Oxygen

Vocabulary/new concepts:

Dissolved oxygen is the amount of oxygen dissolved in a body of water (river, lake, sea, etc.) It is a prerequisite for the survival of aquatic organisms.

Eutrophication – describes the process by which nutrients, wastewater, and other pollutants accumulate in a body of water, resulting in increased blooms of microorganisms and algae. This depletes dissolved oxygen in the water and causes significant environmental degradation.

What is oxygen?

It is a chemical element, a gas with no colour, odour and taste. It always occurs in pairs in our atmosphere, hence its spelling O₂.

Where does most of our planet’s oxygen come from?

Almost half of the Earth’s oxygen comes from underwater organisms such as plankton and algae through a process known as photosynthesis. To measure how much oxygen is produced by said organisms, scientists use satellite images to analyse different aspects of the body of water (temperature, depth, colour, etc.) to approximate the amount of oxygen. They found that:

• The amount of plankton in a water body varies depending on the season, nutrient load, temperature, etc.
• The amount of O₂ in certain locations may vary during the same day.
• Sea dwellers consume almost all the oxygen that is produced in the water.

What is dissolved oxygen?

This is the amount of oxygen dissolved in a body of water (river, lake, sea, etc.). Without it, aquatic organisms would not survive. One way of getting oxygen into a body of water is when molecules of the gas (in our case the gas is oxygen) that are constantly moving can get into the water. Another way is through the photosynthesis of organisms that live underwater.

The factors that influence the amount of oxygen in a particular marine environment are:

• Temperature: a higher temperature means that the molecules move faster, so they can get out of the water more easily. This leads to a lower concentration of D.O. Fish that need less oxygen to survive can live in warmer waters than fish that need more oxygen. Temperature is a critical factor affecting dissolved oxygen levels.
• Fish respiration: Since fish consume oxygen to survive, having more fish means less D.O.
• Decomposition: Bacteria and other microorganisms consume oxygen during the decomposition of organic matter. Organic materials, such as dead plants or animal waste, can increase their oxygen demand during decomposition, resulting in lower D.O. levels.
• Turbulence and water mixing: water movement and turbulence enhance the mixing of oxygen from the atmosphere into the water basin. Wave action, wind and water currents help replenish oxygen levels by increasing gas exchange at the air-water interface. Areas with stagnant or slow-flowing water may have lower D.O. levels due to reduced mixing.

Monitoring and understanding these factors are critical to maintaining healthy aquatic ecosystems and sustaining diverse aquatic life.

What is eutrophication?

Harmful algal blooms, dead zones and fish kills are the result of a process called eutrophication. It occurs when the environment becomes enriched with nutrients, increasing the amount of plants and algae in estuaries and coastal waters. Excessive amounts of nutrients, mainly nitrogen and phosphorus, enter the water body through a variety of sources, such as fertilizers, sewage, and runoff from agricultural fields or urban areas. Increased nutrient availability stimulates rapid algal growth in the water. As the algal population increases, the water becomes more turbid due to the abundance of suspended algae. This reduces the penetration of sunlight into the water column. During daylight hours, algae produce oxygen through photosynthesis. At night, however, or when algal blooms die off, the decomposition of organic matter by bacteria consumes oxygen, resulting in lower dissolved oxygen levels in the water. Excessive algal growth can shade out and smother underwater plants, leading to a decline in their populations. Reduced oxygen levels can harm fish and other organisms that depend on dissolved oxygen. In addition, some algae can produce toxins that are harmful to humans, animals and other aquatic organisms.