Management of the San Marcos Watershed

Water Quality

The health of the San Marcos River is measured using quantitative and qualitative measures.

Dissolved Oxygen

An indicator of how well a body of water can support aquatic plant and animal life.


What is dissolved oxygen?

Dissolved oxygen is the level of oxygen in a body of water, measured in milligrams per liter (mg/L). A level consistently above 5 mg/L is sufficient to support plants and animals. Readings between 3 to 5 mg/L are a "warning" zone, where steps must be taken to restore a body of water back to equilibrium, and levels below 3 mg/L can be harmful to aquatic life.

What affects dissolved oxygen?

  • A higher water temperature will mean lower dissolved oxygen levels in the water, while lower temperature water will support more oxygen.
  • Plants on the surface and aquatic animal life affect dissolved oxygen through photosynthesis and respiration.
  • Chemicals or other foreign substances in the water - specifically, when waters are polluted - will react with oxygen in the water and impact aquatic life, which may cause a drop in dissolved oxygen.

How is it tested?

The dissolved oxygen test for citizen scientists is a three-step process:

  1. Collecting the sample
  2. "Fixing," or stabilizing the sample (Watch on YouTube)
  3. Titrating the sample (Watch on YouTube)

This process removes air bubbles from the sample, so that the oxygen content measured is only the amount of oxygen dissolved within the water itself.

Source: Texas Stream Team Monitoring Manual

San Marcos River, Upstream

The citizen scientist data on dissolved oxygen at the beginning and end of the San Marcos River is made available from the Texas Stream Team water quality database. The average dissolved oxygen content each year stays above 5 mg/L, and therefore is consistently at a healthy level for aquatic plants and animals, including the endagered and threatened species.