Where do algae come from

Fight algae in the pond

Have you ever noticed a greenish shimmer in the water of your garden pond? These are microscopic green or blue algae. However, they do not interfere with the aesthetic impression of the pond system, because the water still remains clear. In addition, these algae are easy to keep at bay with water fleas. The tiny swimming crabs feed on them, so that a biological equilibrium is established over time. In contrast to real fleas, water fleas are completely harmless to humans and also welcome helpers for good water quality in swimming ponds. If green algae multiply too much, they are usually deposited on the surface of the water as a tough slime and can be removed relatively easily.

How can you fight thread algae in the pond?

Pond owners are particularly concerned about the larger thread algae. If they multiply rapidly, they cause the water in the pond to become completely cloudy. After this so-called algae bloom, the plants die and sink to the bottom of the pond. As a result of intensive decomposition processes, the oxygen concentration in the pond water sometimes drops so much that the fish suffocate and the water tips over.

Where do the algae come from?

There are various types of algae in every pond. As long as the nutrient concentration in the water is normal, they live in peaceful coexistence with other plants and fish. But if the phosphate content rises to over 0.035 milligrams per liter, their living conditions improve. If the water temperatures and solar radiation rise, they multiply explosively - so-called algae bloom occurs.

Phosphate and other nutrients get into the garden pond in various ways. The most common sources of phosphate are fish droppings and excess food, which sink to the bottom of the pond and are broken down into their components there. In addition, lawn fertilizers or nutrient-rich garden soil are often washed into the pond when it rains heavily. The leaves that enter the water in autumn also contain small amounts of phosphate and other nutrients that promote algae growth.

Fighting algae: Remove the nutrients from the garden pond

Not only do the algae need phosphate, nitrate and other nutrients to grow, but also the aquatic plants. The more plants live in your pond, the faster the nutrients are bound by the plant growth. In order to remove these from the nutrient cycle of the water, you have to prune the aquatic plants vigorously from time to time. You can then dispose of the clippings on the compost.

Regularly fishing the algae also reduces the nutrients in the pond. The algae can be composted excellently like the aquatic plants. You can also lower the phosphate content of the pond water with mineral binders (phosphate binders). The nutrients are bound by chemical processes so that they cannot be absorbed by either the algae or the plants.

You remove most of the nutrients from the water with a renovation. Remove the so-called sludge layer from fish droppings and rotten plants and replace the old pond soil with new, nutrient-poor substrate. All plants are cut back vigorously, divided and then placed in new, nutrient-poor pond soil or without substrate in special plant baskets or embankment mats.

Tips for the pond system

To ensure that the pond water always remains clear, you must eliminate all sources of phosphate. The course for this is already set when the pond is set up. The body of water looks most natural when it is in a depression - but this harbors the risk that garden soil and fertilizer can be washed into the pond. Therefore, it is better to choose a slightly elevated place or surround the water with a 60 centimeter deep drainage trench, which you fill with coarse-grained construction sand.

The lighting conditions do not affect the phosphate content of the pond water, but sunlight promotes algae growth. Therefore, choose a location that is at least one third in the shade. The amount of water and the depth of the water also play a role. Rule of thumb: The smaller and shallower the garden pond, the more common algae problems are.

Use low-nutrient sand as the pond soil, and use as little of it as possible. You should only use tested tap water as pond water, because many water suppliers enrich the drinking water with up to five milligrams of phosphate per liter in order to reduce corrosion in the pipes. The waterworks often publish their water analyzes on the Internet or send you the relevant documents on request. If the tap water contains too much phosphate, you should treat it with a phosphate binder. Groundwater is generally low in phosphate and therefore generally better suited. Rainwater is optimal because it is free of minerals. Very few hobby gardeners have the appropriate amount available.

Proper care for an algae-free pond

Even in clear garden ponds, nutrient-rich deposits form over time. You can remove these with a special pond sludge vacuum. In addition, it is best to cover smaller ponds with a net in autumn so that no leaves fall into the water. In order to remove floating foreign bodies such as pollen or the like from the pond surface, there are also so-called skimmers, which suck off the water on the surface and guide it into a filter system. Under certain conditions, pond mussels can also be used as natural water filters.

Problem case: fish in the garden pond

The excretions from fish, newts and other aquatic animals naturally also contain phosphate. That is not a problem as long as the animals have to live on what they find edible in the pond. However, if you regularly supply them with fish food, additional nutrients will enter the pond from outside. There are two ways to prevent a fish pond from tipping over: Either you use so few fish that you do not have to feed them, or you install a good filter system that removes algae and excess nutrients from the pond. Especially with large fish such as the magnificent Japanese Koi carp, you cannot do without powerful technology.

Practical video: Creating a mini pond for the garden and balcony

No space for a large pond in the garden? No problem! Whether in the garden, on the terrace or on the balcony - a mini pond is a great addition and creates a holiday flair on balconies. We'll show you how to put it on.

Mini ponds are a simple and flexible alternative to large garden ponds, especially for small gardens. In this video we will show you how to create a mini pond yourself.
Credits: Camera and Editing: Alexander Buggisch / Production: Dieke van Dieken