissolved in water¹
`. The basic
`reaction of aquatic photosynthesis remains:
`CO2 + H2O
` (CH2O) + O2
`As aquatic photosynthesis is light-dependent, the
`dissolved oxygen produced will peak during daylight
`hours and decline at night
`.
`Dissolved Oxygen Saturation
`In a stable body of water with no stratication,
`dissolved oxygen will remain at 100% air saturation.
`100% air saturation means that the water is holding as
`many dissolved gas molecules as it can in equilibrium.
`At equilibrium, the percentage of each gas in the
`water would be equivalent to the percentage of that
`gas in the atmosphere – i.e. its partial pressure ¹³. 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 ³.
`In deeper waters, DO can remain below 100% due to
`the respiration of aquatic organisms and microbial
`decomposition. These deeper levels of water often do
`not reach 100% air saturation equilibrium because
`they are not shallow enough to be aected by the
`waves and photosynthesis at the surface ³. This water
`is below an invisible boundary called the thermocline
`(the depth at which water temperature begins to decline)¹¹.
`What Aects Oxygen Solubility?
`Two bodies of water that are both 100% air-saturated do not necessarily have the same concentration of
`dissolved oxygen. The actual amount of dissolved oxygen (in mg/L) will vary depending on temperature, pressure
`and salinity ¹.
`First, the solubility of oxygen decreases as temperature increases ¹. This means that warmer surface water
`requires less dissolved oxygen to reach 100% air saturation than does deeper, cooler water. For example, at sea
`level (1 atm or 760 mmHg) and 4°C (39°F), 100% air-saturated water would hold 10.92 mg/L of dissolved oxygen. ³
`But if the temperature were raised to room temperature, 21°C (70°F), there would only be 8.68 mg/L DO at 100%
`air saturation ³.
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`Dissolved oxygen concentrations decrease as temperature
`increases
`Dissolved oxygen concentrations decrease as altitude increases
`(pressure decreases)
`Second dissolved oxygen decreases exponentially as
`salt levels increase ¹. That is why, at the same
`pressure and temperature, saltwater holds about 20%
`less dissolved oxygen than freshwater ³.
`Third, dissolved oxygen will increase as pressure
`increases ¹. This is true of both atmospheric and
`hydrostatic pressures. Water at lower altitudes can
`hold more dissolved oxygen than water at higher
`altitudes. This relationship also explains the potential
`for “supersaturation” of waters below the thermocline
`– at greater hydrostatic pressures, water can hold
`more dissolved oxygen without it escaping ¹. Gas
`saturation decreases by 10% per meter increase in
`depth due to hydrostatic pressure ¹². This means that
`if the concentration of dissolved oxygen is at 100% air
`saturation at the surface, it would only be at 70% air
`saturation three meters below the surface.
`In summary, colder, deeper fresh waters have the
`capability to hold higher concentrations of dissolved
`oxygen, but due to microbial decomposition, lack of
`atmospheric contact for diusion and the absence of
`photosynthesis, actual DO levels are often far below
`100% saturation ¹
`. Warm, shallow saltwater reaches
`100% air saturation at a lower concentration, but can
`often achieve levels over 100% due to photosynthesis
`and aeration. Shallow waters also remain closer to
`100% saturation due to atmospheric contact and
`constant diusion ¹
`.
`If there is a signicant occurrence of photosynthesis
`or a rapid temperature change, the water can achieve
`DO levels over 100% air saturation. At these levels, the
`dissolved oxygen will dissipate into the surrounding
`water and air until it levels out at 100% ³.
`How Can Water be More than 100% Saturated?
`100% air saturation is the equilibrium point for gases in water. This is because gas molecules diuse between the
`atmosphere and the water’s surface. According to Henry’s Law, the dissolved oxygen content of water is
`proportional to the percent of oxygen (partial pressure) in the air above it
`. As oxygen in the atmosphere is
`about 20.3%, the partial pressure of oxygen at sea level (1 atm) is 0.203 atm. Thus the amount of dissolved
`oxygen at 100% saturation at sea level at 20° C is 9.03 mg/L ¹
`.
`The equation shows that water will remain at 100% air saturation at equilibrium. However, there are several
`factors that can aect 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. This adds to the dissolved oxygen concentration in the water, potentially bringing
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`Henry’s law determining the dissolved oxygen concentration at
`20 degrees C and 100% air saturation (1 kg water = 1 L water)
`Supersaturation of water can be caused by rapid
`aeration from a dam.
`it above 100% saturation ¹⁴. In addition, the
`equalization of water is a slow process (except in
`highly agitated or aerated situations). This means that
`dissolved oxygen levels can easily be more than 100%
`air saturation during the day in photosynthetically
`active bodies of water ¹⁴.
`Dissolved oxygen often reaches over 100% air saturation due to
`photosynthesis activity during the day.
`Supersaturation caused by rapid aeration is often seen beside
`hydro-power dams and large waterfalls ¹². Unlike small rapids
`and waves, the water owing over a dam or waterfall traps and
`carries air with it, which is then plunged into the water. At
`greater depths and thus greater hydrostatic pressures, this
`entrained air is forced into solution, potentially raising
`saturation levels over 100% ¹².
`Rapid temperature changes can also create DO readings
`greater than 100% ¹⁴. As water temperature rises, oxygen
`solubility decreases. On a cool summer night, a lake’s
`temperature might be 60° F. At 100% air saturation, this lake’s
`dissolved oxygen levels would be at 9.66 mg/L. When the sun
`rises and warms up the lake to 70° F, 100% air saturation
`should equate to 8.68 mg/L DO ³. But if there is no wind to
`move the equilibration along, the lake will still contain that
`initial 9.66 mg/L DO, an air saturation of 111%.
`Typical Dissolved Oxygen Levels
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`Dissolved oxygen concentrations can uctuate daily and seasonally.
`Dissolved oxygen concentrations
`are constantly aected by diusion
`and aeration, photosynthesis,
`respiration and decomposition.
`While water equilibrates toward
`100% air saturation, dissolved
`oxygen levels will also uctuate with
`temperature, salinity and pressure
`changes ³. As such, dissolved
`oxygen levels can range from less
`than 1 mg/L to more than 20 mg/L
`depending on how all of these
`factors interact. In freshwater
`systems such as lakes, rivers and
`streams, dissolved oxygen
`concentrations will vary by season,
`location and water depth.
`Freshwater Fluctuations: Example 1
`In the Pompton River in New Jersey, mean dissolved oxygen concentrations range from 12-13 mg/L in winter and
`drop to 6-9 mg/L in the summer
`. That same river shows daily uctuations of up to 3 mg/L  due to
`photosynthesis production
`.
`Dissolved oxygen levels often stratify in the winter and summer, turning over in the spring and fall as lake temperatures align.
`Freshwater Fluctuations: Example 2
`Studies at Crooked Lake in Indiana show dissolved oxygen concentrations vary by season and depth from 12
`mg/L (surface, winter) to 0 mg/L (32 m depth, late summer), with full lake turnovers in spring and fall equalizing
`DO levels around 11 mg/L for all depths ¹.
`Rivers and streams tend to stay near or slightly above 100% air saturation due to relatively large surface areas,
`aeration from rapids, and groundwater discharge, which means that their dissolved oxygen concentrations will
`depend on the water temperature ¹. While groundwater usually has low DO levels, groundwater-fed streams can
`hold more oxygen due to the inux of colder water and the mixing it causes ¹
`. Standard Methods for the
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`In rivers and streams, dissolved oxygen concentrations are
`dependent on temperature.
`Examination of Water and Wastewater denes
`dissolved oxygen in streams as the sum of
`photosynthetic byproducts, respiration, re-aeration,
`accrual from groundwater inow and surface runo
`¹³.
`Saltwater holds less oxygen than freshwater, so
`oceanic DO concentrations tend to be lower than
`those of freshwater. In the ocean, surface water mean
`annual DO concentrations range from 9 mg/L near the
`poles down to 4 mg/L near the equator with lower DO
`levels at further depths. There are lower dissolved
`oxygen concentrations near the equator because
`salinity is higher ¹
`.
`Dissolved oxygen levels at the ocean’s surface: (data: World Ocean Atlas 2009; photo credit: Plumbago; Wikipedia Commons)
`Some states have Water Quality Standard Acts, requiring minimum concentrations of dissolved oxygen; in
`Michigan, these minimums are 7 mg/L for cold-water sheries and 5 mg/L for warm-water sh
`; in Colorado,
`“Class 1 Cold Water Aquatic Life” needs 6 mg/L, and “Class 1 Warm Water Aquatic Life” requires DO levels of at
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`Minimum dissolved oxygen requirements
`of freshwater sh
`least 5 mg/L
`. In order to mimic ideal environmental systems, freshwater tanks ideally need around 8 mg/L DO
`for optimum growth and marine tank requirements range from 6-7 mg/L DO based on the salinity level ¹
`. In
`other words, dissolved oxygen should be near 100% air saturation.
`Examples of Freshwater Organisms and Dissolved Oxygen Requirements
`Coldwater sh like trout and salmon are most aected by low dissolved
`oxygen levels
`. The mean DO level for adult salmonids is 6.5 mg/L, and
`the minimum is 4 mg/L ¹². These sh generally attempt to avoid areas
`where dissolved oxygen is less than 5 mg/L and will begin to die if
`exposed to DO levels less than 3 mg/L for more than a couple days ¹
`.
`For salmon and trout eggs, dissolved oxygen levels below 11 mg/L will
`delay their hatching, and below 8 mg/L will impair their growth and
`lower their survival rates. ¹
` When dissolved oxygen falls below 6 mg/L
`(considered normal for most other sh), the vast majority of trout and
`salmon eggs will die. ¹
`Bluegill, Largemouth Bass, White Perch, and Yellow Perch are considered
`warmwater sh and depend on dissolved oxygen  levels above 5 mg/L .
`They will avoid areas where DO levels are below 3 mg/L, but generally do
`not begin to suer fatalities due to oxygen depletion until levels fall
`below 2 mg/L
`. The mean DO levels should remain near 5.5 mg/L for
`optimum growth and survival ¹².
`Walleye also prefer levels over 5 mg/L, though they can survive at 2 mg/L
`DO levels for a short time.²⁴ Muskie need levels over 3 mg/L for both
`adults and eggs ²
`. Carp are hardier, and while they can enjoy dissolved
`oxygen levels above 5 mg/L, they easily tolerate levels below 2 mg/L and
`can survive at levels below 1 mg/L ²
`.
`The freshwater sh most tolerant to DO levels include fathead minnows
`and northern pike. Northern pike can survive at dissolved oxygen
`concentrations as low as 0.1 mg/L for several days, and at 1.5 mg/L for
`an innite amount of time ²
`. Fathead minnows can survive at 1 mg/L for
`an extended period with only minimal eects on reproduction and
`growth.
`As for bottom-dwelling microbes, DO changes don’t bother them much. If all the oxygen at their water level gets
`used up, bacteria will start using nitrate to decompose organic matter, a process known as denitrication. If all of
`the nitrogen is spent, they will begin reducing sulfate ¹
`. If organic matter accumulates faster than it decomposes,
`sediment at the bottom of a lake simply becomes enriched by the organic material. ²
`.
`Examples of Saltwater Organisms and Dissolved Oxygen Requirements
`Saltwater sh and organisms have a higher tolerance for low dissolved oxygen concentrations as saltwater has a
`lower 100% air saturation than freshwater. In general, dissolved oxygen levels are about 20% less in seawater
`than in freshwater ³.
`This does not mean that saltwater sh can live without dissolved oxygen completely. Striped bass, white perch
`and American shad need DO levels over 5 mg/L to grow and thrive
`. The red hake is also extremely sensitive to
`dissolved oxygen levels, abandoning its preferred habitat near the seaoor if concentrations fall below 4.2 mg/L
`²
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`Minimum dissolved oxygen requirements
`of saltwater sh
`The dissolved oxygen requirements of open-ocean and deep-ocean sh
`are a bit harder to track, but there have been some studies in the area.
`Billsh swim in areas with a minimum of 3.5 mg/L DO, while marlins and
`sailsh will dive to depths with DO concentrations of 1.5 mg/L ³
`.
`Likewise, white sharks are also limited in dive depths due to dissolved
`oxygen levels (above 1.5 mg/L), though many other sharks have been
`found in areas of low DO ³³. Tracked swordsh show a preference for
`shallow water during the day, basking in oxygenated water (7.7 mg/L)
`after diving to depths with concentrations around 2.5 mg/L ³⁴. Albacore
`tuna live in mid-ocean levels, and require a minimum of 2.5 mg/L ³
`,
`while halibut can maintain a minimum DO tolerance threshold of 1 mg/L
`³
`.
`Many tropical saltwater sh, including clown sh, angel sh 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). Higher dissolved oxygen
`concentrations are generally found around coral reefs due to
`photosynthesis and aeration from eddies and breaking waves ³
`. These
`DO levels can uctuate from 4-15 mg/L, though they usually remain
`around  5-8 mg/L, cycling between day photosynthesis production and
`night plant respiration ³
`. In terms of air saturation, this means that
`dissolved oxygen near coral reefs can easily range from 40-200% ³
`.
`Crustaceans such as crabs and lobsters are benthic (bottom-dwelling)
`organisms, but still require minimum levels of dissolved oxygen.
`Depending on the species, minimum DO requirements can range from 4
`mg/L to 1 mg/L ¹³. Despite being bottom dwellers, mussels, oysters and
`clams also require a minimum of 1-2 mg/L of dissolved oxygen
`, which
`is why they are found in shallower, coastal waters that receive oxygen from the atmosphere and photosynthetic
`sources.
`Consequences of Unusual DO Levels
`If dissolved oxygen concentrations drop below a certain level, sh mortality rates will rise. Sensitive freshwater
`sh like salmon can’t even reproduce at levels below 6 mg/L ¹
`. In the ocean, coastal sh begin to avoid areas
`where DO is below 3.7 mg/L, with specic species abandoning an area completely when levels fall below 3.5 mg/L
`²
`. Below 2.0 mg/L, invertebrates also leave and below 1 mg/L even benthic organisms show reduced growth and
`survival rates ²
`.
`Fish kill / Winterkill
`A shkill occurs when a large number of sh in an area of water die o. 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. A winterkill is a
`sh kill caused by prolonged reduction in dissolved oxygen due to ice or snow cover on a lake or pond ²
`.
`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-o.
`Fish kills are more common in eutrophic lakes: lakes with high concentrations of nutrients (particularly
`phosphorus and nitrogen) ⁴¹. 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-
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`Dissolved oxygen depletion is the most common
`cause of sh kills
`Sockeye salmon with gas bubble
`disease
`depleted, environment where sh and other organisms cannot
`survive. Such nutrient levels can occur naturally, but are more
`often caused by pollution from fertilizer runo or poorly treated
`wastewater ⁴¹.
`Winterkills occur when respiration from sh, plants and other
`organisms is greater than the oxygen production by
`photosynthesis ¹. They occur when the water is covered by ice,
`and so cannot receive oxygen by diusion 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 o.
`In these situations, sh, plants and decomposition are all using
`up the dissolved oxygen, and it cannot be replenished, resulting
`in a winter sh kill. The shallower the water, and the more
`productive (high levels of organisms) the water, the greater the
`likelihood of a winterkill ²
`.
`Gas Bubble Disease
`Just as low dissolved oxygen can cause problems, so too can high concentrations.
`Supersaturated water can cause gas bubble disease in sh and invertebrates ¹².
`Signicant death rates occur when dissolved oxygen remains above 115%-120%
`air saturation for a period of time. Total mortality occurs in young salmon and
`trout in under three days at 120% dissolved oxygen saturation ¹². Invertebrates,
`while also aected by gas bubble disease, can usually tolerate higher levels of
`supersaturation than sh ¹².
`Extended periods of supersaturation can occur in highly aerated waters, often
`near hydropower dams and waterfalls, or due to excessive photosynthetic
`activity. Algae blooms can cause air saturations of over 100% due to large
`amounts of oxygen as a photosynthetic byproduct. This is often coupled with
`higher water temperatures, which also aects saturation. ¹² At higher
`temperatures, water becomes 100% saturated at lower concentrations, so higher
`dissolved oxygen concentrations mean even higher air saturation levels.
`Dead Zones
`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. Dead zones often occur near heavy human populations, such as estuaries and
`coastal areas o the Gulf of Mexico, the North Sea, the Baltic Sea, and the East China Sea. They can occur in large
`lakes and rivers as well, but are more well known in the oceanic context.
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`Hypoxic and anoxic zones around the world (photo credit: NASA)
`These zones are usually a result of a fertilizer-fueled algae and phytoplankton growth boom. When the algae and
`phytoplankton die, the microbes at the seaoor use up the oxygen decomposing the organic matter ³¹. These
`anoxic conditions are usually stratied, occurring only in the lower layers of the water. While some sh and other
`organisms can escape, shellsh, young sh and eggs usually die ³².
`Naturally occurring hypoxic (low oxygen) conditions are not considered dead zones. The local aquatic life
`(including benthic organisms) have adjusted to the recurring low-oxygen conditions, so the adverse eects of a
`dead zone (mass sh kills, sudden disappearance of aquatic organisms, and growth/development problems in
`sh and invertebrates) do not occur ³¹.
`Such naturally occurring zones frequently occur in deep lake basins and lower ocean levels due to water column
`stratication.
`Dissolved Oxygen and Water Column Stratication
`Stratication 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 stratication 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 signicant division between freshwater and saltwater sources.
`Lake Stratication
`The uppermost layer of a lake, known as the epilimnion, is exposed to solar radiation and contact with the
`atmosphere, keeping it warmer. The depth of the epilimnion is dependent on the temperature exchange, usually
`determined by water clarity and depth of mixing (usually initiated by wind) ¹¹. Within this upper layer, algae and
`phytoplankton engage in photosynthesis. Between the contact with the air, potential for aeration and the
`byproducts of photosynthesis, dissolved oxygen in the epilimnion remains near 100% saturation. 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.
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`Lake stratication
`Below the epilimnion is the metalimnion, a transitional layer that
`uctuates in thickness and temperature. The boundary between the
`epilimnion and metalimnion is called the thermocline – the point at
`which water temperature begins to steadily drop o ¹¹. Here, two
`dierent outcomes can occur. If light can penetrate beyond the
`thermocline and photosynthesis occurs in this strata, the metalimnion
`can achieve an oxygen maximum ¹¹. This means that the dissolved
`oxygen level will be higher in the metalimnion than in the epilimnion. But
`in eutrophic or nutrient-rich lakes, the respiration of organisms can
`deplete dissolved oxygen levels, creating a metalimnetic oxygen
`minimum ⁴².
`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. ¹¹.  While lab conditions would
`conclude that at colder temperatures and higher pressures water can
`hold more dissolved oxygen, this is not always the result. In the
`hypolimnion, bacteria and fungi use dissolved oxygen to decompose
`organic material
`. This organic material comes from dead algae and
`other organisms that sink to the bottom. The dissolved oxygen used in
`decomposition is not replaced – there is no atmospheric contact,
`aeration or photosynthesis to restore DO levels in the hypolimnion ¹¹.
`Thus the process of decomposition “uses up” all of the oxygen within this
`layer.
`If the lake in question is a holomictic “mixing” lake, all the layers mix at
`least once per year (usually spring and fall) when lake strata temperatures align. This turnover redistributes
`dissolved oxygen throughout all the layers and the process begins again.
`Ocean Stratication
`Stratication in the ocean is both horizontal and vertical. The littoral, or coastal area is most aected by estuaries
`and other inow sources.⁴⁴ It tends to be shallow and tidal with uctuating dissolved oxygen levels. 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 uctuate as much as they do in the littoral zone.
`This is the zone where many coral reefs grow, and DO levels remain near 100% air saturation due to eddies,
`breaking waves and photosynthesis
`. This zone is also where most oceanic benthic (bottom-dwelling)
`organisms exist. Oceanic benthic sh do not live at the greatest depths of the ocean. They dwell at the seaoor
`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.
`In the open ocean, there are ve major vertical strata: epipelagic, mesopelagic, bathypelagic, abyssopelagic, and
`hadalpelagic ⁴⁴. The exact denitions 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 200 m and is bordered by a collection of clines.
`45
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`⁶
`
`⁵
`
`Stratication in the ocean
`These clines can overlap or exist at separate depths.
`Much like in a lake, the thermocline divides oceanic
`strata by temperature. The halocline divides by salinity
`levels and the pycnocline divides strata by density ¹
`.
`Each of these clines can aect the amount of
`dissolved oxygen the ocean strata can hold.
`The mesopelagic, meaning “twilight” zone, stretches from 200-1000 m. Depending on water clarity, some light
`may lter through, but there is not enough for photosynthesis to occur ⁴⁴. Within this strata, the oxygen
`minimum zone (OMZ) can occur. The OMZ develops because organisms use the oxygen for respiration, but it is
`too deep to be replenished by photosynthetic oxygen byproducts or aeration from waves. This zone tends to
`exist around a depth of 500 m ⁴
`. The mesopelagic zone is bordered by chemoclines (clines based on chemistry
`levels, e.g. oxygen and salinity) on both sides, reecting dierent dissolved oxygen and salinity levels between
`the strata.
`Below the mesopelagic is the aphotic zone(s). These strata have lower dissolved oxygen levels than the surface
`water because photosynthesis does not occur but can have higher levels than the OMZ because less respiration
`occurs.
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`⁶
`
`⁷
`
`Dissolved oxygen stratication in an estuary is dependent on salinity (expressed in
`PSU).
`Dissolved oxygen unit conversions at 21° Celsius (70° F) and 1
`atmosphere (760 mmHg)
`The bathypelagic, “midnight” zone exists between 1000-4000 m, and many creatures can still live here. The
`bottom layer of the ocean is the abyssopelagic, which exists below 4000 m. The hadopelagic is the name for the
`zone of deep ocean trenches that open below the abyssal plain, such as the Mariana Trench ⁴⁴.
`Estuary Stratication
`Estuary stratications are based on
`salinity distributions. Because
`saltwater holds less dissolved
`oxygen than freshwater, this can
`aect aquatic organism
`distribution. The stronger the river
`ow, the higher the oxygen
`concentrations. This stratication
`can be horizontal, with DO levels
`dropping from inland to open
`ocean, or vertical, with the fresh,
`oxygenated river water oating
`over the low DO seawater ⁴
`. When
`the stratication is clearly dened, a
`pycnocline divides the fresher
`water from the salt water,
`contributing to separate dissolved oxygen concentrations in each strata.
`Dissolved Oxygen Units and Reporting
`Dissolved oxygen is usually reported in milligrams per
`liter (mg/L) or as a percent of air saturation. However,
`some studies will report DO in parts per million (ppm)
`or in micromoles (umol). 1 mg/L is equal to 1 ppm.
`The relationship between mg/L and % air saturation
`has been discussed above, and varies with
`temperature, pressure and salinity of the water. One
`micromole of oxygen is equal to 0.022391 milligrams,
`and this unit is commonly used in oceanic studies ⁴
`.
`Thus 100 umol/L O2 is equal to 2.2 mg/L O2.
`Calculating DO from % Air Saturation
`To calculate dissolved oxygen concentrations from air saturation, it is necessary to know the temperature and
`salinity of the sample. Barometric pressure has already been accounted for as the partial pressure of oxygen
`contributes to the percent air saturation . Salinity and temperature can then be used in Henry’s Law to calculate
`what the DO concentration would be at 100% air saturation
`. However, it is easier t