What's presented here is a very elementary account, but it is really useful to understand some aspects of pond ecology if you are going to manage your garden pond for wildlife. If you want to learn more we recommend The Pond Book2
by the Freshwater Habitats Trust
and an old but still very useful article by the same team in British Wildlife3
. A very academic and complete account is available from two Swedish scientists "The Biology of Lakes and Ponds"4
Freshwater habitats generally
Freshwater habitats are often clased in two categories. "Lentic" is where the water is essentially still, including ponds, lakes and wetlands. The other category is "lotic" where the water is moving, as in streams and rivers. Some habitats such as ditches can be a bit of both at different times of water flow. We can also note the brackish water habitats of estuary rivers and wetlands, but these special sites aren't relevant to gardens.
Most garden ponds, and many countryside ponds too, are essentially closed ecosystems. They do recieve inputs of material from outside such as leaves and rainfall, and they do export adults of creatures which have larval stages in them, but generally what goes into a pond stays there, be it nutrients or debris, unless someone fishes it out. This makes them more sensitive to pollutants such as excess nutrients than do most other garden habitats.
Certain physical factors are important for ponds and the creature that live in them.
Clean water is an essential component of a healthy wildlife pond. The best water to fill a pond is rainwater. Avoid diverting water from a ditch or stream, because at some times of year they may well be contaminated with pollutants, fertiliser or silt - and you would need a license to extract water. Tap water and run-off often contain nutrients in which make your pond more susceptible to algal blooms. Tap water also contains chlorine, although it naturally degrade over a few days. If at all possible, prevent the run-off of water into the pond that has passed through fertilised soil. This important topic is explained further in our leaflet by Ian Thornhill Clean water is key for wildlife ponds.
Oxygen and temperature
These can be considered together, because the warmer water becomes, the less oxygen it can hold, and so the more stress animals can experience. At 5°C water holds 9.1 ml
oxygen per litre, by 30°C it has fallen to 5.9ml/l. In hot weather you can often see carp gasping an air and water mix at the pond surface, and if the water gets too hot, fish can be killed by oxygen lack. However, most invertebrate animals living in lentic (pond) conditions are adapted to low and fluctuating oxygen levels, so this isn't quite the problem some people assume.
Temperature of course depends on sunlight, so shaded ponds heat less than sunlit ponds, while shallow ponds heat up faster than deeper ones. An ideal wildlife pond could have shaded and sunlit areas, and a variety of (shallowish) depths, so creatures can move to the conditions that suit them.
Oxygen levels are also affected on a daily level by photosynthesis in the day driving up levels to saturation, and respiration by plants and animals at night bringing it back down. Windy days cause ripples which help to oxygenate the water, while decay of dead material at the bottom of a deeper pond can reduce local oxygen levels to zero. In the winter, ice and snow can build up on the top of the pond which acts as a duvet preventing further chilling - deep water stays at 4°C - but also prevents oxygen exchange at the surface, so deeper water over organic detritus can get rather anoxic, occasionally killing overwintering creature like frogs. However, as Jeremy Biggs recently discovered
, in a clean shallow pond ice can actually raise oxygen levels, by limiting loss
of oxygen without preventing photosynthesis!
In ponds with fish, we are often recomended to create "breathing holes" in ice, by floating a large ball, or putting a hot saucepan of water on the ice to melt through. In a hard winter these are at best very temporary solutions, and especially if you haven't got fish, there is probably little or no benefit.
There are different parts of the pond with different creatures using them
Surface film habitat. Water has very high surface tension, making it possible for small creature like pond skaters to walk on the surface film where they feed on plants and debris. Conversely, the film is very difficult for small aquatic creatures to break through, so snails can hang upside down from surface film feeding on material accumulating just below he surface. Gnat and mosquito larvae use the film to support them while they breathe air through snorkels on their bottoms.