Dissolved oxygen (DO) is the concentration of oxygen gas in water. It is a fundamental indicator of water quality, central to sustaining aquatic ecosystems. Aquatic life, including fish, invertebrates, and microorganisms, relies on sufficient DO for metabolic processes like respiration. Adequate DO levels are necessary for the growth, reproduction, and health of aquatic organisms.
Understanding How Dissolved Oxygen Becomes Excessive
Water can sometimes contain more dissolved oxygen than it typically holds at equilibrium, a condition known as supersaturation. This occurs when water becomes oversaturated with gases. Several factors contribute to this phenomenon. Rapid temperature changes, such as cold water warming quickly, can reduce the water’s capacity to hold dissolved gases, forcing excess oxygen out of solution.
Strong aeration and turbulent water flow, found in areas like waterfalls, rapids, or spillways below dams, can significantly increase the absorption of oxygen from the atmosphere into the water. Intense photosynthesis by aquatic plants or algae during daylight hours can also produce oxygen more quickly than it can be released into the atmosphere, leading to supersaturation. When these conditions persist, the water’s total gas pressure can exceed 100% saturation.
Harmful Effects on Aquatic Life
Excessive dissolved oxygen primarily harms aquatic life by causing Gas Bubble Disease (GBD). This non-infectious condition occurs when supersaturated gases enter the bloodstream and tissues of fish and other aquatic animals. As the water temperature or pressure changes, these dissolved gases come out of solution, forming bubbles within the body.
These bubbles can form in various tissues, including the eyes, skin, gills, and fins. In severe cases, microbubbles combine to create larger bubbles, obstructing blood flow and damaging tissues. This can lead to symptoms such as emphysema, where small gas bubbles are visible under the skin, or exophthalmia, commonly known as “pop-eye,” where one or both eyes bulge outwards due to gas accumulation behind them.
Affected fish may exhibit abnormal buoyancy, disorientation, or lethargy. Bubbles in the gills can hinder oxygen diffusion, leading to respiratory distress. While sensitivities vary among species and life stages, prolonged exposure to high DO can lead to reduced feeding, slow growth, increased disease susceptibility, and mortality.
Broader Impacts on Water Quality and Systems
Beyond the direct harm to aquatic organisms, excessive dissolved oxygen can also influence water chemistry and impact man-made systems. High DO levels can accelerate the oxidation of compounds, potentially affecting nutrient cycles. For instance, an imbalance can disrupt the natural equilibrium of nutrient processing.
In industrial and municipal water systems, high dissolved oxygen can contribute to infrastructure degradation. It can accelerate pipe corrosion, particularly in drinking water distribution and industrial cooling systems. Industries often aim for very low DO levels in water used for boilers to minimize such corrosive effects.
In wastewater treatment plants, while aerobic bacteria require oxygen to break down organic matter, excessively high DO levels can sometimes lead to increased power consumption for aeration systems. Maintaining appropriate dissolved oxygen concentrations is necessary for efficient treatment processes and water quality management.