What Are The Impacts Of Dissolved Oxygen In Ocean Water?

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Measuring dissolved oxygen (DO) in ocean water is a key factor in understanding water quality. When DO levels start to drop in ocean water, consequences include decreased biodiversity algal blooms and eutrophication events, a reduction or displacement in fishery resources, and shifts in species distributions. If low DO levels continue for a prolonged period, it can have major implications for the ocean’s food web and ecosystem services. 

Covering more than 70% of the Earth’s surface, the ocean is home to an estimated 1.4-1.6 billion marine species on Earth. This vital blue planet provides us with 50-80% of the oxygen we breathe and now it is as crucial as ever to protect it. By understanding the role of dissolved oxygen (DO), we can protect the oceans and the life that call it home.

All aquatic organisms require dissolved oxygen to survive, and while DO concentrations mustn’t be too high or low, aquatic life and ecosystems are impacted most when levels are dangerously low. So, let’s take a look into why this happens, and what other impacts DO has on our precious oceans. 

What Is Dissolved Oxygen?

Dissolved oxygen is a non-compound or free oxygen (O2) that is found within water or other liquids. Despite water molecules (H2O) being compromised of one oxygen atom, organisms living in the ocean are unable to consume these molecules to obtain the oxygen needed to survive. This is because H2O is a bonded molecule that doesn’t count towards DO levels in the water. So, for aquatic organisms to survive, they must locate free oxygen molecules, which is a challenge in itself, as only a tiny amount of oxygen in the water naturally dissolves and forms into dissolved oxygen. 

What Causes Dissolved Oxygen To Enter Ocean Water?

Dissolved oxygen enters ocean water via the air or as a byproduct of photosynthesis from various plant species.

When DO comes from the surrounding air, the oxygen slowly diffuses along the water’s surface or when mixing occurs via aeration. Aeration in water occurs from wind, waterfalls, streams, groundwater discharge, and other forms of running water that create turbulence in the water. It is the most effective way to increase DO levels in water, which is why man-made structures and systems are often installed. A good example of this principle is an aquarium air pump. 

When DO is a byproduct of photosynthesis, oxygen is introduced by aquatic plants such as seaweed, algae, phytoplankton, and other aquatic plants. Although DO is produced by algae and seaweed, if these aquatic plants grow excessively, the water becomes eutrophic, resulting in critically low DO levels entering the ocean water. 

Dissolved Oxygen Concentrations & Zones In Ocean Water

The highest concentration of dissolved oxygen is at the ocean’s surface, also known as the sunlit upper layer, or epipelagic zone. This is where oxygen is dissolved from the atmosphere and where the most oxygen production takes place via phytoplankton. The epipelagic zone is also where high rates of reproduction occur, however, the rate of photosynthetic oxygen is much greater than the removal of oxygen via respiration. 

We must however note, that despite the epipelagic zone being oxygen-rich in a sense, there is significantly less oxygen in the ocean’s surface water than oxygen levels in the air. For example, atmospheric air contains approximately 210 mg O2/L compared to the ocean’s surface water containing 8mg O2/L.

As we move further down the water column, the level of DO declines, dropping off to a bare minimum between a few hundred meters and 1000m in depth. This section of the ocean is called the oxygen minimum layer (OMZ), often referred to as the shadow zone due to the lack of light reaching the greater depths. At these depths, atmospheric exchange no longer takes place and the lack of light means that photosynthesis is no longer supported. Therefore, in the OMZ, there is little to no oxygen in the ocean water. 

In addition to these oxygen-depleted ocean depths, oxygen is removed from the water via respiration from deep water organisms and the decomposition of marine snow (organic material that sinks from the upper water layers). 

Continuing down, below the OMZ an increase of DO can be found because the O2 consumption rates are much lower and they are supplied with cold, oxygen-rich waters from Earth’s polar regions. When water is exposed to lower temperatures and higher pressure, the amount of DO is greater than warmer temperatures and lower pressure. 

The polar regions also absorb more O2, pushing the cold, oxygen-rich water below the ocean’s surface and sinking it to the bottom ocean water layers, a process known as thermohaline circulation. This is because the water is denser in the polar regions, taking the DO with it, and distributing it in bottom waters over thousands of years until it ends up in one of the five major ocean basins (Pacific, Atlantic, Indian, Southern, and Arctic. 

What Are Hypoxia Zones?

In some areas of the oceans where the O2 is not circulated, DO levels are too low to sustain marine life in the bottom or near waters. These so-called hypoxic zones (or dead zones) are defined as having O2 levels lower than 2mg/L. These zones largely impact and alter biogeochemical cycles which change the productivity of ocean ecosystems and coastal communities due to changes in the cycling of elements such as phosphorous (P). In the oceans, phosphorous has a pivotal role in promoting the production of microbes and phytoplankton. When DO levels are low in ocean water, the recycling of P becomes enhanced, resulting in increased availability which can increase productivity and the oxygen demand in deeper water. This cycle increases the growth of algae communities and other aquatic plants, resulting in eutrophication and thus, hypoxic zones.

Another issue with low DO levels in ocean water is nitrogen loss which impacts photosynthetic organism growth.

The Gulf of Mexico is a good example of an area that experiences (seasonal) hypoxic zones every year. This is caused by excessive nutrients such as nitrogen and phosphorous from the Mississippi River. Throughout the year, other parts of the ocean can experience seasonal or temporary hypoxia periods, often resulting in mass die-offs of marine organisms that cannot survive without oxygen.

When O2 levels drop below 0.5mg/L, they become anoxic, creating a zone without oxygen. A classic example of a marine anoxic basin is The Black Sea, a large inland sea situated between Europe and Asia. Here the water has an oxygenated surface layer that sits on an anoxic deep layer. This forms due to the strong density stratification (where the upper layers do not mix with deeper water) in the water column. As a result, more than 90% of the deep water in The Black Sea contains anoxic water. 

What Impact Does Dissolved Oxygen Have In The Ocean?

Oxygen is fundamental for the survival of aquatic life. Yet, the level of dissolved oxygen in the ocean not only directly affects aquatic life but it also affects the water quality

How Does Dissolved Oxygen Affect Aquatic Organisms?

Life as a marine organism can be a challenge! Extracting oxygen from ocean water is more difficult than in air because water contains 20-40 times less O2 by volume than air, and ocean water (seawater) holds less O2 than freshwater. As O2 diffuses into ocean water, this process takes 10,000 times longer than in air.

Unfortunately, many factors (temperature, salinity, atmospheric pressure, currents and upwelling, respiration, decomposition rates, nutrient discharge from wastewater or agricultural/urban run-off) can affect DO levels in ocean water, and the slightest decrease in DO levels can have detrimental effects on not only marine organisms but also entire ecosystems, threatening those on land who rely on aquatic ecosystems. 

The majority of aquatic organisms occupying the oceans need DO to survive. Organisms such as invertebrates, bacteria, fish, mammals, and plants, rely on the DO for respiration. For example, fish and crustaceans like crabs allow water to pass their gills to breathe, accessing DO from the ocean water. Plants and phytoplankton on the other hand use DO to respire and drive the photosynthesis process when light is limited.

When aquatic organisms cannot survive in their particular area of the ocean, the motile ones (able to move) will seek other habitats where DO is higher, and unfortunately, the sessile species (unable to move) will likely perish and die. This movement of species can cause issues such as increased competition in different areas, often resulting in outcompeting smaller or weaker organisms and disturbing the food web. 

While marine organisms need DO to survive, the level of DO for survival differs between species. For example, fish that occupy shallow waters, require 4-15 mg/L of oxygen to survive, whereas crabs, aquatic worms, and shellfish like oysters require much less DO (only 1-6 mg/L). 

Decomposers such as fungi and bacteria also need oxygen, but they use the DO to play a key role in breaking down organic material that sinks in the water column. Without these decomposers, there would be too much decaying organic material in the ocean water, which would lead to declines in DO levels. 

How Does Dissolved Oxygen Affect Water Quality?

Water quality is essential in protecting marine ecosystems and preventing large marine species’ loss, and for those people whose livelihoods depend on it. Not only does measuring the level of DO tell us about the oxygen content available for species, but it is also a good indicator of water quality.

Earlier we mentioned that ocean water can become eutrophic if aquatic plants overgrow, causing DO levels to plummet. These dangerous hypoxic conditions can kill fish and plants like seagrass which support bivalve mollusk populations such as scallops and oysters. Without these bivalves, nutrients that are usually removed from ocean water through their filter-feeding activities will increase, which could result in mass mortalities. If eutrophication in the ocean occurs, the water appears with a green hue as a result of algal blooms (overpopulation of algae species). 

Eutrophic conditions are more common in the summer when the ocean’s surface temperatures are too warm for DO to be absorbed. Unfortunately, even though the bottom water still contains DO during the early stages of eutrophication, the DO is soon consumed by aquatic organisms and the majority of fish will die off. 

Does Dissolved Oxygen In Ocean Water Affect Fish Stocks?

The fishing industry is huge, with an estimated 0.97-2.7 trillion fish being caught in the oceans every year globally. In ocean areas suffering from low DO levels, the fish are fleeing negatively affecting species diversity and the size of fish caught due to lower susceptibility to diseases, slower growth rates, reduced fertilization rates, and slower embryonic development.

With such a demand on the fishing industry, fish will continue to be caught in extensive numbers, and these catches will not only reduce due to low DO levels in the ocean water but smaller fish with stunted growth and poor health will be served on our plates. Verbeck, an eco-physiologist from the Netherlands (Radboud University) has said: “Overfishing has already had the effect of decreasing the number of large fish in the ocean; deoxygenation looks set to exacerbate that effect”.

How To Measure Dissolved Oxygen In Ocean Water?

Measuring DO levels in ocean water identifies the current water quality; when DO levels are too high or low, it usually indicates poor water quality, and therefore, aquatic organisms will find living conditions challenging.

You’d be surprised to learn that measuring dissolved oxygen in ocean water is the same method as how to measure dissolved oxygen in wine – with a dissolved oxygen sensor/meter!

When measuring DO levels in ocean water, we always recommend using a standard DO sensor. Dissolved oxygen sensors provide accurate readings typically ranging from 0-20 mg/L. At Atlas Scientific, our dissolved oxygen kits have a range of 0-100mg/L, covering a wide range of applications from standard lab use and environmental monitoring to field use like testing ocean water. 

Can We Increase Dissolved Oxygen In The Oceans?

With the ocean losing its means to breathe, we must act now. Fortunately, the actions we take can contribute to restoring the ocean’s oxygen levels while reducing the impacts of climate change. 

We can:

  • Clean up our act when it comes to sewage by treating wastewater at source to remove excess nitrogen and phosphorous from entering the oceans through waterways such as rivers and streams. 
  • Make the agricultural industry more efficient by reducing harmful chemicals like fertilizers and pesticides that contribute to eutrophication due to run-off.
  • Conserve wetlands and forests to absorb nutrient pollution before they enter the ocean water. 
  • Deploy more real-time monitoring devices along coastlines to monitor DO levels. 
  • Utilize fish science – inside a fish’s ear, calcium growths called otoliths (ear stones) record the chemistry as the fish grows. Allowing scientists to study these can identify if the fish have passed tough areas with low DO levels. 
  • Governments should set stricter size limits to protect vulnerable species or implement protected areas where fishing is restricted in certain areas or seasons when low DO levels are known to occur. 

Summary

Dissolved oxygen (DO) is an important water quality indicator in ocean water, but it goes far beyond that, as DO also affects the metabolism of aquatic organisms and provides us with a greater understanding of ocean water and ecosystem health. Reductions of DO in ocean water can have detrimental effects on marine life, leading to dead zones and possible species extinctions.

Should you have any questions about dissolved oxygen in water or would like more information on our range of DO sensors, do not hesitate to contact the world-class team at Atlas Scientific. ​​

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