The Garden Pond Blog

The old pond

Following heavy rain both ponds are now as full as they get.

The old pond has been clear since the tadpoles became less abundant in the summer.

The new pond has only just cleared – until recently single-celled free floating algae were colouring the water. I expect these will return in spring until there is a reasonable population of water fleas, and other plants get going to compete for nutrients.

Conductivity of the old pond is still very low: just 70 micro Siemens. This is really very good.

The new pond is a bit higher: 155 micro Siemens.

The new pond: good points - the water is clear, plants are beginning to take, there's plenty of animals including some quite uncommon things; bad points - the butyl is showing in places and its hard to keep it covered.

Conductivity is a rough guide to how clean the water is – but you do have to be careful in interpreting the measurements. Water draining from soft rocks such as clay or chalk, will naturally have more chemicals dissolved in it, giving a higher conduvtivity, than water draining from hard rocks e.g granite. But if the water is free of pollutants this doesn’t matter. In any given area – where its unsual to see big differences in background conductivity – then high conductivities are often a clue to the presence of pollution. In the south you start to get suspicious when conductivity is over 250-300.

I don’t actually know why the conductivity is higher in the new pond. It maybe that some soluble chemicals have washed in from the grass turves while they were bare, or it maybe something from the gravel (we did wash it, but it may be that it still had some soluble compounds on it). These may be harmless: calcium, sodium, magnesium. But there may be nutrients too. Only a proper laboratory analysis of the water will allow me to tell.

Interestingly we have never had any blanketweed or filamentous algae in the ponds: I put this down to the presumed low nutrient levels. Although I don’t know if this is the reason for certain, I’m sure it must help.

Richard asks:

“Perhaps you could settle an argument between my wife & I – our pond is quite small (less than 1 square metre) – should we clear out leaves? My wife says yes to stop algae building up and I say no because it’s a habitat for small pond critters.

Who, if anyone is right?”

Well, the picture above is a clue to my answer.

Leaves and debris are indeed habitat: in my pond the place I see most Broad-bodied Chaser dragonfly larvae is the leafy overhung bit. But my pond is quite a bit bigger than Richard’s so maybe it’s unsual.

At the other extreme is this pond.

This one is probably a bit overwelmed by leaves.

But do you need to remove leaves to stop algae building up? Well the truth is no-one really knows.

Leaves might cause the growth of algae by adding nutrients to the water – which is why removing them might be something you need to do.

But although plenty of reputable people suggest this is necessary, the amount of fertiliser in the leaf as it fallls off the tree is quite small (trees take them out of the leaves before dying – no fools trees). I did some rough, back of the envelope, calculations here on the quantity of nutrients in dead leaves.

My conclusion is that you need a lot of leaves to add a signficant amount of nutrients to the pond.

So probably the truth is somewhere in between. I make no effort to stop leaves getting into my pond but if you’ve got a lot falling in, then you might do.

Until there is a better answer – which needs some careful observations of garden pond sized ponds with different quantities of leaves in them – the best guide is the animals.

If there are animals living amongst the leaves, and the rest of the pond has a reasonable variety of animals too, then things are probably OK.

And don’t forget the algae might be there because of polluted water – nothing to do with the leaves.

And another thing: leaves may themselves stop the growth of algae by releasing tannins into the water.

My answer creates domestic harmony

In case you don’t look too closely at the comments, I’m afraid I couldn’t resist pulling out this quote:

“If only I had found this website last year when I started my pond!” (Liz)

As it happens, we’re off to survey Liz’s pond this morning as part of our detailed study of what makes garden ponds tick.

You can tell winter is approaching by keeping an eye out for the ‘winter pond advice’ columns.

So I wasn’t surprised to see this latest offering which advises on how to protect wildlife when you clean out the pond.

But is the key bit of advice…..

‘Minimise the damage to wildlife by removing the accumulated debris in the autumn or late winter.’

…..actually true?

Well, the simple answer is, we dont really know.

But here are a few thoughts:

As I noted the other day there are lots of animals in the pond all through the winter, though some others do come and go.

So dragonflies, damselflies, mayflies, caddis flies: all will have young stages in the pond over winter. There are likely to be adult water bugs and water beetles too. There’s a good chance that anything that can’t fly will be overwintering as well.

Some things are more seasonal: most young amphibians will have left the pond. Some of your water beetles will have flown to deeper more permanent ponds.

But most things will still be there.

So removing accumulated sediments and leaves could well remove the habitat of these animals.

So what’s to do? Well maybe remove a little bit of debris regularly each year rather than having a blitz every 5 years. And carefully wash the animals out of anything you remove, then return them (although you will of course have removed their homes). Putting debris on the side doesn’t work for most soft bodied creatures – its easy to see that they simply get stuck in the muck and die where you leave them.

In more natural environments than gardens a bit of gentle trampling and grazing by cattle, horses or sheep is a good way to manage ponds. We could all do worse than simulating something like this in our ponds at home.

It’s easy to assume that as the weather gets colder life shuts down under water. The bulrushes are dying back now, and it won’t be long before we have our first frosts.

But dipping the pond today shows just how much is going on over winter.

I can see swarms of tiny, recently hatched Smooth Ram’s-horn snails (Gyraulus laevis – pictures here: dead shells show best the distinctive glossy appearance) – there seem to be hundreds amongst the grasses at the pond edge. With them are full grown Whirlpool Ram’s-horns.  The Smooth Ram’s-horn is the most unusual animal in the pond at present: it’s a specialist of new ponds and only patchily distributed across the country.

Lurking in the dense cover of trailing ivy, fallen leaves and submerged mosses at the back of the pond are mating Spotted Backswimmers (Notonecta maculate), Broad-bodied Chaser dragonfly larvae (Libellula depressa) and quite a number of full grown Large Red Damselfly larvae.

Numbers of Pond Olive mayflies are rising now too – there are many hundreds in the pond, including tiny specimens that hatched in the autumn and will grow over the winter to emerge in the spring.

Water beetles are usually harder to find in autumn and winter when many species migrate away from smaller ponds but I did find a Common Black Scavenger Beetle (Hydrobius fuscipes – there’s a rather beautiful picture of it here on this Russian website showing the distinctive rows of fine dots on the wing cases), a Helochares lividus – another edge loving scavenger beetle found amongst the trailing grasses – and the larvae of the Common Black Water Beetle (Agabus bipustulatus) and of small diving beetles in the genus Hydroporus (these are not really identifiable in the field).

There are still pond skaters but no sign of any lesser water boatmen at present.

And in some ways most surprising of all: a couple of frog tadpoles quietly sheltering amongst the marginal grasses. It’ll be interesting to see whether they make it through the winter.

Having crowed about seeing a Grey Wagtail visiting the pond, I’m quickly taken down a couple of notches when two correspondents point out they’ve both had them too.

So I look up the Big Garden Bird Watch results for 2009 and find that Grey Wagtails were the 45th most abundant bird recorded, seen in 1.6% of all gardens – in other words every 62nd person watching saw a Grey Wagtail in their gardens.

You might ask what took me so long!

But they’re still one of my absoulte favourite birds.

Anthony commented a day or two ago:

“Another good thing about bulrushes is that female Aeshnea cyanea (Southern Hawker) dragonflies lay their eggs in the emerging stems of bulrushes. They have a sickle shaped ovipositor that they insert just above the water level….I’m waiting ’til next spring to watch them emerge and drop into the water where they spend up to 4 years before metamorphosing into adults when they will use the bulrushes to climb out and dry their wings before taking off and starting the process all over again.

And the picture above is the evidence from his garden pond in La Montagne in Brittany.

Anthony also comments:

“Our wildlife pond is just over two years old and is coming on strong… we already have frogs breeding and this summer we found some Palmate Newts in the garden and one newt tadpole in the pond!! Lots of dragonflies and damselflies using the facilities.”

Finally, for those of you more interested in your stomachs than your wildlife you might like to check out just how much of the bulrush plant you can eat.

http://www.northernbushcraft.com/plants/cattail/notes.htm

Now you know it makes sense to dig up the veg. patch and replace it with a nice pond full of bulrushes.

The English Fairy shrimp (Chirocephalus diaphanus). Photo copyright Jean-François Cart.

Exiting news from correspondent John Woolliams of Fairy Shrimps in Oxfordshire.

These inch long animals are rare inhabitants of temporary ponds that dry out in summer – usually in the least polluted, semi-natural landscapes – well-known spots for these animals are the heathland ponds of the New Forest and pools made by tanks on the extensive chalk grasslands of Salisbury Plain. Both places are notable for having escaped the 20th centuries intensification of farming and its associated water polluting chemicals.

Over the country as a whole Fairy Shrimps have probably been seen in less than 50 ponds over the last 50 years – and they are so distinctive its hard to mistake them for anything else so we can be pretty sure they really are this rare.

What’s exiting about the new find is this is the only place in Oxfordshire where they are currently known – it’s on the edge of ancient woodland in the north-west of the county.

Fairy shrimps – the species is almost certainly Chirocephalus diaphanus [ky-ro-ke-fa-luss di-a-fa-nus], the only species known from the UK at present – are dependent completely on ponds: they never occur in permanent lakes or rivers where they would quickly be snapped up by fish or washed away.

They glide gracefully through the water on their backs, filtering tiny particles from the water. As their preferred ponds dry out in summer the eggs produced by the adults drop to the bottom to lie in the mud, waiting to hatch when the water returns.