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Parameters Measured

Surface Dissolved Oxygen

Dissolved oxygen (DO) is a measure of the amount of oxygen in the form of microscopic bubbles dissolved in water. DO is measured in milligrams per liter (mg/L). The concentration of DO is a single, easy-to-measure characteristic of water that correlates with the occurrence and diversity of aquatic life. A water system that can support diverse, abundant life is a good indication of excellent water quality.

Two major sources of DO are absorption from the atmosphere and the release of oxygen during photosynthesis by aquatic plants and algae. Over a 24-hour period, DO fluctuation occurs naturally in most water bodies. Normally, as the sun rises, photosynthetic organisms begin to produce oxygen and the DO concentration increases through the day (production is greater than consumption). As the sun sets, photosynthesis essentially ceases and DO levels drop due to respiration by plants and animals (consumption is greater than production). The lowest DO levels usually occur just before dawn. In general, oxygen consumption (respiration) can be considered a constant drain on an aquatic system, day and night.

The introduction of nutrients and organic waste also increases the drain on an aquatic system. Nutrients, mainly nitrogen and phosphorus, are essential for plant growth. However, in large quantities (primarily from wastewater discharge, urban and agricultural runoff, and leaking septic systems), nutrients can cause algae to grow at an accelerated rate and create algae blooms. During algae blooms, DO levels in the daylight hours can be extremely high. However, during the night when oxygen production (photosynthesis) ceases and respiration (consumption) begins, the DO levels can drop to levels that lead to fish kills. The effect of an algal bloom can be compounded on overcast days. Decreased sunlight causes a reduction in oxygen production which results in a net loss of DO. As algae die, decomposition by oxygen consuming bacteria occurs. Inflow of organic waste (grass clippings, animal waste, raw sewage) can increase the amount of oxygen consumed by bacteria.