Water Quality Measurement | Physical Parameter
Lake Pend Oreille | Pend Oreille River
Dissolved Oxygen (DO) is essential for all fish, aquatic plants, and aerobic bacteria. The majority of oxygen comes from rooted aquatic plants, which release oxygen during photosynthesis.
Low dissolved oxygen levels are indicative of excessive oxygen demand, which indicates excessive productivity linked to unusually high nutrient levels. Low DO levels are stressful to most aquatic organisms and may cause species death, weakening or relocation (US EPA 2010).
Dissolved Oxygen also reflects temperature patterns since colder water can hold more oxygen than warmer water. This results in both seasonal and daily patterns. Generally, DO levels are lowest in the summer months and higher in winter.
Adequate dissolved oxygen (DO) is essential to sustain most freshwater life. The majority of dissolved oxygen in a lake comes from the atmosphere and from rooted aquatic plants, which release oxygen during photosynthesis. High nutrient concentrations can influence DO concentrations by causing increased plant and algal growth. Bacteria that decompose plants and algae consume oxygen during their metabolic processes. When more oxygen is consumed at a greater rate than it is produced, DO declines. In other words, low dissolved oxygen levels can be indicative of excessive oxygen demand. Low DO levels are stressful to many aquatic organisms (US EPA 2010).
The propensity for water to hold dissolved oxygen is temperature and pressure dependent. Low DO levels may reflect thermal pollution or increased salinity. Cold water can hold more oxygen than warm water. Accordingly, DO concentrations follow diurnal and seasonal patterns associated with natural air temperature changes.
Dissolved oxygen also changes with depth. Temperate oligotrophic lakes such as Lake Pend Oreille tend to stratify in the summer months; dissolved oxygen increases with depth (as temperature decreases) (Wetzel 2000). The lake “turns over” in in the fall, allowing recirculation of DO throughout the water column.
Some sources of dissolved oxygen
- Atmospheric inputs (e.g., turbulent, flowing water)
- Byproduct of photosynthesis by aquatic plants and algae
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