Pond Algae: Common Types, Causes & Effective Solutions

Last Updated on February 3, 2019

The most common problem that ponds experience surprisingly does not relate to fish, but to plants, – and unwanted ones at that, such as green water and blanketweed. You can take a little comfort in knowing that you are not alone if your pond becomes plagued with unwanted plant growth.

Ponds are usually afflicted with either green water or blanket weed (and sometimes both) at sometime throughout the season.

If you take early action, you can reduce the impact that unwanted algae, particularly green water, will have on your pond.

Types of Pond Algae

Green water, – is it really all that bad?

Green water offends the eye, creating a green blot on our aquatic landscape. It is regarded as the water gardening equivalent to leaving your garden to become over-run with nettles, dandelions and goose grass. It makes your wonderful creation look neglected and second-rate, reflecting badly on your abilities as a pond keeper. But take some solace in the fact that green water offers many benefits to your fish and pond life promoting improved health and colour. In fact, commercial breeders and farmers of ornamental pond fish such as goldfish and koi positively encourage green water to flourish in their ponds, recognising that fish benefit from living in such conditions.

Blanketweed what is it?

Blanketweed is a collective term given to a number of very similar algae that both look and behave identically. The most common genera are Cladophora and Spirogyra. Cladophora means branched plant and when viewed under the microscope, it is possible to see the regular-branding filaments, each of which is divided by cross walls. Absorption of light and nutrients is very efficient in such small structures and so growth can be incredibly rapid. They reproduce both sexually (releasing gametes that unite and develop into new plants), and asexually (releasing small motile spores or simply smaller fragments that break off from the main body).

We dont resent blanket weeds grip on our ponds because it poses a direct threat to the health of our pond fish, as in fact, it can actually lead to improved water conditions. When there is a thin, beardlike covering on areas of the pond, pond fish will browse and graze on the soft, lush growth. However, pond fish find it less appealing when the beard has grown into lengths of weed several feet in length (hence its other names such as hair or thread algae). Blanketweed will also provide an excellent nursery, both feeding and protecting developing pond fish fry.

Furthermore, blanketweed is beneficial to a pond in that it will very actively take up minerals and nutrients from the pond water (just like a vegetable filter), the only difference being that this one is in the pond! So vigorous is the growth and uptake of nutrients by blanketweed that should we find a way of confining it to a vegetable filter, it would be our number one plant choice. Unfortunately, like all other weeds, blanketweed does not know its right place and freely enters any garden pond, doing so at its own risk, as its presence is likely to be challenged.

What causes algae in ponds?

The 3 key factors that stimulate such rampant algae growth are:

1. Sunlight

Ponds in full sunlight are most likely to be affected. Light is required to fuel the process of photosynthesis, which allows these algae to produce organic molecules for new tissue growth. Growth is also particularly rampant in shallow areas where the water is exposed to intense sunlight.

2. Nutrients

Algae will readily absorb nitrates and phosphates to satisfy their need for nitrogen and phosphorous as they grow. These are readily available in tap water or indirectly through fish metabolism. Wherever nutrients abound, so will this opportunistic algae, being the first to capitalise on ideal growth conditions.

3. Warmth

A high temperature will accelerate algae growth considerably and blanketweed growth will be greatest in the shallower areas such as cascades and waterfalls and along the pond perimeter.

Preventing Algae + Blanketweed from taking over

Blanketweed and algae will find your pond. Many pond owners are fortunate in that, nuisance algae does not appear to proliferate in their ponds (and they cannot explain why). However, the vast majority of us will find that our ponds are very hospitable to algae or blanketweed, forcing us to look for ways of preventing it from causing problems.

Does pond algae die in winter?

Most ponds enjoy an unprecedented degree of clarity over the cold winter months. Such clarity makes it possible to see down to the bottom of even the deepest ponds and is a factor of the cold water temperatures and very short day lengths. Under these conditions, algae growth is retarded, and any algae that survived through last year is no longer able to thrive. However, as soon as the day lengths start to increase and the suns rays get hotter, pond water temperatures will also rise, creating ideal conditions for the primitive microscopic plant life that create green water.

The accumulation of nutrients in the water over winter (particularly nitrates and phosphates) fuels the growth of these remarkable, opportunistic plants. Because of their size and mode of reproduction, algae can soon multiply out of control, blooming from a few dormant cells or cysts into a pond of green soup, being the first plants to capitalise on these favourable conditions, dominating the ponds biology and appearance.

The transformation of smaller ponds is fastest, as these warm up a lot quicker than the larger, deeper ponds. The microscopic single celled algae become so dense that the clarity is soon lost, in a similar way to the density of water droplets can transform a fine mist into a dense fog.

Recognising that green water is an increasing probability at this time of year, and that we want to keep our ponds as clear as possible so we can see our fish (thats what we bought them for!), what can we do to prevent it from taking hold? Learning from tackling weed problems in the garden, if possible we need to find an equivalent to the methods of weed control, such as ground covering mulches, competitive planting or selective weed killers.

Getting rid of Algae in Pond

There are a multitude of algae control methods available for the stillwater pond other than draining. Each method comes with the added complication of whatever comes into contact with the algae, can also affect other pond life, including our desirable pond plants and fish. This is particularly true when considering chemical treatments. Following are different approaches for solving green water problems, each with their own benefits (and costs).

1. Shading

A significant number of pond keepers who have tried many different methods of controlling green water (and blanket weed) have found that shading the pond is very effective. Erecting a pergola, and stretching green house shading across the top can cut out a great deal of direct sunlight that causes algae to thrive. As it does not involve treating the water, the fish and most aquatic plants will continue to thrive. Shading can also be achieved within the pond by adding natural vegetable dyes to the pond water, cutting out the sunlight. They change the appearance of the pond water to dark blue, but offer a long term, effective solution to algae problems.

2. Chemical treatments

These include algicides that actively kill algae and blanket weed. These chemicals must be used correctly and accurately because overdosing can lead to desirable pond plants being affected. If your pond is largely free of plants, this does not present a problem.

3. Competitive Planting

The first of 3 greener or natural remedies involves using desirable pond plants to out-compete the algae for sunlight and nutrients. A pond can take several years to become fully balanced with desirable pond plants eventually winning the battle. This is the reason why natural, mature water bodies are rarely plagued with algae.

4. Barley Straw.

A second natural method of controlling green water is to add barley straw to the pond. The natural decomposition of barley straw causes the slow release of compounds that lead to the pond water becoming mildly toxic to primitive plants. This method can take several months to work as its effectiveness relies on the bacterial breakdown of the straw.

5. Bacteria.

A more recent solution to the green water problem is to add a supply of beneficial bacteria that breakdown the nutrients on which the algae thrive. With regular, weekly dosing, these bacteria will continue to digest and utilise the nutrients that would otherwise have fuelled algae growth.

6. Uvc. (For the recirculating pond.)

The final solution to green water problems is guaranteed to work by most manufacturers and is the only method of those discussed that requires a pond pump for it to work. The unit is called a Uvc (Ultra-Violet clarifier), and uses ultra violet light to control green water. A pond pump is required to pump the green pond water through the Uvc (which is positioned outside the pond), where it clumps together and is removed by the ponds filter. The running costs are minute compared to other treatments and as long as the pond is fitted with the correct sized UV, it will be completely effective against green water. Its bulb will need changing every 12 months, with springtime being the best time to install a new one, with the subsequent 4-5 months presenting it with its hardest work.

Depending on your ponds circumstances, you have a range of alternative methods for controlling green water to choose from. Even if you get caught out this spring with green water, a guaranteed method of controlling green water is available in a Uvc. However, never succumb to the temptation of substituting your green pond water with tap water as this nutrient-rich water will take you back to square-one, turning green again in a matter of weeks.

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