Oxygen exists underwater, but not in the gaseous form found in the air we breathe. Instead, it is dissolved within the water, and this dissolved oxygen sustains most aquatic life. Its presence is fundamental for the survival of many aquatic organisms.
Understanding Dissolved Oxygen
Dissolved oxygen (DO) refers to the concentration of oxygen gas molecules incorporated into water. This is not the oxygen atom chemically bonded within water molecules (H₂O). Aquatic organisms utilize these free oxygen molecules, which are measured in units like milligrams per liter (mg/L) or parts per million (ppm).
Oxygen enters water through two main mechanisms. One is direct absorption from the atmosphere, a process called diffusion. This absorption is enhanced by turbulence, such as wind and wave action or swift-moving streams. The other source is photosynthesis by aquatic plants and algae. During daylight hours, these organisms release oxygen as a byproduct, directly contributing to DO levels.
How Aquatic Organisms Breathe Underwater
Aquatic organisms have specialized respiratory systems to extract dissolved oxygen from water. Fish, for example, use gills, which are efficient for gas exchange. Gills consist of numerous comb-like filaments, each with thousands of tiny folds called lamellae. These lamellae are richly supplied with capillaries, which are small blood vessels.
Fish actively pull oxygen-rich water through their mouths and pump it over their gills. Within the gill lamellae, countercurrent exchange takes place. Blood flows through the capillaries in a direction opposite to the water flow over the gills. This opposing flow maintains a continuous concentration gradient, allowing oxygen to diffuse from the water into the fish’s blood. This system enables fish to extract up to 80% of the available oxygen. Some other aquatic creatures, like certain amphibians and invertebrates, can also absorb dissolved oxygen directly through their skin.
Factors Influencing Underwater Oxygen Levels
The concentration of dissolved oxygen in water is influenced by several environmental factors. Water temperature is a significant factor, as colder water holds more dissolved oxygen than warmer water. Salinity, or dissolved salts, also affects DO levels; higher salinity reduces oxygen solubility. Additionally, atmospheric pressure and altitude influence oxygen solubility, with lower pressures at higher elevations leading to less dissolved oxygen.
Biological activity impacts oxygen availability. During the day, aquatic plants and algae produce oxygen through photosynthesis, leading to higher DO levels. At night, photosynthesis stops, and both plants and animals consume oxygen through respiration, causing DO levels to drop. The decomposition of organic matter by microorganisms also consumes large amounts of dissolved oxygen, known as biochemical oxygen demand (BOD).
Low dissolved oxygen levels, termed hypoxia, can severely impact aquatic ecosystems. Oxygen concentrations below 5 mg/L can stress fish and other aquatic organisms, impairing their growth and reproduction. Levels below 1-2 mg/L for even a few hours can result in large-scale fish kills. Hypoxia can lead to disrupted food chains, altered behavior like fish gathering near the surface, and increased vulnerability to pollution, degrading aquatic habitats.
Why Humans Cannot Breathe Underwater
Humans are unable to breathe underwater because our respiratory system is adapted for extracting oxygen from air. Human lungs process gaseous oxygen, which is far more concentrated in the atmosphere than dissolved oxygen in water. Our lungs lack the specialized structures, like the filaments and lamellae in fish gills, needed to absorb the much lower concentration of oxygen from water.
Water is denser and more viscous than air, making it harder for human lungs to move and process the volume of fluid necessary to extract sufficient oxygen. As warm-blooded animals, humans also have a higher metabolic rate and oxygen demand compared to cold-blooded fish, highlighting the inadequacy of our lungs for aquatic respiration. Marine mammals like whales and dolphins, though aquatic, still breathe air and must surface periodically to take in oxygen, demonstrating this fundamental difference.