Dissolved oxygen (DO) describes the amount of oxygen (O2) molecules that are dissolved in water. Dissolved oxygen is affected by both anthropogenic (human activities) and natural factors. Naturally, water conditions such as salinity and temperature, and too many aquatic organisms can affect DO levels. Humans also have an impact on DO levels in water from clearing land, runoff, and sewage waste.
The level of dissolved oxygen in water can tell us a lot about water chemistry, which is why it is an important characteristic to measure in many applications, from environmental water monitoring and wastewater treatments to the beverage industry such as brewing beer.
Aquatic plants and animals require oxygen to survive, therefore a change in DO concentrations in water can affect aquatic communities and ecosystems, leading to illnesses and even death. Dissolved oxygen concentrations also affect the chemistry and nutrient availability in water. For example, nutrients such as phosphorus may accumulate and over-fertilize water bodies in the absence of oxygen.
In this article we will go into more detail on the effects different DO concentrations have, what affects the levels of DO in water, and how you can test DO.
What Affects Dissolved Oxygen Levels In Water?
Dissolved oxygen is affected by natural and anthropogenic factors, which affects both aquatic communities and water quality.
Natural Factors That Affect DO Levels
Aquatic Life: Animals that live in water consume dissolved oxygen, for example, as bacteria in water decomposes organic material, they consume oxygen. Therefore, when water contains a lot of decomposing material, a drop in DO concentrations is recorded. This process is known as biological oxygen demand (BOD).
Vegetation: During photosynthesis, plants release oxygen into the water, which directly affects DO levels (it increases). Vegetation also indirectly affects DO levels in the water, creating shaded areas that can decrease water temperatures, therefore increasing DO levels in the water.
Salinity: Saltwater contains less oxygen than freshwater bodies.
Temperature: Warmer water contains less DO than colder water.
Elevation: Water bodies that are at higher altitudes will have lower oxygen levels than water further away from the atmosphere.
Water Flow: The more turbulent the water flow is, the more oxygen the water will contain.
Anthropogenic Factors That Affect DO Levels
Clearing Land: When land is cleared, excess organic matter can find its way into different water bodies, where it is decomposed by microorganisms that require oxygen. This means, if there is an excessive amount of organic matter, the DO level in the water will decrease.
Deforestation: Tree & plant removal along water bodies decreases shading contributing to increased water temperatures. As warmer water contains less oxygen, it can indirectly reduce DO levels in the surrounding water.
Sewage Waste: Improper disposal of waste or during heavy rainfall, excess nutrients and organic matter from the waste can flow into nearby streams, altering the DO levels in water and creating a BOD.
Agricultural & Urban Runoff: When nutrients such as phosphorus or nitrogen runoff into the surrounding water, the nutrients can over-fertilize the water, enhancing plant growth. When these plants die and start decaying, they consume more oxygen, contributing to a drop in DO levels in the water. Organic waste from land animals or landfill sites can increase DO levels.
Channel Alterations: When water channels are altered, turbulence is often reduced and channels may deepen, shifting DO levels.
Are Low Dissolved Oxygen Levels An Issue?
As DO is key for health and reproduction in many fish and invertebrates, prolonged exposure to low DO levels increases stress and diseases, and in some cases, leads to organism death. Lethal DO levels for fish are between 1 and 3 mg/L, which is why it is important to test DO levels in different water bodies.
Another example is fish that can physiologically adapt to low-DO levels in the water by increasing how much water flows over their gills. This lowers their metabolic oxygen demand by increasing oxygen-carrying blood levels. Both these examples of adaptations cannot happen overnight, it takes long-term exposure to low or even sublethal DO levels in water, and while some aquatic organisms can adapt, many cannot and will die in such conditions.
Microbes, like bacteria, also require DO; they use DO for the decomposition of organic matter, which is critical for nutrient cycling in water. When there is an excess of decaying organic matter, the microbes must consume more in a shorter time period, thus consuming more DO in the water to drive the process.
Hypoxic & Dead Zones
Hypoxia is the term used for an oxygen-depleted area, and a dead zone is an area that has little to no DO in the water. Dead zones are common near large human communities that contribute to anthropogenic factors affecting DO levels in different water bodies. This is more common in coastal areas, but it can also be seen near large lakes and rivers too.
Waters that have hypoxic environments or dead zones, are usually due to fertilizers fuelling the overproduction of algae and phytoplankton, creating blooms. Algal blooms create anoxic conditions lowering the DO levels in water, trapping juvenile fish and eggs, which usually results in killing them.
Areas, where hypoxia has occurred naturally, are not considered dead zones as most local aquatic life have adjusted to the low-DO levels over time.
Where Does Dissolved Oxygen Originate From?
Dissolved oxygen comes from main two sources: via the air/atmosphere, or as a byproduct of plants.
Oxygen in the atmosphere slowly is either diffused across the surface of water bodies or is mixed via aeration. Both these can occur naturally (wind, groundwater, or water discharge) or via man-made processes (aquarium pumps, water dams, or waterwheels).
Dissolved oxygen also comes from waste products of algae, seaweed, and other aquatic plants when they undergo photosynthesis. During photosynthesis in plants, oxygen becomes a byproduct that dissolves in water; it can be seen in the following simplified reaction of aquatic photosynthesis below:
CO2 + H2O → (CH2O) + O2
Aquatic photosynthesis is light-dependent, therefore DO levels in the water will be higher during daylight hours and will drop at night when it is dark. This is obviously dependent on geographical location as some areas experience the midnight sun and polar night phenomenon.
Water Stratification & Dissolved Oxygen Levels
Stratification is what separates a water body into layers or zones. This occurs due to temperature or dissolved substances such as salt and oxygen (DO). Stratification layers differ between different water bodies (lakes, oceans, or estuaries).
Stratification In Lakes
Epilimnion: Dissolved oxygen varies depending on the temperature and how much photosynthesis occurs in the water.
Metalimnion: Dissolved oxygen levels are higher than the epilimnion layer if sunlight is penetrated to this layer, however, in eutrophic lakes, excessive respiration can deplete DO levels in the water.
Hypolimnion: As the oxygen used for decomposition in this layer cannot be replaced, DO levels in this layer are used up and cannot be restored by aeration, atmospheric contact, or photosynthesis.
Monimolimnion: If the hypolimnion layer doesn’t mix with the upper layers, it is known as the monimolimnion.
Stratification In The Ocean
In the ocean, stratification occurs both horizontally and vertically.
Littoral (Coastal Zone)/Epipelagic (Photic Zone): Dissolved oxygen levels fluctuate due to estuaries and other water inflows.
Sublittoral (Coastal/Demersal Zone)/Mesopelagic: Dissolved oxygen also varies in this zone, however, DO levels do not fluctuate as much as the littoral zone. Most coral reefs grow in this area, and it is also home to most bottom-dwelling fish, as DO levels are near 100% air saturated from photosynthesis and ocean movement (waves) that creates aeration.
Bathyal/Bathypelagic, Abyssal/Abyssopelagic, & Hadal/Hadalpelagic: In this zone, there are low DO levels as photosynthesis does not occur, however, respiration can, therefore DO levels can still exist in this zone.
Stratification In Estuaries
In estuaries, water stratification is based on salinity levels. As DO levels are lower in saltwater than freshwater, the distribution of aquatic organisms in estuaries is affected. If an estuary has a strong river inflow, the DO concentration in the water will usually increase.
Stratification in estuaries can also be horizontal or vertical:
Horizontal Stratification: Dissolved oxygen levels decrease from inland to the open ocean.
Vertical Stratification: Freshwater from rivers that are oxygenated, flows over seawater that has a low DO level.
The mixing of saltwater and freshwater creates a brackish environment where only a selected number of aquatic species can live.
How To Test Dissolved Oxygen In Water
Dissolved oxygen can be directly measured in the water body or immediately after collecting a water sample. It is important to quickly test samples as oxygen concentrations can change when exposed to temperature changes.
The easiest way to measure DO is with a calibrated DO probe/sensor that is connected to a meter with a readable display. Sensors must be calibrated before each use, as DO levels in water are affected by temperature.
You can find more information on how to test DO in water using DO probes, plus other methods (titrimetric, optical dissolved oxygen, and colorimetric) here.
Dissolved oxygen is the amount of oxygen dissolved in water, which is affected both naturally and by human activities. Dissolved oxygen can also be affected by different water stratification levels and the presence of hypoxic water conditions and dead zones.
If you have any questions regarding dissolved oxygen or are unsure which DO kit is best for you, do not hesitate to reach out to the world-class team at Atlas Scientific.
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