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River water must contain sufficient dissolved oxygen levels to sustain a healthy and balanced environment for aquatic life, including plants, fish, and other animals. In rivers, these organisms require oxygen to respire, and without it, they are unable to live.
Clean freshwater is essential for both humans and other organisms to survive. Rivers are a precious water source that provides many people with drinking water around the globe. Many people also depend on rivers for their livelihoods. For example, in many countries, rivers are essential for agriculture and fishing industries. When rivers become polluted either by industries or poor water management, it can be life-threatening, therefore we must ensure that rivers are closely monitored.
Freshwater habitats like rivers also contain some of the richest biodiversity in the world.
One of the most important parameters to measure in rivers is dissolved oxygen (DO). Dissolved oxygen is the measure of how much oxygen (O2) is dissolved in the river water, and therefore available for drinking water purposes or for living aquatic organisms. From plants to animals, dissolved oxygen in rivers is essential, and in this article, we are going to explain this in greater detail.
Rivers and streams contain dissolved oxygen levels to sustain the lives of many. Plants and animals require DO for respiration, similar to how we require O2 on land. On land, we can breathe O2 from the surrounding air, but O2 that exists in water like rivers must be dissolved due to the water (H2O) molecule being too bound to give up its oxygen molecule – this is where dissolved oxygen comes in!
For fish, DO in river water is used for daily respiration via their gills. As for plants and phytoplankton species, DO in river water is used for respiration when there is limited or no light for photosynthesis. Other organisms such as bacteria and fungi also need DO to survive.
If DO levels drop too low in river water, fish and other aquatic organisms may suffocate and eventually die. Healthy river water should have a dissolved oxygen level/concentration above 6.5-8 mg/L.
The definition of a river is a naturally flowing watercourse, such as a freshwater stream. So, the movement of the river can affect the amount of dissolved oxygen.
River water that has a rapid speed will contain more DO than slower-flowing rivers or rivers that are stagnant. Rivers that are located by mountain streams or are larger, typically contain more DO as they can flow much faster. Therefore both the size and landscapes that rivers transverse will affect the amount of DO in the river water.
Rivers that have high amounts of decaying organic matter will have noticeably lower levels of DO due to the bacteria consuming the organic matter. The bacteria require DO to power the process so they consume the oxygen in the river water.
If organic matter increases in the river water, eutrophic conditions can occur, where the water becomes very rich in nutrients. While it is true that nutrient-rich water is beneficial for plants, eutrophic conditions result in too many nutrients and minerals in the river water for the animals to survive.
During eutrophic conditions, algae grow rapidly, often smothering the river’s surface. This makes it very challenging for aquatic life to get the DO they require to thrive. These conditions are common during the summer months when the water temperature increases and DO levels drop.
Good oxygen levels are important for the health of the river system as most aquatic species do not come up to the surface for air, therefore, they must consume dissolved oxygen in the river water. The amount of DO levels varies from organism to organism, and the use of the river water.
Typically, most fish will thrive within a DO level of 4 to 5 mg/L. For freshwater fish that rely on rivers as part of their lifecycle, such as salmon, DO levels must be above 6 mg/L for them to reproduce. Whereas bottom feeders such as oysters can survive on lower DO levels between 1 and 6 mg/L.
When DO levels drop below 4 mg/L, fish will swim to the surface, lose their appetite, swim slower, and breathe rapidly. If DO levels in river water continue to drop, fish mortality rates will increase. For species that can move, if the DO levels are too low for a long period, the aquatic organisms will move away from their current habitat and seek a more desirable habitat.
Some fish are more tolerable, such as tilapia. These fish can tolerate DO levels slightly lower than the norm, but it does affect their growth rate.
Plants also rely on dissolved oxygen in rivers. As plants carry out photosynthesis, the plant’s roots need DO to respire and release energy for root growth and nutrient uptake, this process is cellular respiration. During cellular respiration, aquatic plants absorb DO through their stems, roots, and leaves. Generally speaking, aquatic plants require DO levels >3 mg/L to perform cellular respiration.
In return, the plants survive and provide other aquatic organisms with oxygen as a byproduct of photosynthesis.
If DO levels are too low in the river water, the following can happen:
Physical signs of low DO levels in plants:
To function and perform their important tasks, bacteria need enough dissolved oxygen in the river water. The optimum DO level for bacteria is 4 to 8 mg/L. If the DO level drops below 2 mg/L, the nitrification process will stop.
Nitrification is extremely important in converting ammonia from fish waste into nitrites and nitrates. For nitrification to happen, the DO level in the river water must be at least 3 mg/L.
As dissolved oxygen is an important water parameter in rivers, it must be monitored.
To measure DO in water there are specific sensors that will allow you to get reliable and high-accurate DO readings, which include the EZO-Dissolved Oxygen Kit.
Dissolved oxygen probes work by measuring the amount of oxygen that passes across a permeable or semi-permeable membrane into a probe/sensor. Once oxygen is detected by the sensor, a chemical reduction reaction occurs, producing an electrical signal. This signal is read by the DO probe and is displayed on a meter.
Colorimetry (test strips) and titrations can also be used to measure DO, however, in rivers, a portable dissolved oxygen meter/probe is best.
Both natural and anthropogenic factors can lead to low levels of DO in river water. The main cause is the decomposition of excessive organic matter. Oxygen depletion that leads to DO depletion results from microbial decay of organic matter like leaves, algae, or dead organisms accumulated in the water. Nutrient runoff from urban areas or agricultural activities often worsens the situation by causing algal blooms.
As mentioned, increased temperatures also reduce the capacity of water to contain oxygen hence leading to reduced DO levels, especially in warmer seasons or shallow, slow-moving waters.
Human activities such as wastewater, effluents and industrial discharges may introduce pollutants and organic materials into surrounding rivers leading to an uptake in oxygen demand due to their decay. Additionally, urban stormwater runoff carries with it sediments and pollutants worsening levels of oxygen, depleting DO levels in rivers.
Less spoken about are the physical characteristics like stratification or low flow rates which can also decrease DO by inhibiting mixing between surface waters rich in O2 and deeper layers.
There are several ways of increasing the levels of dissolved oxygen in water. These methods include promoting wind and wave action, introducing aquatic vegetation, and adding purified oxygen. However, adding purified oxygen can lead to excess oxygenation known as supersaturation. We recommend using portable splash or spray-type aerators to help maintain optimum DO levels.
When struggling with low DO readings in a river, we can:
Constant monitoring is important because it allows early intervention without resorting to drastic remedies like changing the entire water source. When these measures are implemented, they create an ecosystem that supports aquatic life and provides safe drinking water to communities.
It is essential to keep oxygen dissolved in river water at levels that are sufficient because this is very important in supporting plants, fish, and other organisms. Freshwater rivers also provide fresh drinking water for humans as well as serve as a base where agriculture and fishing industries take place.
To learn more about dissolved oxygen and what dissolved oxygen meters we have to offer, do not hesitate to reach out to the world-class team at Atlas Scientific.
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