Pond eutrophication: causes, phosphates and solutions
35 µg/L of total phosphorus: start of eutrophication
100 µg/L of total phosphorus: hyper-eutrophication beyond this
× 0.33 to convert mg/L of PO₄ to mg/L of phosphorus
95 % apparent phosphorus retention in French pond heads
Eutrophication: what are we talking about?
CNRS researchers define it as an increase in primary production linked to an excessive input of nutrients. It is accompanied by an increase in respiration and decomposition, which can lead to a lack of oxygen in the water. In a pond, this results in green or murky water, filamentous algae, duckweed, thickening sludge, odors, and sometimes suffocated fish.
Phosphorus is often the nutrient that limits growth in freshwater, hence the attention paid to phosphates. Nitrogen also plays its part: in more than a quarter of the studies reviewed, biomass only responds to a combined input of nitrogen and phosphorus. Therefore, addressing both sources remains the safest approach.
Point 4 of the cycle is the one that is forgotten. In the oxidized layer of the sediment, phosphates are attached to iron oxides. When the bottom falls into anoxia, these oxides dissolve and release the phosphate into the water. For this to happen, the absence of oxygen is not enough: a reducing agent is also needed, such as sulfides or fresh organic matter. A bottom loaded with decomposed leaves and deprived of oxygen is exactly this scenario.
Where do the phosphates in your pond come from?
| Source | Why it is a problem | What to do |
|---|---|---|
| Leaves and plant debris | They decompose at the bottom and feed the sludge | Autumn net, regular collection |
| Fish feeding | Remains and excrement enrich the water | Give what the fish consume quickly |
| Animal excrement | Direct input of phosphorus (waterfowl, livestock with access to the water) | Fence off access for livestock, provide a separate watering trough |
| Runoff, drains, fertilizers | Input of phosphorus and nitrogen from the watershed | Grass buffer strip or buffer ditch, no fertilizer near the water |
| The sludge itself | Internal stock released when the bottom lacks oxygen | Aeration and bacteria |
| Filling water | Variable content depending on the source | Test before filling |
Phosphorus accumulates more than it is removed. In French pond heads, apparent retention reaches 95% (± 29) according to the work cited by the CNRS, especially in the sediment. Agricultural diffuse inputs there are estimated at around 0.31 kg of phosphorus per hectare per year: little per hectare, but a pond is a nutrient trap. We also explore this link in why duckweed keeps coming back.
Measuring: how is your pond doing?
Limnologists classify bodies of water according to their total phosphorus content. Tests sold for ponds measure phosphates (PO₄) rather, expressed in mg/L: the table below gives the equivalent.
| Class | Total phosphorus | PO₄ equivalent (approx.) |
|---|---|---|
| Oligotrophic | 4 to 10 µg/L | 0.01 to 0.03 mg/L |
| Mesotrophic | 10 to 20 µg/L | 0.03 to 0.06 mg/L |
| Meso-eutrophic | 20 to 35 µg/L | 0.06 to 0.11 mg/L |
| Eutrophic | 35 to 100 µg/L | 0.11 to 0.31 mg/L |
| Hyper-eutrophic | more than 100 µg/L | more than 0.31 mg/L |
Conversion: 1 mg/L of PO₄ contains about 0.33 mg/L of phosphorus. A drop test gives an order of magnitude: always compare measurements among themselves, at the same place and in the same season.
Our free water analysis helps you interpret your results, and the details of phosphates can be found in phosphates in the pond: combine SeDox Max and aeration.
The solutions, in the right order
1. Cut off inputs
This is the most sustainable and cheapest solution: leaf net, measured feeding, fenced livestock access, grass buffer between fields and water. Without this, everything else empties a bucket that is being refilled.
2. Oxygenate the bottom
An oxygenated bottom keeps its phosphorus trapped in the sediment, and organic matter degrades better there. A compressor and a diffuser are enough for many ponds: see our aeration kits, our diffusers and the article bottom aeration or surface oxygenator.
3. Digest organic matter
Once oxygen is present, bacteria work better. Here are the two Aquipond solutions we use on this point, with the dosages from their data sheets.
| Product | Role | Data sheet dosage |
|---|---|---|
| Bactovase (powder) | Bacteria that degrade the organic fraction of the sludge; does not replace the removal of mineral deposits | 200 g/m² (200 m² or less) or 150 g/m² (beyond that), 2 applications per year |
| Liquid Bactovase | Liquid bacteria, especially useful in autumn | 1 L for 20 m³ at startup, 1 L for 40 m³ for monthly maintenance; water between 6 and 25 °C |
| Boost & Clean | Active oxygen biological cleaner: loosens organic deposits, which rise | 25 g/m³ from 15 °C, at least 15 days between two applications |
Aquipond Boost & CleanLoosens organic deposits from the bottom; 25 g per m³from €39.95
Aquipond Bactovase 5 kgBacteria to reduce organic sludge, 25 to 50 m²from €149.95
Attention: with Boost & Clean, remove the materials that rise with a skimmer, otherwise they fall back to the bottom. Keep the filtration and aeration running. With Liquid Bactovase, turn off the UV lamps for the 24 hours following the application.
4. Make plants consume the nutrients
A belt of marsh plants and aquatic plants absorbs nutrients and competes with algae. To really export them, cut and remove the aerial parts regularly at the end of the season.
Regulated species: never plant species from the European Union list (EU regulation 1143/2014), for example water primrose, Brazilian waterweed, giant waterweed, cabomba or water hyacinth. If you spot any in your body of water, report it to the competent authorities in your region instead of moving them.
5. Fix the remaining phosphate
Finally, a phosphate fixer can accompany the process. OASE PhosLess Direct products fix the phosphate in the water and reduce algae growth, according to their data sheet. Use them as a supplement: if inputs or sludge continue to feed the water, the phosphate will return. To monitor the evolution, a Sera PO₄ drop test remains easy to use.
Sera Test PO₄Drop test to monitor phosphatesfrom €14.95
AquaActiv PhosLess Direct 5 LFixes the phosphate in the water, according to the product data sheetfrom €119.95
In most ponds, we find the same pattern: we treat green water, but the stock at the bottom continues to feed it. Our working order is therefore always the same: measure, cut off inputs, oxygenate, then digest. For a pond of a few dozen m³, a compressor and diffuser kit may be enough; professional sets are reserved for large bodies of water.
Want to know where your pond stands?
Our online water analysis is free. Free delivery from €149 (excluding bulky products).
Frequently Asked Questions
What is the eutrophication of a pond?
It is an excessive enrichment of the water in nutrients, especially phosphorus and nitrogen. It increases the production of algae and plants, then respiration and decomposition, which can lead to a lack of oxygen in the water.
Phosphates or nitrates: which one makes algae proliferate?
Phosphorus is often the nutrient that limits growth in freshwater, hence the attention paid to phosphates. Nitrogen can also limit: in more than a quarter of the studies analyzed by researchers, biomass only responds to a combined input of nitrogen and phosphorus. Therefore, addressing both sources remains the safest.
From what phosphate level is a pond eutrophic?
According to the classification of Environment and Climate Change Canada, a body of water is eutrophic between 35 and 100 µg/L of total phosphorus, and hyper-eutrophic beyond 100 µg/L. In phosphates (PO₄), this corresponds to about 0.11 and 0.31 mg/L. A drop test gives an order of magnitude, not a laboratory value.
Can we go backward?
Yes, but gradually. You must first reduce inputs, then empty the accumulated stock at the bottom, which often takes several seasons. No treatment replaces this fundamental work: without reducing inputs, the phosphate returns.
Do aquatic plants help?
Yes: as they grow, they absorb nutrients and compete with algae. To really export these nutrients, you must cut and remove the aerial parts regularly. Never plant species from the European Union list (EU regulation 1143/2014), such as water primrose or Brazilian waterweed.
Are bacteria enough without aeration?
No. The data sheets of our products recommend combining them with good aeration of the body of water to maximize bacterial activity. Without oxygen, organic matter degrades poorly and the bottom remains a nutrient reservoir.
Sources consulted: CNRS-INEE, collective expertise on eutrophication, chapter 5 · Environment and Climate Change Canada, Phosphorus in aquatic ecosystems · List of invasive alien species of concern for the European Union
To go further
- Natural pond: the guide for healthy water all year round
- How to treat sludge in a natural pond?
- Duckweed in a pond: eliminate it sustainably
The Foudebassin experts, experts since 1998
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