Dissolved Oxygen, Stratification and Algae in Lagoons
The diurnal swing, and why the time you sample matters
In a facultative pond, algae photosynthesise in daylight and release oxygen, and they respire
around the clock and consume it. The result is a daily cycle:
- Oxygen accumulates through the day, peaking in mid to late afternoon, sometimes to
supersaturation.
- It falls overnight as respiration continues with no photosynthesis, reaching its **minimum
around dawn**.
An operator who always reads dissolved oxygen at 2 p.m. will record healthy values from a pond
that goes anoxic every night. The dawn reading is the one that tells you whether the pond is
in trouble. If you can only take one reading, take the early one, and record the time with
every reading so the record is interpretable.
pH swings with the same cycle
Photosynthesis consumes carbon dioxide, whic