A garden pond is a water feature constructed in a garden or designed landscape, normally for aesthetic purposes, to provide wildlife habitat, or for swimming.
Garden ponds can be excellent wildlife habitats and may contribute to the protection of freshwater wildlife. Invertebrate and amphibian animals such as dragonflies and water beetles can colonize new ponds quickly. Garden pond owners have the potential to make many original and valuable observations about the ecology of small waterbodies, which garden ponds replicate.
There are several problems that can result from garden ponds. In particular, they can be pathways for the spread of invasive plants. In the UK, the invasive species Crassula helmsii and Myriophyllum aquaticum are both escaped from garden ponds. These plants cause considerable practical problems in protecting freshwaters.
Amphibious species such as frogs, toads, and newts are common in garden ponds, especially in overgrown areas with algae and reeds. Without proper precautions being taken by pond owners, such areas can harbour excessive frogspawn and may pose a challenge.
Cases of high rodent activity are also common in garden ponds, with field mice, rats, and voles taking refuge in freshwater areas such as garden ponds. Consuming pondwater contaminated with rat urine can cause infections such as Weil's disease or leptospirosis.
In tropical climates, garden ponds may become breeding sites for mosquitoes and increase the spread of diseases carried by these insects.
Ponds may be created by natural processes or by people; however, the origin of the hole in the ground makes little difference to the kind of wildlife that will be found in the pond. Much more important is whether the pond is polluted or clean, how close it is to other wetlands and its depth, particularly whether it dries out from time to time and how many fish (if any) there are.
Naturally, ponds vary more in their physical and chemical conditions from day to day than other freshwaters, like rivers. People often install pumps in garden ponds to counter these natural tendencies, particularly to maintain higher levels of dissolved oxygen. Although this is generally unnecessary for wildlife, it may be essential to keep fish in a garden pond. For ponds with polluted nutrient-rich tapwater added to them, filters can be used to reduce the abundance of algae.
Ponds outside of gardens are fed by four main water sources: rain, inflows (springs and streams), surface runoff, and groundwater. The wildlife value of ponds is greatly affected by the extent to which these water sources are unpolluted. Garden ponds are generally not fed by inflows or groundwater, except in the larger and rural gardens. Usually, the pond will be filled by a combination of tap water, rainwater, and surface runoff – and lost to evaporation.
In soils that lack natural clay, additional water loss to drainage and permeation is prevented by a liner. Pond liners are PVC or EPDM foils that are placed between the soil of the pond bed and the water. Liners can also be made from puddled clay, and ponds on free-draining soils can even be self-sealing with fine sediments washed into the pond.
Ponds may dry out if more water is lost to evaporation than is gained from other sources. This drying out is more common in Summer, when weather may become hotter and rainfall may decrease. These "temporary ponds" (which usually dry out once per year) are an important natural habitat worldwide, and many freshwater plant and animal species are well adapted to such periods of drought. While fish are less likely to survive the droughts common in temporary ponds, their tendency to eat tadpoles means that amphibians may benefit from water drying out. Provided that their larvae emerge before the pond dries out, the drought presents no problems for amphibians.
Ponds or swimming ponds can be constructed by an isolating membrane or membranes or (on an organic model sometimes called natural pools) contained by a layer of loam. For all variants no chemicals or devices disinfect or maintain water, the water is instead cleared through the use of biological filters, aquatic plants, or other organisms used in water purification (zooplankton).
The first such pools were built in the early 1980s in Austria, where they are known as "Schwimmteiche". The first was built by Werner Gamerith and Richard Weixler in Gamerith's private garden in the 1980s.: 16 The market slowly grew, and by 2016 there were around 20,000 such swimming ponds in Europe.: 19
The first public swimming pool in North America built and maintained in this way, was finished at Webber Park in Minneapolis in 2015.
An organization called International Organization for Natural Bathing Waters (IOB) sets standards for such pools.