DO

APES Section 1 — Dissolved Oxygen (DO)

1) The Core Concept (Most Important)

Dissolved Oxygen (DO) = the amount of oxygen in water available to aquatic organisms.

Why it matters:

• Aquatic organisms (fish, insects, etc.) need oxygen to survive

• DO is a major indicator of water quality and ecosystem health

Typical test phrasing:

• “Low dissolved oxygen indicates poor water quality”

• “High dissolved oxygen supports aquatic life”

2) Key Vocabulary You Should Know

These are very quizable terms:

• Dissolved Oxygen (DO) — oxygen dissolved in water

• Hypoxic — low oxygen

• Anoxic — no oxygen

• Eutrophication — excess nutrients causing algal growth

• Stratification — layers form in water due to temperature differences

• Turnover — mixing of water layers

• Photosynthesis — adds oxygen

• Respiration / decomposition — uses oxygen

Example from the packet:

Water with less than about 0.2 mg/L oxygen is considered anoxic (virtually no oxygen).

3) The Big Cause–Effect Relationships (APES Loves These)

What increases dissolved oxygen:

• Photosynthesis

• Mixing / aeration

• Cooler temperatures

What decreases dissolved oxygen:

• Warmer water

• Decomposition

• Eutrophication

• Respiration

• High salinity

• High altitude

Important detail:

Warmer water holds less oxygen.

4) Daily DO Cycle (Very Common Question)

According to the graph on page 2, dissolved oxygen follows a daily pattern:

Daytime:

• Photosynthesis happens

• Oxygen increases

Night:

• Photosynthesis stops

• Respiration continues

• Oxygen decreases

Lowest DO occurs:

Early morning — organisms are most vulnerable then.

5) Seasonal Lake Changes (Stratification & Turnover)

Spring:

• Water mixes

• Oxygen is evenly distributed

Summer:

• Layers form (stratification)

• Top has more oxygen

• Bottom has less oxygen

Fall:

• Water mixes again (turnover)

Winter:

• Ice forms

• Oxygen decreases with depth

The diagram on page 2 shows oxygen decreasing as depth increases during stratification.

6) Eutrophication and DO (Very Testable Chain Reaction)

Nutrients increase

→ Algae grow (algal bloom)

→ Algae die

→ Decomposition increases

→ Oxygen decreases

→ Fish die

Your packet explains that decomposition of dead algae reduces dissolved oxygen.

7) Measurement Facts (Short but Testable)

• DO is measured in mg/L

• Normal range: 0 to about 12 mg/L or higher

• Lowest readings usually happen: Early morning

1. Dissolved oxygen measures water quality

2. Warm water holds less oxygen

3. Photosynthesis increases oxygen

4. Decomposition decreases oxygen

5. Eutrophication leads to low oxygen and fish death