Maritech CV

Dissolved Oxygen (DO) refers to the concentration of oxygen gas (O2) dissolved in water, which is essential for the survival of aquatic organisms and the health of aquatic ecosystems. It is measured in milligrams per liter (mg/L or ppm) or as a percentage of saturation.

DO is influenced by various factors such as:

• Temperature: Cold water holds more oxygen than warm water.
• Pressure: Higher atmospheric pressure increases oxygen solubility.
• Salinity: Freshwater holds more oxygen than saltwater.
• Biological Activity: Photosynthesis increases DO, while respiration and decomposition decrease it.

Examples of Extreme DO Levels

1. High DO Levels (>14 mg/L)

Occurs under specific conditions, such as:

• Extreme Photosynthesis:
  • In dense algal blooms, intense photosynthesis during daylight can supersaturate the water with oxygen.
  • Example: Shallow, sunlit ponds with excessive nutrient input.
• Cold, Turbulent Water:
  • Fast-moving streams or waterfalls in cold climates can reach very high DO levels due to continuous aeration and low water temperatures.
  • Example: Mountain rivers or streams in alpine regions.

Effects of High DO:

• Positive: Beneficial for most aquatic life.
• Negative: Can lead to gas bubble disease in fish and aquatic invertebrates if levels are excessively high for prolonged periods.

2. Normal DO Levels (5–14 mg/L)

• Typical in healthy aquatic systems where water temperature, flow, and biological activity are balanced.
• Examples:
  • Rivers, lakes, and oceans in temperate climates.

Effects:

• Supports diverse aquatic ecosystems with good biodiversity.

3. Low DO Levels (2–5 mg/L)

• Occurs due to:
  • High organic matter decomposition, consuming oxygen.
  • Stratified water bodies with limited mixing.
  • Nutrient-rich water leading to algal blooms and subsequent decay.

Examples:

• Stagnant lakes or reservoirs during summer stratification.
• Polluted rivers receiving untreated sewage.

Effects:

• Stressful for many aquatic species, especially oxygen-sensitive fish like trout.

4. Extremely Low DO Levels (<2 mg/L) – Hypoxia

• Causes:
  • Decomposition of organic matter consuming oxygen.
  • Limited water mixing in stratified lakes or dead zones in oceans.
• Examples:
  • Gulf of Mexico Dead Zone: Seasonal hypoxic zone caused by agricultural runoff, leading to algal blooms and oxygen depletion.
  • Deep layers of eutrophic lakes during summer.

Effects:

• Suffocation of aquatic life, leading to fish kills.
• Dominance of anaerobic organisms (e.g., certain bacteria).

5. Zero DO (0 mg/L) – Anoxia

• Causes:
  • Complete oxygen depletion in stagnant or heavily polluted waters.
  • High levels of organic waste or prolonged decomposition.
• Examples:
  • Black Sea: Deep anoxic layers due to stratification and organic matter buildup.
  • Heavily Polluted Rivers: Urban rivers with untreated sewage (e.g., the Ganges in certain regions).

Effects:

• Complete die-off of aerobic organisms.
• Only anaerobic life (e.g., sulfate-reducing bacteria) can survive.

DO Levels and Aquatic Life

Monitoring and Managing DO Levels

1. Monitoring Tools:

   • DO meters or optical sensors for real-time measurements.
   • Winkler titration method for precise lab testing.

2. Management Strategies:

   • Aeration systems in ponds or reservoirs.
   • Reducing nutrient runoff to prevent eutrophication.
   • Promoting water circulation and mixing in stagnant areas.