There is no doubt that the amount of dissolved oxygen in the water has a major impact on fish behaviour and survival. What actually is dissolved oxygen though, where does it come from and what happens if there is insufficient available?

The most comprehensive and easy to understand article that I’ve read on dissolved oxygen is from the Fundaments of Environmental Measurements website and can be accessed by clicking on the following link:- http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/  I’ve summarised the key points below:-

“Dissolved oxygen refers to the level of free, non-bonded oxygen present in water or other liquids. The red arrows indicate the free oxygen (O2) in the image below. It is an important parameter in assessing water quality because of its influence 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.  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. Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen (1-6 milligrams per Litre of water or 1-6 mg/L), while shallow water fish need higher levels (4-15 mg/L).

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. From the air, oxygen can slowly diffuse 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. 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 100 metres below the surface. In clear water there is no longer enough light for photosynthesis to occur beyond 200 metres 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 water. 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.

In a stable body of water with no stratification, 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. 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.

In deeper waters, dissolved oxygen 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 affected by the waves and photosynthesis at the surface. This water is below an invisible boundary called the thermocline (the depth at which water temperature begins to decline).

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.

First, the solubility of oxygen in water decreases as temperature increases. 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. However, if the temperature were raised to room temperature, 21°C (70°F), there would only be 8.68 mg/L of dissolved oxygen at 100% air saturation.

Second dissolved oxygen decreases exponentially as salt levels increase. That is why, at the same pressure and temperature, saltwater holds about 20% less dissolved oxygen than freshwater. 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. 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. Gas saturation decreases by 10% per metre increase in depth due to hydrostatic pressure. 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 diffusion and the absence of photosynthesis, actual dissolved oxygen levels are often far below 100% saturation. 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 diffusion.

If there is a significant occurrence of photosynthesis or a rapid temperature change, the water can achieve dissolved oxygen 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%.

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. This adds to the dissolved oxygen concentration in the water, potentially bringing it above 100% saturation. 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.

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. At greater depths and thus greater hydrostatic pressures, this entrained air is forced into solution, potentially raising saturation levels over 100%.

Rapid temperature changes can also create dissolved oxygen readings greater than 100%. As water temperature rises, oxygen solubility decreases. On a cool summer night, a lake’s temperature might be 15° Celcius. 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 21° Celcius, 100% air saturation should equate to 8.68 mg/L of dissolved oxygen. But if there is no wind to move the equilibration along, the lake will still contain that initial 9.66 mg/L dissolved oxygen, an air saturation of 111%.

Dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration and decomposition. While water equilibrates toward 100% air saturation, dissolved oxygen levels will also fluctuate with temperature, salinity and pressure changes. 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. The graph above, where dissolved oxygen is the black line, shows how dissolved oxygen levels vary with the seasons.

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 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. In the ocean, surface water mean annual dissolved oxygen concentrations range from 9 mg/L near the poles down to 4 mg/L near the equator with lower levels at further depths. 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 6.5 mg/L, and the minimum is 4 mg/L. These fish generally attempt to avoid areas where dissolved oxygen is less than 5 mg/L and will begin to die if exposed to levels less than 3 mg/L for more than a couple days. 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. When dissolved oxygen falls below 6 mg/L (considered normal for most other fish), the vast majority of trout and salmon eggs will die.

Bluegill, Largemouth Bass, White Perch, and Yellow Perch are considered warmwater fish and depend on dissolved oxygen  levels above 5 mg/L. They will avoid areas where dissolved oxygen levels levels are below 3 mg/L, but generally do not begin to suffer fatalities due to oxygen depletion until levels fall below 2 mg/L. The mean dissolved oxygen levels should remain near 5.5 mg/L for optimum growth and survival.

Walleye also prefer levels over 5 mg/L, though they can survive at 2 mg/L levels for a short time. Muskie need levels over 3 mg/L for both adults and eggs. 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.

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 0.1 mg/L for several days, and at 1.5 mg/L for an infinite amount of time. Fathead minnows can survive at 1 mg/L for an extended period with only minimal effects on reproduction and growth.

Saltwater fish 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.

This does not mean that saltwater fish can live without dissolved oxygen completely. Striped bass, white perch and American shad need dissolved oxygen 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 seafloor if concentrations fall below 4.2 mg/L.

 

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 3.5 mg/L dissolved oxygen, while marlins and sailfish will dive to depths with concentrations of 1.5 mg/L. 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 dissolved oxygen. Tracked swordfish 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. Albacore tuna live in mid-ocean levels, and require a minimum of 2.5 mg/L, while halibut can maintain a minimum tolerance threshold of 1 mg/L.

Many tropical saltwater fish, especially those surrounding coral reefs, require higher levels of dissolved oxygen. 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. These dissolved oxygen levels can fluctuate from 4-15 mg/L, though they usually remain around  5-8 mg/L, cycling between day photosynthesis production and night plant respiration. In terms of air saturation, this means that dissolved oxygen near coral reefs can easily range from 40-200%.

Crustaceans such as crabs and lobsters are benthic (bottom-dwelling) organisms, but still require minimum levels of dissolved oxygen. Depending on the species, minimum dissolved oxygen requirements can range from 4 mg/L to 1 mg/L. 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.

Dissolved Oxygen and 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. The depth of the epilimnion is dependent on the temperature exchange, usually determined by water clarity and depth of mixing initiated by wind. 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 dissolved oxygen 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. The boundary between the epilimnion and metalimnion is called the thermocline – the point at which water temperature begins to steadily drop off . Below the metalimnion is the hypolimnion. This deep, cold layer has a more gentle temperature drop toward the bottom. Generally most fish will hold at or about 1 -2 metres above the thermocline.

Estuary stratifications are based on salinity distributions. Because saltwater holds less dissolved oxygen than freshwater, this can affect aquatic organism distribution. The stronger the river flow, the higher the oxygen concentrations. This stratification can be horizontal, with dissolved oxygen levels dropping from inland to open ocean, or vertical, with the fresh, oxygenated river water floating over the low dissolved oxygen seawater. When the stratification is clearly defined, a pycnocline divides the fresher water from the salt water, contributing to separate dissolved oxygen concentrations in each strata.

What does all this mean?

If dissolved oxygen concentrations drop below a certain level, fish mortality rates will rise. Sensitive freshwater fish like salmon can’t even reproduce at levels below 6 mg/L. In the ocean, coastal fish begin to avoid areas where dissolved oxygen is below 3.7 mg/L, with specific species abandoning an area completely when levels fall below 3.5 mg/L. Below 2.0 mg/L, invertebrates also leave and below 1 mg/L even benthic organisms show reduced growth and survival rates”.

In summer, when the water temperature increases, river fish will hold in deeper, shaded areas where the water is colder, areas where cooler side streams enter the main river, in pockets of cooler water or in riffly water as this is where the dissolved oxygen levels are higher. Their feeding behaviour may also change to take advantage of the higher dissolved oxygen levels and cooler temperatures around dawn and dusk. In lakes and reservoirs many prey species and predators will aggregate around the thermocline particularly during the daytime. This zone represents the coolest water in the lake that contains adequate dissolved oxygen for use by fish. It also harbors more zooplankton (micro-crustaceans) during daylight hours and thus provides high food abundance for prey fish. Many modern fish finders can detect the thermocline as the density change in the water is strong enough to deflect sonar. Learning to find the thermocline is a critical skill for lake and reservoir anglers. Also in lakes fish will hold at the mouths of rivers and streams entering the lake as this is where the water is often cooler and contains higher levels of dissolved oxygen. Some anglers carry a thermometer to measure water temperature as this provides clues as to where the dissolved oxygen levels are higher and thus where fish may be holding.

The chart below shows how the dissolved oxygen levels change with salinity and temperature. Typically seawater has a salinity of 35 grams of salt per kilogram (3.5%) whereas freshwater has 0 grams per kilogram. When fresh and saltwater streams meet the freshwater will rise above the saltwater as it is less dense and forms a layer. Fish in estuaries will hold downstream of a freshwater inflow especially when the tide is running strongly and mixing the two layers. This is because the dissolved oxygen level of the mixed streams is higher due to the lower salinity.

As you can see Dissolved oxygen is one of the many factors which helps anglers identify where fish are likely to be holding and this is why it is very important to have a basic understanding of it.