Have you ever noticed a greenish glow in your garden pond? These are microscopically small algae. However, they do not interfere with the aesthetic impression of the pond, as the water remains clear. Also, these algae are easily contained by water fleas. The tiny floating crayfish feed on them so that a biological equilibrium develops over time. In contrast to real fleas, water fleas are completely harmless for humans and are also welcome helpers in the swimming pond for good water quality.
If green algae reproduce too much, they will first deposit on the water surface as viscous mucus and are relatively easy to remove.
Algae are not the same as algae
Pond owners are particularly concerned about the more abundant thread algae. They cause the water to become utterly cloudy in case of a substantial increase. After this so-called algal bloom, the plants die and sink to the bottom of the pond. Due to intensive decomposition processes, the oxygen concentration in the pond water sometimes decreases to such an extent that the fish suffocate and the pH-number shifts.
Where do the algae come from?
In each pond, there are different types of algae. As long as the nutrient concentration in the water is balanced, they live in peaceful coexistence with other plants and fish. However, if the phosphate content rises to more than 0.032 milligrams per liter, their living conditions improve. If the water temperature and solar radiation rise, they multiply explosively – resulting in the so-called algae bloom.
Phosphate and other nutrients enter the garden pond in various ways. The most common sources of phosphate are fish droppings and excess food, which sink onto the bottom of the water body and are broken down into its components. In case of heavy rainfall, lawn fertilizers or nutrient-rich garden soil are often rinsed into the pond. The foliage that enters the water in autumn also contains small amounts of phosphate and other nutrients that promote algae growth.
Extracting nutrients from the pond water
Not only do algae need phosphate, nitrate and other nutrients to grow, but also the aquatic plants. Regular fishing of algae also reduces the nutrients in the pond. The algae, like the aquatic plants, can be easily composted. You can also reduce the phosphate content of the pond water with mineral binders. The nutrients are bound by chemical processes so that neither the algae nor the plants can absorb them.
Most nutrients are removed from the water body by renovation. Remove the so-called mulch layer from fish droppings and rotted plants and replace the old pond soil with new, nutrient-poor substrate. All plants are pruned back strongly, divided and then placed into new, nutrient-poor pond soil or without substrate into special plant baskets or embankments.
Tips for the pond system
To keep the pond water clear, you must eliminate all phosphate sources. The course for this will be set when the pond is laid out. The water is most natural when it lies in a depression – but this carries 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 body with a 60 cm thick drainage ditch, which you fill with coarse-grained building sand.
The light conditions do not affect the phosphate content of the pond water, but sunlight does promote algae growth. Therefore, select a location that is at least one-third shaded. Water volume and water depth also play a role. Rule of thumb: The smaller and flatter the water body, the more common are algae problems.
Use nutrient-poor sand as pond soil, and as little of it as possible. As pond water, you should only use tested tap water, because many water suppliers enrich the drinking water with up to five milligrams of phosphate per liter to reduce corrosion in the pipes. Waterworks often publish their water analyses on the Internet or send you the relevant documents on request. If the tap water contains too much phosphate, you should treat it with phosphate binders. Groundwater is usually low in phosphate and therefore generally more suitable. Rainwater is ideal because it is free of minerals.
Correct care for an algae-free garden pond
Nutrient-rich deposits also form in clear garden ponds over time. These can be removed with a special pond suction cleaner. Also, you should cover smaller ponds with a net in autumn so that no leaves fall into the water. To remove floating foreign bodies such as pollen or similar outside materials from the pond surface, you can also use so-called skimmers that suck the water off the surface and channel it into a filter system.
Problem case fish pond
The excretions of fish and other aquatic animals naturally also contain phosphate. It’s no big deal, as long as the animals have to live off what they find in the pond. However, if you feed them regularly with fish food, additional nutrients enter the pond from the outside. There are two ways to prevent a fish pond from tipping over: Either you use so few fish that you don’t have to feed them, or you install a good filter system that removes algae and excess nutrients from the pond.