Oxygen Bottoms Out at 4 AM — Never Trust a Daytime DO Reading

You walk out in the morning and find dead shrimp — but yesterday's midday DO reading was 8 mg/L and looked perfectly fine. This is the single most common accident pattern in shrimp farming, and the cause is the midday reading itself. ■ How oxygen moves through the day Pond oxygen is not steady; it swings hard. During daylight, phytoplankton photosynthesize and produce oxygen, peaking around 2–4 PM. Measure then and you get 8 mg/L, sometimes supersaturation. After sunset photosynthesis stops, but the plankton, the shrimp and the microbes all keep respiring. For the whole night oxygen is only consumed, never produced. The result is the daily minimum just before sunrise, around 4–6 AM. A pond reading 8 mg/L at noon dropping to 2 mg/L at dawn is not unusual. ■ What makes it worse The denser the plankton bloom, the higher the daytime peak — and the deeper the night-time low, because a bloom that produces a lot of oxygen also consumes a lot after dark. Deep green water is not reassurance; it is a reason to watch the dawn more closely. Higher water temperature reduces how much oxygen the water can hold at all. Summer nights combine high consumption with low solubility — the worst pairing. High stocking density, a heavy feeding day, cloudy or rainy weather (weak photosynthesis) and a sudden plankton crash are all additional risk factors. ■ How to prepare First, move your measurement to dawn. If you only measure once a day, that once must be at 4–6 AM. A midday value tells you almost nothing useful for management. Second, run aerators from sunset through to after sunrise. The principle is to run them through the window when oxygen will fall, not to switch them on after it has already fallen. Third, read the shrimp. If at dawn they gather near the surface or along the pond edge, oxygen is already short. At that point run aerators at maximum and stop feeding immediately — digestion consumes oxygen too. Fourth, cut feed in advance when cloudy weather lasts more than two days. With photosynthesis weak, leftover feed consumes still more oxygen as it breaks down. ■ Records are what make prediction possible Log dawn DO at the same time every day and you will see the minimum drifting down over several days. A run of 6.0 → 5.6 → 5.2 is still within range, but the next dawn is likely to reach the danger zone. Mortality looks sudden, but in ponds with records there was almost always a warning. Shrimp365 stores your dawn readings per tank, plots the trend, and alerts you the moment a value drops below range.