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Phosphate problems
Do you have persistent problems with blanketweed or duckweed in your pond? Phosphate (a form of phosphorus) may be the problem.
What is Phosphorus/Phosphate?
Phosphorus is an essential element for life. Usually it is found in the form of phosphate (bound with oxygen and other minerals). It forms a major part of animal bones (calcium phosphate).
High levels of phosphate in ponds act as a potent nutrient for green water and blanketweed, and can also cause nuisance levels of duckweed. copyright www.watergardensolutions.co.uk
You may have heard of phosphate as one of the main ingredients of garden fertilisers (“NPK” - Nitrogen, Phosphorus & Potassium). In balanced ponds and lakes there are enough of these nutrients to support moderate growth of waterplants. However, if there are high levels of these nutrients (especially phosphates) then problems with algae and blanketweed are more likely.
Where do phosphates in my pond come from?
- the soil in planting containers, can be a source. Commercial aquatic soils do not contain high amounts of phosphates (unlike standard potting compost), but most soils unavoidably contain some.
- aquatic fertilisers. These are generally only needed for waterlilies to help them to flower better. Such specialist fertilisers usually incorporate slow release phosphate, applied in pellets close to the roots, to limit any leaching into the pondwater, but some may still get into the water.
- tap water. Phosphates are not only found in the rivers and aquifers used for water supplies, but in the past thirty years, phosphates have been deliberately added to tap water to reduce pipe corrosion**.
- fish food. This contains some phosphorus as it is an essential part of the fish diet. A large proportion will pass through the fish and be released in the fish waste. This waste releases phosphates into the water, and also sinks to the pool base, storing phosphates within the sediments.
- run-off from lawns, borders, and surrounding paving. This run-off can include phosphates, especially if the garden has had fertilisers applied, or if there are many bird droppings on the paving.
What can I do to prevent high levels of phosphates in my pond?
Top up with rainwater, rather than tapwater. Rainwater has little to no phosphate in it. It is generally safe to use directly, especially in e.g. countryside with clean air. In regions with lots of air pollution the rainwater may need to be filtered to remove contaminants. Some rainwater downpipe collectors incorporate a strainer to keep out larger debris like leaves. More dubious rainwater can be routed via a storage tank, where further debris will settle out. The rainwater can then be passed through filter carbon (a ‘low-phosphate’ type) before it reaches the pond. Following a long dry spell, water collected from roofs can initially be directed elsewhere, so that dust on the roof isn’t washed straight into the pond.
Rainwater is considered as ‘soft’ - it is missing the mineral salts that make water ‘hard’. If your tap water is particularly hard, you will notice more scale in your kettle and on your taps. Some hardness minerals are essential for healthy fish and for maintaining active biological filters. For this reason, especially in ponds with higher stocks of fish, you should not rely 100% on rainwater for topping up. Instead, either:
- top up occasionally with a typical tap water that contains some hardness (use an appropriate dechlorinating treatment),
- or add a commercial “KH/GH buffer” suited to ponds, to make up for any depletion of the essential carbonate hardness (KH) and general hardness (GH) minerals in the pond. Test kits are available to allow you to simply check if your pond has sufficient levels of these two types of hardness.
Do not overfeed fish. Use good quality fish foods. Only feed higher-protein ‘growth’ formulas during the summer months, when they can be properly digested by the fish. Some fish foods list the phosphorus levels on the label - ideally foods should have 1% or a bit less. Poorly formulated foods can contain more than double this amount.
Prevent run-off by designing the pool edges so that rain does not tend to wash debris into the pond.
Keep out waterfowl. Do not feed waterfowl in or beside the pond.
Have sufficient pond plants. As well as being decorative and encouraging wildlife, pond plants such as waterlilies, submerged plants, floating plants, irises and other marginal plants, all help to consume the phosphates in the water, and in the sediments, helping the pond to reach a better balance. Remove old and damaged leaves from plants as the season continues.
Clear dirt from filter systems. Regularly clean out the captured debris from any filter system. Maintaining a filter in this way can remove those phosphates that have become bound to fine particles in the water. If debris is cleaned out regularly, it can halve the phosphate levels in the pond.
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