Ponds full of life

Habitats, 21 April 2019
Lacs de Bastan, Néouvielle

A pond is a small lake. It's a small, shallow body of water. The water is still and fresh. There are natural ponds everywhere, from mountain tops to the desert oasis. They're most common in wetland areas such as river deltas, and on tundra in summer.

A pond is an excellent thing to have in a park or garden, especially if you're a naturalist or a photographer.

In summer it will have interesting plants (colourful plants like the marsh marigold, Caltha palustris, and tall, elegant plants like the yellow flag, Iris pseudacorus). The plants, algae and water attract dragonflies, frogs and birds.

Even a very small pond makes a garden much more interesting, and more wildlife-friendly.



Some definitions

A variety of ponds

So, a pond is a small, shallow area of still, fresh water:

How small? A pond is much bigger than the puddles you see after rain, but it's smaller than a lake. A pond could be just 2 metres x 3 metres like the one in my mother's garden. If it's much bigger than 100 metres x 100 metres (one hectare), it's probably a lake, but there's no definite rule.

How deep? Ponds are shallow, which is the opposite of deep. Most ponds are between 1 metre and 3 metres deep, which means that sunlight can always get to the bottom, and plants can grow everywhere underwater.

The water in a pond is still. When we say that a pond is "still water" we mean there's no flow, there's no current. In other words, the water doesn't go from one place to another. The bottom of a pond is a very quiet place, which is perfect for a lot of small animals to live and lay eggs.

Ponds are also "still" because they're too small to have any real waves. If a body of water is big enough to have big waves when there's a strong wind, it's a lake. A lake will have different vegetation and animals round the edge, because of the waves. Even delicate, fragile plants can live around the edge of a pond.

The water in a pond is fresh water, which is water without salt. The adjective is "freshwater", so you find freshwater life in fresh water. Sea water is about 3.5% salt. Between fresh water and sea water there is brackish at between 0.5% and 3% salt. A small, still body of sea water isn't a pond. It's a lagoon.

A pond is usually man-made, but it could be natural. Some natural ponds are made by beavers (Castor fiber and Castor canadensis). Other ponds just happen, because of the way the landscape is changed by erosion and deposition, rivers and glaciers.

A pond isn't a sterile place like a swimming pool; it has mud in the bottom, lots of plants in the water and around the edge, and lots of animal life.

A pond is a very sheltered environment in many ways, but it's not perfect. Because it's only a small volume of water, it will easily get covered in ice in cold weather. In a really cold winter, a shallow pond may freeze all the way to the bottom. In a hot summer, the water in a shallow pond can get too hot for a lot of animals unless there is some shade from trees or buildings. Shade is protection from the sun. Some natural ponds dry up completely in summer. The biggest problem for pond life, though, is oxygen. We'll come back to this.

(*In America, a pond can be very big, it's usually natural, and it can even have salt water. The island of Martha's Vineyard, in the Atlantic off the US state of Massachusetts, has 27 saltwater "ponds".)



Ponds and oxygen

In a very small pond, or a pond on a fish-farm, lack of oxygen can be a real problem.

In a sense, water is full of oxygen. H2O means that every water molecule is two hydrogen atoms bonded to one oxygen atom. However, it's a strong bond. You can't burn that hydrogen, and you can't breathe that oxygen. The oxygen a fish breathes is dissolved oxygen. It's free oxygen from the air that is temporarily dissolved in water, in the same way that you can dissolve sugar into your coffee. The Earth's atmosphere is about 21% oxygen, so that's where dissolved oxygen comes from. The process of putting free oxygen into water is called aeration.

If water has 10 mg/L or 10 ppm (parts per million) of oxygen, that's fine for life of all kinds. The water is highly oxygenated. Some fish that live in mountain rivers need at least 8 mg/L or 8 ppm when they're young. Some adult river fish need at least 6 mg/L. In water with less than 5 mg/L most fish start to have health problems and many die. The water is poorly oxygenated. If the oxygen level goes below 2 mg/L most things will die.


"[Photosynthesis] happens swiftly, in silence, at the temperature and pressure of the atmosphere, and gratis. Dear colleagues, when we learn to do likewise we will be sicut Deus, and we will have also solved the problem of hunger in the world." - Primo Levi, The Periodic Table

How does a pond get aerated?

  • At the surface of the water. Anywhere water touches air, oxygen dissolves into the water.
  • From a stream (a very small river). Water that flows usually has more oxygen than still water.
  • From rainwater. A raindrop has a big surface area for a small volume of water, and it absorbs a lot of oxygen.
  • From water plants. Photosynthesis is the process by which green plants and algae use sunlight to turn carbon dioxide and water into sugar and oxygen. It's very clever; human scientists can't do it. So, green plants and algae produce oxygen to aerate your pond, but only when the sun is shining.
  • By technology. Freshwater fish farms often have to add oxygen to the water. They can do this by pumping air bubbles into the water, or with fountains, or by hitting the water with a mechanical device.


How does a pond lose its oxygen?

Fish gasping for air

Animals use oxygen. If you put a lot of fish into a small pond, they may die from lack of oxygen.

Water plants use oxygen. Yes, green plants and algae also need oxygen all the time, just like us. The difference is that when the sun is shining, a green plant produces more oxygen than it uses. At night, plants and algae don't produce oxygen but they still take oxygen from the water.

Warm water holds less oxygen. Cold water like the North Atlantic or the Antarctic can hold a lot of oxygen. At sea level, water at 4°C can hold 10.92 mg/L of oxygen, but at 21°C it can only hold 8.68 mg/L. If the water in a pond gets too warm, it will make oxygen problems worse. Also, algae grow more strongly in warm water.

Decomposition (or decay or rot) uses oxygen. In spring, the leaves on a tree are fresh green; in summer they're darker green; in autumn, they turn brown and die; in winter they turn black as they decay. A lot of dead leaves fall into ponds. Bacteria do a lot of the work of decomposition, and most bacteria need oxygen.

Eutrophication. This literally means "good food supply", but it's a problem for life in rivers, lakes and ponds. When there are too many nutrients in the water, plants grow too fast and microscopic algae turn the water into thick green soup, with very low oxygen levels at night. That's why it's important to keep chemical fertilisers and dead leaves out of ponds. Eutrophication can happen to an entire lake or river when it is polluted by chemical fertilisers from farms or organic waste from cities.



A very small pond

Ornamental basin and a small pond

This stone bowl at Hatfield House, near London, is elegant but it's too small to be a pond.

A pond only 2 metres by 3 metres can be full of interesting small animals. In Britain, teachers who work with children call these invertebrates "minibeasts". They live in a world of miniature forests, where a raindrop may be bigger than they are.

My mother's garden pond is small and simple. In summer it always has not only minibeasts like pond skaters (Gerris lacustris) and water boatmen (Notonecta glauca) but also smooth newts (Lissotriton vulgaris), darter dragonflies (Libellulidae) and damselflies (Zygoptera). It also gets visits from wild ducks (the mallard, Anas platyrhynchos), a heron (Ardea cinerea), a grass snake (Natrix natrix), frogs (Rana temporaria) and toads (Bufo bufo).

It's in direct sun for part of the day, which is not good. However, there are trees that give it some shade for most of the day. Also, the tall green plants (the yellow flag, Iris pseudacorus) get taller in summer, and give it some shade.

My mother sometimes has to add water in a hot summer, and she has to take some water plants out of the pond once or twice every year. We put a net over the pond in autumn, to stop dead leaves. That's all the maintenance it needs.

There are ramshorn snails (Planorbarius corneus) which work all year to control the algae and keep the water clear. There are enough small predators in there to control mosquito larvae, so we never get mosquitoes from the pond.

I'm writing this at the end of April. The smooth newts, like little dragons, have all just reappeared in the pond. There are also pond skaters (some people call them water striders) on the surface, and water boatmen (some people call them backswimmers) just under the surface.

I know that there are monsters in the deep. Well, not very deep, the pond's only about 90cm deep in the middle! The monsters are the nymphs (an aquatic larval stage) of several species of dragonfly and damselfly (Odonata). They are fierce predators. With the dragonfly nymphs, the pond skaters and the water boatmen, mosquito larvae have no chance.

When it's sunny, we often sit by the pond to eat lunch, and we may find that a common frog is in the pond looking back at us. Frogs sometimes lay frogspawn (frog eggs) in the pond, but things eat the spawn before it can become tadpoles or adult frogs. We only get one or two common frogs at a time, and they're almost silent. We don't have the noisy marsh frog from eastern Europe (Pelophylax ridibundus) that somebody introduced to south-east England in 1935.

A grass snake sometimes spends a week in the pond. It should be called a water snake, not a grass snake, because it's Europe's version of the anaconda. It is often 1 metre long (sometimes 1.5 metres or even 2 metres), it loves water, and it eats mainly frogs, toads, fish and small mammals. It is not venomous.

Garden birds like blackbirds (Turdus merula) like to have a bath at the edge of the pond. They roll around in the water, putting their heads underwater and making a shower with their wings. Other birds come to drink, and most years we have visits from two or three wild ducks. Last year, a mother duck arrived at the pond with 13 new ducklings. They didn't stay long; the mother had to move her family to a larger body of water (because predators like cats and foxes can get too close to birds in a small pond) but it was nice to have them for a day.

Honey bees (Apis mellifera mellifera) come to the pond and sit on wet moss to drink water.

There aren't any fish in my mother's pond. Every time we put fish in there, a heron comes and takes them. That's quite impressive, because a heron is about 1 metre tall with a wingspan of nearly 2 metres, and the pond is just outside the living-room windows.


Making a pond

Pond fauna

It's easy to make a small pond; you just make a hole and add water. 90cm is about the minimum depth for a wildlife pond, especially if winter temperatures will go below 0 centigrade.

Unless your pond is in a very wet area, you need a pond liner to hold the water. The cheapest pond liner is a single large piece of damp-proof membrane (polyethylene plastic). A better solution is a pond liner made of reinforced rubber. A butyl rubber liner can last 50 years or more. Some people line a pond by pouring cement or concrete into it, but this is not a very good solution. Cement soon starts to leak, unless you use swimming pool technology.

The best pond liner is puddled clay. It's not only a great pond liner, it is also also the best solution for the environment. Clay is a natural mineral that you can buy from specialist companies. "Puddling" means walking up and down on wet clay. You put the clay in your hole, add water, and walk on it until it makes a waterproof liner. Some people mix the clay with soot or lime, to stop earthworms such as Lumbricus terrestris from making holes in the liner.

If you're making a large pond, you probably need professional advice. One cubic metre of water (1.3 cubic yards) weighs about 1 metric tonne. One or two hundred tonnes of water can do a lot of damage if it escapes.

Make at least one place where small animals, from newts to hedgehogs (Erinaceus europaeus), can climb out of the water; and where birds can splash in water that's only 2-5cm (an inch or two) deep. A little "beach" of marsh, moss or wet meadow at the edge of your pond is good for biodiversity. The barn swallow (Hirundo rustica) may come and live near you if there is mud at the edge of your pond, because they use it to build their nests on cliffs and on the walls of barns and houses. They make very good neighbours.

Every pond needs oxygen for its animals. Rainwater contains a lot of oxygen, and some people use pipes so that rainwater from the roof of their house goes into their pond.

You may get too much algae if the pond is in direct sun all day. You can put tall plants around the southern half to create more shade.

If you have a small pond and lots of trees, you probably need to put a net over your pond at the end of autumn, or it will fill with leaves. As the leaves decay, they take oxygen from the water, which becomes stagnant (dead and smelly).


What to put in your pond

What plants and animals do you put in your pond? Because water plants and pond animals are so quick to move into new environments, you don't have to put anything into your pond at all, except water. Within three or four years, you will have a full collection of local water plants, invertebrates and even fish.

How can you help the process? The answer depends on the size of your pond, on the amount of work you want to do every year, and the part of the world where you live.

The most important rule is: Don't put any non-native water plants in your pond. Thousands of attractive plants have become invasive species after some gardener took them to another part of the world. For example, kudzu (Pueraria) from Japan was taken to the USA, where it is now called "the plant that ate the South". It covers and kills other plants, including large trees, and it damages buildings, bridges and power supplies. Water plants and water animals very often become invasive in new environments. For example, water hyacinth (Eichhornia crassipes) from South America became an ecological disaster in parts of south-east Asia, India and Africa.

The question of "what to put in and around your pond" is too big to cover in a blog article, but there is some good advice online:


Pond books

There are some great books on pond life, design, construction, planting and management.
All of these books, except perhaps The Pond Book, are easy to get at low prices on Amazon, eBay and Abebooks:

The Ponds Book cover image The Pond Book: A Guide to the Management and Creation of Ponds.
Penny Jane Williams, J Biggs, M. Whitfield, A. Thorne, S. Bryant, G. Fox, P. Nicolet, P. Williams, A. M. Julian.
The Freshwater Habitats Trust, see Links page
111 pages, large paperback, 2010, ISBN-13: 9780953797110.
I don't personally have this book, which costs nearly £30, but it has excellent reviews.
The Pond cover image The Pond
Gerald Thompson, Jennifer Coldrey, George Bernard
Oxford Scientific Films / Collins
256 pages, large hardback, 1984, ISBN-13: 9780002191456
Level: Perfect if you want to become an amateur naturalist.
A really excellent introduction to biology.
Very good large colour photos.
Waterways and Water Life of Great Britain Waterways and Water Life of Great Britain
Heather Angel, Pat Wolseley
Peerage Books
192 pages, large hardback, 1986, ISBN-13: 9781850520559
Level: Child and family (also published as The Family Water Naturalist).
Includes lots of easy biology projects. Looks at ponds, streams, rivers, sea shores, the open sea.
Very good illustrations in colour and black and white
Collins Gem - Pond Life cover image Collins Gem: Pond Life
Richard Manuel, Chris Shields (illustrator)
240 pages, very small, wipe-clean cover, 1991, ISBN-13: 9780004588254
Level: Child and family
Good illustrations (colour paintings, black and white line drawings).
Collins Nature Guides - Pond Life cover image Collins Nature Guides: Pond Life of Britain & Europe
Richard Manuel, Chris Shields (illustrator)
255 pages, small, wipe-clean cover, 2012 ISBN-13: 9780007915354
Level: Child, family, amateur naturalist
Good illustrations (colour paintings, black and white line drawings).
Ponds - Creating and Maintaining Ponds for Wildlife cover image Ponds: Creating and Maintaining Ponds for Wildlife
Chris McLaren
National Trust Books
104 pages, small hardback, 2009, ISBN-13: 9781905400751
Level: Amateur gardener
Black and white illustrations.
Ponds and Streams : A Nature Guide cover image Ponds and Streams: A Nature Guide
John Clegg
British Naturalists’ Association
128 pages, medium paperback, 1989, ISBN-13: 9781852232337
Level: Amateur naturalist.
Good illustrations (colour, black and white, line drawings).
Ponds and Pools - Oases in the Landscape cover image Ponds and Pools - Oases in the Landscape.
Klaus Kabisch, Joachim Hemmerling
Christopher Helm
260 pages, medium hardback, 1984, ISBN-13: 9780709915454
Level: First year biology at university, but it's still accessible to the general reader.
The text is centred on Germany, but looks at the whole planet.
Very good illustrations (colour, black and white, line drawings).
Collins New Naturalist - Reptiles and Amphibians cover image Reptiles and Amphibians in Britain
Deryk Frazer
Collins New Naturalist
256 pages, medium hardback, 1983, ISBN-13: 9781870630047
Level: Amateur naturalist
Illustrations (black and white, line drawings)
Collins Field Guide to Freshwater Life Collins Field Guide to Freshwater Life
Richard Fitter, Richard Manuel
382 pages, medium hardback, 1986, ISBN-13: 9780002191432
Level: Professional naturalist. Not easy to read.
Britain and north-west Europe.
Good illustrations (colour, line drawings).
A Guide to the Restoration of Nutrient-Enriched Shallow Lakes A Guide to the Restoration of Nutrient-Enriched Shallow Lakes
Brian Moss, Geoff Phillips, Jane Madgwick
The Broads Authority
180 pages, large paperback, 1996, ISBN-13: 9780948119293
Level: Professional manager, but still accessible to the general reader
How to deal with eutrophication.
Good illustrations (colour and line drawings)


Ponds aren't just pretty

Sewage plant and constructed wetland

I spent 2003 and 2004 in the French Alps, and I often heard about ponds for processing what humans put down the toilet. (Most people call this sewage but engineers call it effluent or blackwater.) Every village in the area had a system for "phytoépuration", which means purification with plants. Even some private homes had one. In German, this system is called "Pflanzenkläranlage", and in English it's called "constructed wetlands". The top photo shows a traditional sewage plant, the bottom photo shows a constructed wetlands facility, under construction in Morocco.

This method of treating sewage is not well known in Britain, but everybody in the French Alps knew about it because it works and it's cheap. Also, it can look like a garden, not like an industrial facility.

The system uses a series of ponds. If possible, they're on a slope so that water flows from one pond to the next by gravity, with no mechanical pumps. Blackwater (from the lavatory) and greywater (from the kitchen and bathroom) go into the top pond, and clean water comes out of the final pond. Instead of chemicals and mechanical devices, these facilities use sand and gravel as a filter, and each pond has water plants growing in it, especially reeds (Phragmites australis) and species of Typha (reed mace, bulrushes, cattails). It's the plants that do most of the work.


And some English...

Mill pond

"Pond" is an everyday English word, but it isn't a major part of the language.

There's "millpond", which we often use to talk about very calm water. "The sea's like a millpond today." A millpond is the body of water above a water-mill. The miller controls it to make sure there is always the right amount of water to turn the mill-wheel.

And there's "fishpond" ... a pond with fish .... and "duckpond" ... a pond with ducks ... and a beaver pond, which is made by beavers.



Picture credits

1: Lacs de Bastan, Plateau de Néouvielle, Jorge Franganillo, Creative Commons BY 2.0
2: Erciyes pond by Ömer Büyükakten
3: Forest lake, Natalia Kollegova, Наталья Коллегова
4: Blue iris at pond, Alison Day, Creative Commons BY-ND 2.0
5: Red water lily pond, Bishnu Sarangi
6: Koi gasping for air, PatternPictures
7: Dead fish (tilapia), faungg's photos, Creative Commons BY-ND 2.0
8: Ducks in a fountain, Jerry Pank on Is Beer A Vegetable, Creative Commons attribution 3.0 unported (CC BY 3.0)
9: My mother's garden pond, Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
10: My mother's garden pond, Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
11: My mother's garden pond, Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
12: Azure damselflies (Coenagrion puella), einszweifrei
13: Hawker dragonfly (Aeshna) with exuviae, sternbea
14: Smooth newt (Lissitriton vulgaris); male with breeding crest, Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
15: Grass snake (Natrix natrix), VPhoto
16-26: Book covers, Nicholas of OnePlanet and Linguetic, Creative Commons BY 2.0
27: Traditional sewage plant, Commune du Val d'Ajol, Creative Commons BY 2.0
28: "Constructed wetland" facility in Morocco, SuSanA Secretariat, Creative Commons BY 2.0
18: Millpond, image by Falco