Reading Pond Water Colour — Sudden Clearing Is a Warning

When cloudy water clears over a few days it is tempting to read that as an improvement. In a shrimp pond it usually means the opposite. Sudden clearing is the signal that the algal bloom has collapsed, and oxygen falls that same night. ■ Plankton is equipment, not decoration Phytoplankton makes oxygen through photosynthesis during the day. At the same time it takes up ammonia as a nutrient. You have an oxygen generator and a water treatment unit floating in the pond. So the amount of plankton is the state of the pond. Too little is a problem and too much is a problem. Secchi depth is how you measure that every day. ■ Secchi depth — 30 to 40 cm Lower a Secchi disk slowly and note the depth where it disappears. A white disk is enough; without one, the depth at which your palm vanishes gives you a rough read. 30 to 40 cm is the working range. At that density plankton produces oxygen without consuming too much of it overnight. Measure at the same time and the same spot each day. The difference between yesterday and today tells you more than the absolute number. ■ Below 25 cm — too much Below 25 cm you have far too much plankton. The danger there is not daytime but night. More plankton means more oxygen burned through respiration after dark. Ponds that read supersaturated in the afternoon and 2 mg/L before dawn are usually in this state. A daily pH swing above 0.5 is the same signal. If you confirm an overbloom, cut feed and consider a water change. Plankton keeps growing as long as nutrients keep arriving. ■ Sudden clearing — that is a crash This is the most dangerous signal. If yesterday read 30 cm and today reads 60 cm, plankton has died off in bulk. Two things arrive together. The oxygen producer is gone, and the dead cells consume more oxygen as they break down. Supply stops while demand rises, which makes that night the most dangerous of the cycle. If you confirm a crash, run aerators at maximum and stop feeding immediately. And be at the pond before dawn that night. ■ Reading the colour Light green and light brown are good. Bright, lively colour means the cells are young and growing. Deep blue-green means the balance has tipped toward cyanobacteria. They produce oxygen poorly, consume it at night, and collapse all at once when they go. A grey cast means the plankton has aged and lost vigour, or that uneaten feed and bottom mud are suspended. Once colour starts to dull, treat it as a crash warning. ■ Records make prediction possible Log Secchi depth daily and the trend before a crash becomes visible: 38, then 34, then 30, then suddenly 50. Catch that inflection and you can prevent a pre-dawn kill. Best of all is Secchi depth and pre-dawn dissolved oxygen written side by side on the same date. Shrimp365 logs clarity and dissolved oxygen per tank and alerts you the moment a value leaves its range. Free to use.