Spectra Scientific | Professional Grade Groundwater Monitoring
Dissolved Oxygen
Measures the amount of free oxygen dissolved in water, crucial for supporting aquatic life and assessing water quality.
I. Introduction to Dissolved Oxygen
What is Dissolved Oxygen (DO)?
Measures the amount of free oxygen gas (O2) in
water, essential for aquatic life.
Why is DO Important?
Influences water quality, biological processes, and ecosystem health. Typically measured in mg/L or % saturation.
II. Sources of Dissolved Oxygen
- Atmospheric Exchange: Oxygen diffuses from air to water based on concentration gradients; aeration and turbulence enhance this process.
- Photosynthesis: Aquatic plants, algae, and phytoplankton release oxygen during daylight hours.
III. Dissolved Oxygen Saturation
- 100% Saturation: Maximum DO a body of water can hold under equilibrium conditions.
- Supersaturation: Occurs when DO exceeds 100% due to rapid photosynthesis or turbulence.
- Stratification: Lower DO in deeper waters due to limited mixing and microbial respiration.
IV. Factors Affecting Dissolved Oxygen
- Temperature: Inversely affects DO; colder water holds more oxygen.
- Pressure: Higher atmospheric pressure increases DO solubility.
- Salinity: Higher salinity reduces DO solubility.
- Photosynthesis & Respiration: Daytime DO increases due to photosynthesis; respiration continuously consumes DO.
- Turbulence & Mixing: Enhances gas exchange and DO distribution.
- Nutrient Levels: Excess nutrients cause algal blooms, leading to oxygen depletion upon decomposition.
- Pollution: Organic waste and chemicals can lower DO levels, stressing aquatic life.
V. Why Measure Dissolved Oxygen?
A. Surface Water & Aquaculture
- Optimal DO Levels: Fish require 5-12 mg/L; stress occurs below 4 mg/L.
- Hypoxia: Occurs when DO falls below 2-3 mg/L, leading to fish kills.
- Stratification & Organic Decomposition: Microbial breakdown depletes oxygen in deeper layers.
- Harmful Algal Blooms (HABs): Cause fluctuations in DO, leading to hypoxia.
- Thermal Pollution & Salinity: Warm water holds less DO; road salt runoff decreases solubility.
B. Groundwater
- Groundwater Investigations: DO indicates well stability and water quality.
- Chemical Reactions: DO regulates metal solubility and microbial metabolism.
- Microbial Activity: DO depletion signals contamination or microbial respiration.
C. Wastewater Treatment
- Microbial Activity & Treatment Efficiency: Oxygen supports organic matter breakdown.
- Nutrient Removal: DO monitoring optimizes biological nutrient removal (BNR) for nitrogen and phosphorus removal.
VI. Typical DO Levels
- Freshwater: Seasonal and depth-dependent variations (e.g., lakes, rivers, streams).
- Saltwater: Lower DO due to salinity; polar waters hold more oxygen than equatorial waters.
- Regulatory Standards: Cold-water fisheries: 7 mg/L, warm-water fisheries: 5 mg/L.
VII. Consequences of Unusual DO Levels
- Fish Kills & Winterkills: Low DO causes mass mortality in aquatic organisms.
- Gas Bubble Disease: Occurs when DO exceeds 115-120% saturation.
- Dead Zones: Oxygen-deprived areas due to eutrophication, disrupting ecosystems.
VIII. Consequences of Lack of DO Monitoring – Aral Sea
- Environmental Disaster: Water diversion reduced freshwater inflow, increasing salinity and ecosystem collapse.
- Lessons Learned: Highlights the need for DO monitoring to prevent ecological damage.
IX. Dissolved Oxygen: Key to Water Quality & Ecosystems
Dissolved oxygen is essential for aquatic ecosystems and water quality. Monitoring DO prevents hypoxia, supports biodiversity, and ensures environmental sustainability.