Water that sits still for too long can develop problems that are not always visible on the surface. From unpleasant odours to reduced oxygen levels, stagnant water can affect water quality and the health of aquatic life.
Understanding what happens when water stops moving is an important step in maintaining healthier water systems.
1. Dissolved Oxygen Levels Can Drop
Water needs dissolved oxygen (DO) to support fish, beneficial microorganisms, and other aquatic organisms. When water remains stagnant, oxygen replenishment from the atmosphere may be limited, especially when there is little surface movement.
At the same time, bacteria and other microorganisms consume oxygen as they break down organic matter, such as leaves, algae, and waste. If oxygen consumption exceeds replenishment, dissolved oxygen levels can fall, putting aquatic life under stress.
2. Unpleasant Odours Can Develop
When organic matter accumulates in stagnant water, microorganisms begin breaking it down. If oxygen becomes depleted, certain bacteria can continue decomposition under low-oxygen conditions, producing compounds that cause unpleasant smells.
This is why stagnant ponds, drainage areas, and poorly circulated water systems may develop a rotten or sulphur-like odour.
3. Algae Growth Can Increase
Stagnant water can encourage algae growth when sunlight, nutrients, and suitable temperatures are available. Excess nutrients from fertilisers, organic waste, and runoff can make the problem worse.
When algae grow excessively and later die, their decomposition consumes dissolved oxygen. This can create a cycle in which water quality deteriorates further, particularly in ponds and lakes with limited circulation.
4. Sediment and Organic Waste Can Accumulate
Without sufficient water movement, suspended particles and organic matter can settle on the bottom. Over time, this accumulation can contribute to sludge formation and create conditions that encourage oxygen depletion near the sediment.
The breakdown of accumulated organic material can also release nutrients back into the water, potentially contributing to further algae growth.
5. Water Quality Can Become Less Stable
Stagnant water may develop uneven conditions throughout a pond, tank, or reservoir. For example, surface water may contain more oxygen than deeper water, where oxygen can become depleted.
Temperature, pH, dissolved oxygen, and nutrient levels can vary depending on the depth, weather, and amount of organic matter present. These changes can affect aquatic organisms and make water management more challenging.
How Can Stagnant Water Be Managed?
Managing stagnant water starts with understanding the conditions causing the problem. Depending on the application, useful measures may include:
- Improving circulation: Water movement can help distribute oxygen and reduce areas with poor circulation.
- Managing organic waste: Removing excessive leaves, sludge, and other organic material can reduce oxygen demand.
- Controlling nutrient inputs: Limiting excessive fertiliser runoff and other nutrient sources can help reduce algae growth.
- Monitoring water quality: Regularly checking dissolved oxygen, temperature, and other relevant parameters helps identify changes before they become more serious.
Aeration and other oxygen-transfer methods may also help where dissolved oxygen is low. However, the appropriate solution depends on the water body’s size, depth, oxygen demand, and overall condition.
Why Water Monitoring Matters
Water can look clear and still have low dissolved oxygen or other water quality issues. Visual checks alone cannot provide a complete picture of what is happening beneath the surface.
Monitoring key parameters, especially dissolved oxygen and temperature, helps operators understand changing conditions and make better-informed decisions about water management.
Healthy water requires more than a clean-looking surface. By understanding the effects of stagnation, maintaining appropriate circulation, and monitoring water quality, we can better protect aquatic life and maintain healthier water systems.
