03/15/2022 | Environmental | 8 MINUTE READ

What Contributes the Most Dissolved Oxygen to the Ocean?

Dissolved oxygen is free oxygen that can be found in water or other liquid solutions. Even though water molecules are comprised of an oxygen atom, any organisms that are present in the water won’t be able to consume these molecules to obtain the oxygen they require to survive. Instead, these organisms must locate free oxygen molecules. An exceedingly small amount of the oxygen in water will naturally dissolve and form into dissolved oxygen.

Because many different organisms require dissolved oxygen to survive, it’s possible for a body of water to not contain enough dissolved oxygen for the aquatic life that exists within. By measuring dissolved oxygen, you can identify the current water quality. If DO levels are too high or too low, there’s a good chance that the water quality is bad, which means that any aquatic life will have difficulty surviving.

If you want to measure dissolved oxygen, you can do so with a dissolved oxygen sensor. When you use the right sensor, you’ll obtain a reading that’s displayed as milligrams per liter or mg/L. Most dissolved oxygen sensors provide readings that range from 0-20 mg/L.

Even though it’s important that dissolved oxygen concentrations are never too high or too low, it’s more dangerous for aquatic ecosystems when the dissolved oxygen concentrations are at low levels. In this situation, the fish won’t be able to obtain enough oxygen to thrive. It’s common for DO levels to drop in bodies of water that contain ample amounts of algae. This guide provides a detailed examination of dissolved oxygen and the factors that contribute to different DO levels.

Ocean fish around coral

Dissolved Oxygen, Water Quality, and How It Affects Aquatic Life

Dissolved oxygen affects both water quality and aquatic life. In regards to aquatic life, many organisms in the water consume dissolved oxygen in order to survive. These organisms include fish, bacteria, plants, and invertebrates. All of these organisms depend on dissolved oxygen for respiration. Crustaceans and fish are able to gain access to oxygen in the water with their gills. On the other hand, plants and phytoplankton use this oxygen for respiration in the event that they don’t have access to ample amounts of light for the photosynthesis process.

Different organisms require different amounts of dissolved oxygen to survive. For instance, worms, crabs, and oysters only need access to around 1-6 mg/L of oxygen. On the other hand, shallow water fish require 4-15 mg/L of oxygen to live.

Any fungi and bacteria in the water also need dissolved oxygen. These particular organisms use dissolved oxygen to properly decompose any organic material that’s present at the very bottom of the water. If ever there’s too much decaying organic material in the water, the dissolved oxygen will be used up quickly.

As for water quality, dissolved oxygen levels are a good indicator of what the current water quality is. When water is moving rapidly in a river or stream, dissolved oxygen levels will be higher. In comparison, DO levels are lower in relatively stagnant water. It’s common for lakes to contain very low DO levels when too much organic material is introduced to the water. In this situation, the decaying organic material will cause DO levels to drop since bacteria requires DO to decompose organic materials.

If the DO levels get too low, eutrophication can occur, which essentially means that the body of water will die because of an inability to support most aquatic life. When a lake is experiencing eutrophic conditions, the water will have a greenish hue to it as a result of an increase in algae concentration.

These conditions are common during the summer when the temperatures at the top of the water are too warm for dissolved oxygen. Even though the water towards the bottom of the lake will be cool enough to support dissolved oxygen, not enough dissolved oxygen reaches these depths in the first place. The majority of fish in a body of water will die off if eutrophic conditions occur.

How Does Oxygen Enter the Ocean Water?

Dissolved oxygen can enter water by air or as a byproduct from various plants. If the oxygen comes from the air, it will very slowly diffuse along the water’s surface or by being mixed into the water via aeration. The aeration process results from ground water discharge, wind, waterfalls, and other types of running water. There are also a couple of man-made systems that can cause aeration, the primary of which include a hand-turned waterwheel and an aquarium air pump.

Photosynthesis in the ocean

What Are the Sources of Dissolved Oxygen Already in Ocean Water?

Dissolved oxygen is a byproduct that’s produced during photosynthesis, which means that dissolved oxygen can be introduced to the water by seaweed, phytoplankton, algae, and similar aquatic plants. Even though dissolved oxygen can be produced by algae and seaweed, too much algae means that the water could become eutrophic, which would result in not enough dissolved oxygen entering the water.

Dissolved Oxygen from Photosynthesis

As mentioned previously, some of the dissolved oxygen that enters ocean water is introduced by various types of plants that live in the water. A majority of the photosynthesis process will take place along the water’s surface because of the presence of algae and shallow water plants. However, a considerable amount of the process will still occur underwater by phytoplankton, seaweed, and sub-surface algae.

Light is able to penetrate through water, which allows photosynthesis to take place. However, the depth that light reaches largely depends on how many dissolves solids are present in the water. If the concentration of dissolved solids is high, the light will scatter as opposed to going deeper, which will make it difficult for plants at lower depths to perform the photosynthesis process.

The depth that light penetrates also dictates which wavelengths plants will receive. Blue wavelengths can be visible even past 100 meters. On the other hand, red wavelengths are absorbed at a quicker rate. When the water is perfectly clear, light can reach a depth of 200 meters. Past this point, the photosynthesis process can’t take place since plants won’t have the sunlight they require to perform this process.

If the water is turbid and cloudy, the light-penetrating zone is considerably shallower. Because aquatic photosynthesis depends on availability of light, dissolved oxygen production is always at the highest during the daytime.

Ocean water bubbles

Measuring Dissolved Oxygen

Dissolved oxygen is measured as milligrams per liter, which is displayed as mg/L. Keep in mind that the amount of dissolved oxygen that dissolves in water is very low. Only around 10 molecules of oxygen dissolve for every million of water. Lab and field meters have been used to measure dissolved oxygen levels for an extended period of time. Over the years, these meters have become smaller and far more portable, which makes it easy to measure how much dissolved oxygen is currently in a sample of water.

It’s highly recommended that you measure DO levels with a standard DO sensor, of which there are a couple different types to select from. The two types of sensors available to you include galvanic DO sensors and optical DO sensors. A galvanic dissolved oxygen sensor is comprised of a cathode and anode, both of which are placed into electrolytes. An electric signal travels directly to the anode from the cathode to start measuring the amount of dissolved oxygen in the water.

Optical DO sensors are entirely different than galvanic ones and work by using light wavelengths to accurately identify dissolved oxygen levels. Sensorex sensors include galvanic and optical DO sensors. You can use Sensorex sensors like the DO6400 to obtain precise DO readings that will allow you to react accordingly.

If you work in an industrial facility, the make-up water needs to consist of low DO levels to ensure that scale buildup and corrosion don’t occur. A DO sensor will allow you to identify what the current DO levels are and if they need to be lowered. Keep in mind that scale buildup can lead to a costly boiler malfunction. While low DO levels are required for many industrial applications, high DO levels are necessary for drinking water. When DO levels are high, drinking water invariably tastes better.

Dissolved oxygen is an important aspect of water quality and the continued health of aquatic life. Even small changes in dissolved oxygen concentrations can lead to issues in every type of water. Low DO levels usually indicate that water is more contaminated, which means that the water will taste bad and that aquatic life won’t receive the amount of DO they need to survive. On the other hand, high levels of dissolved oxygen mean that industrial systems and equipment can become corroded. No matter the intended application, dissolved oxygen sensors can provide you with accurate measurements in a matter of minutes.

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Posted by Dominic O'Donnell on March 15, 2022

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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