Un-ionised ammonia toxicity, Henderson-Hasselbalch calculation, EA salmonid and cyprinid standards, and ammonia management protocols for managed fisheries.
Species-specific DO requirements, Weiss equation, overnight DO sag calculation and aeration sizing for salmonid and coarse fisheries.
Emergency aeration protocols for acute fishery DO events — paddlewheel deployment, liquid oxygen injection, EA notification and post-event review.
DO management, ammonia control and emergency aeration for fishery and aquaculture waterbodies — species requirements, EA standards and emergency response.
Phosphorus management for eutrophic lake restoration — Vollenweider model, Phoslock, alum dosing, hypolimnetic oxygenation, internal loading, WFD compliance.
Ammonia in a fishery waterbody exists in two forms: ammonium ion (NH₄¹, non-toxic at normal concentrations) and un-ionised ammonia (NH₃, UIA, toxic). The Henderson–Hasselbalch equation governs their equilibrium: the fraction of TAN present as UIA (f_UIA) = 1 / (1 + 10^(pKa−pH)) where pKa = 0.09018 + 2729.92/T(K). Because pKa decreases with increasing temperature, both warming and pH increase shift the equilibrium toward the toxic UIA form. This means that an algal bloom — which drives afternoon pH above 9.0 — can convert a harmless TAN concentration into an acutely toxic UIA level in the same afternoon that the overnight DO crash occurs.
EA water quality standards for ammonia in salmonid waters: UIA < 0.021 mg/L (annual 95th percentile); UIA < 0.025 mg/L (annual maximum). For cyprinid waters: UIA < 0.021 mg/L (95th percentile); UIA < 0.050 mg/L (maximum). These standards apply to rivers under the WFD but are used as guidance targets for managed fisheries under Environmental Permits. Exceeding them constitutes a reportable water quality incident to the EA.
UIA calculation example: TAN = 3 mg/L; pH = 8.5; T = 22 °C (295K). pKa = 0.09018 + 2729.92/295 = 9.35. f_UIA = 1 / (1 + 10^(9.35−8.5)) = 1 / (1 + 10^0.85) = 1 / (1 + 7.08) = 0.124. UIA = 3 × 0.124 = 0.37 mg/L — 17× the EA salmonid standard. These conditions arise routinely in warm eutrophic fishery ponds during summer bloom events.
| pH | UIA fraction at 15°C (%) | UIA fraction at 20°C (%) | UIA fraction at 25°C (%) | TAN producing 0.021 mg/L UIA at 20°C (mg/L) |
|---|---|---|---|---|
| 7.0 | 0.6% | 1.0% | 1.7% | 2.1 |
| 7.5 | 1.8% | 3.1% | 5.3% | 0.68 |
| 8.0 | 5.6% | 9.3% | 15% | 0.23 |
| 8.5 | 16% | 25% | 37% | 0.084 |
| 9.0 | 38% | 53% | 67% | 0.040 |
Deploy online pH probe co-located with DO probe. Collect TAN (total ammonia nitrogen) samples weekly during summer (May–September) using Nessler or phenate colorimetric method. Calculate UIA for each sample using measured pH and temperature. Alert threshold: UIA > 0.015 mg/L (approaching EA salmonid standard).
In most fishery ponds: fish excretion (0.03–0.05 g TAN/kg fish/day) and uneaten feed decomposition are the dominant sources. In rivers and lakes with external inputs: agricultural runoff, sewage effluent discharge, or septic tank seepage. Distinguish internal (stocking-density-driven) from external (catchment-driven) sources before selecting management.
Destratification and surface aeration prevent the afternoon pH peaks (> 9.0) associated with dense algal blooms. By keeping Chl-a < 20 µg/L through effective spring mixing, the pH-driven conversion of TAN to UIA is greatly reduced. Aeration does not remove ammonia but prevents the pH conditions that make it toxic.
If stocking density is the primary TAN source: reduce fish loading by > 30% or implement partial water exchange (5–10%/day). Feed at or below 1.5% body weight/day; use high-protein, low-waste feed (FCR < 1.2 for trout). Remove uneaten feed within 2 hours. Sludge removal from ponds every 2–3 years reduces sediment ammonia flux.
If UIA approaches the EA standard during a bloom event: initiate partial water exchange (20–30% volume). Replace with cooler, lower-pH, lower-TAN water if available. Simultaneously increase aeration to maintain DO > 6 mg/L (hypoxia reduces fish ammonia excretion tolerance further). Reduce or suspend feeding immediately.
Any UIA event causing fish stress or mortality must be reported to the EA emergency line (0800 807060) and documented. Submit post-event report within 14 days: TAN/UIA time series, pH records, temperature, fish response observed, corrective actions taken. Review stocking density and feeding management annually to prevent recurrence.
DO management — hypoxia and ammonia toxicity frequently co-occur during bloom events in fishery waterbodies.
Read MoreWhen UIA reaches toxic levels, emergency aeration, water exchange, and EA notification are the immediate responses.
Read MoreReducing P reduces algal blooms which drive the pH swings that convert TAN to toxic UIA.
Read MoreOverview of all fishery waterbody management topics: DO, ammonia, and emergency aeration.
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