The Pea-Soup Pond: Ammonia Toxicity Under a Dense Bloom
A 0.8 ha pond in Nadia district, average depth 1.6 m, has been cropped for four years without being dried. The farmer stocks 8,000 rohu and catla fingerlings averaging 25 g in July and feeds a home-mixed mash of rice bran and mustard oilcake at about 5% of body weight. By late August the water is pea-soup green with a surface scum that smells faintly of paint, and Secchi depth is 15 cm. At 06:00 the readings are DO 3.1 mg/L, pH 7.4, temperature 29 °C; at 14:30 they are pH 9.5 and DO 11 mg/L. Total ammonia nitrogen is 2.0 mg/L. The bottom sludge is 45 cm deep, black, and smells of sulphide. Fish gasp at dawn and refuse feed by mid-morning. The farmer now proposes to broadcast 400 kg of raw cow dung plus 30 kg of urea to 'keep the water green', and 250 kg of agricultural lime because a neighbour limed his pond last week. He wants to know whether to do this before or after the fish stop gasping.
1. Interpret the water quality data. What is happening between dawn and afternoon, and how dangerous is the 2.0 mg/L total ammonia figure?
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The pond is dominated by a dense cyanobacterial bloom (paint-like smell, Secchi 15 cm, very wide pH and DO swing). Two processes matter.
Oxygen: DO falls to 3.1 mg/L at dawn because the bloom respires all night; during the day photosynthesis pushes it to 11 mg/L. The dawn value is already below the comfortable 5 mg/L, and the sludge adds a continuous oxygen demand, so any cloudy, still night can cause a kill.
Ammonia: only un-ionised NH3 is acutely toxic, and its share depends on pH and temperature. Using the fraction 1/(1+10^(pKa−pH)) with pKa ≈ 9.1 at 29 °C, at pH 9.5 roughly 0.6–0.7 of the total ammonia is un-ionised. So 2.0 mg/L TAN means about 1.2–1.4 mg/L NH3, against a safe ceiling of about 0.05 mg/L for carp ponds and a lethal level around 0.5 mg/L. The fish are therefore in a chronic-to-acute ammonia crisis that peaks in the afternoon, exactly when pH is highest. At dawn, when pH is 7.4, the toxic fraction is only a few per cent, which is why the early-morning gasping is mainly an oxygen symptom and the afternoon distress is ammonia.
The black, sulphide-smelling 45 cm sludge is the source of the nitrogen and the oxygen demand; four years without drying means the pond has no chance to mineralise it aerobically.
2. Evaluate the farmer's plan of cow dung, urea and 250 kg lime. Would you approve it, modify it or refuse it, and why?
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Refuse the plan as written, for three separate reasons.
Raw cow dung and urea are adding to the problem, not solving it. The bloom is already excessive; more nutrients will thicken it, deepen the night-time oxygen crash and generate more ammonia. Raw dung also carries its own ammonia and a large oxygen demand as it decays on an already overloaded bottom. Fertilisation should stop until the bloom is brought under control and ammonia is below about 0.5 mg/L.
Lime at high pH is dangerous. Agricultural lime raises pH, and at pH 9.5 an increase of even a few tenths of a unit sharply increases the un-ionised NH3 fraction and converts a serious ammonia level into a lethal one. Liming is a soil-correction practice for acidic ponds (typically 200–300 kg/ha when soil pH is below about 6.5), applied to an empty or freshly filled pond, not a mid-season rescue measure in an alkaline, bloom-heavy pond. With a two-year-old sludge layer the right sequence is drain, dry, remove sludge, test soil, and lime then if the test justifies it.
Volume and timing are wrong. 250 kg on 0.8 ha is 312 kg/ha, in the normal liming range in quantity but not in timing, and 400 kg of dung plus 30 kg urea in August is a heavy dose for a pond already carrying 2.0 mg/L TAN.
3. Set out a 72-hour action plan and the medium-term correction. Include the intermediate step that must be checked before any lime is ever applied.
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First 72 hours
- Stop all fertilisation and cut feeding to zero, or at most half ration, until DO at dawn is above 3 mg/L and TAN is falling.
- Run any available aerator or lift pump through the night and early morning; if none exists, flush with 10–15% fresh water and use a splash board or pump to break the surface.
- Do not apply copper sulphate or other algicides to 'clear' the bloom: a dying bloom collapses oxygen overnight and releases ammonia and sulphide.
- Physically remove surface scum with a net or by skimming at the outlet, and optionally harvest part of the stock if the pond has a marketable grade.
- Check pH and ammonia each afternoon and DO each dawn; keep a written log.
Medium term (end of the cycle)
- Drain, sun-dry until the bottom cracks, excavate the accumulated sludge and use it on the bunds or as field manure.
- Take a soil pH and organic carbon test. Lime at 200–300 kg/ha only if soil pH is below about 6.5, and apply it to the dry bottom before filling. This soil test is the check that must precede every future lime decision.
- From the next cycle, fertilise on a schedule driven by Secchi depth (keep it at 25–35 cm), use pond-tested organic manure in split doses rather than raw dung in bulk, and keep a night aeration option for the August–September period.