The Garden Pond Blog

This muddy pool looks, well, let's face it, pretty unattractive

It’s always useful to see the ways in which ponds are made naturally as this usually will gives valuable clues to the best designs for wildlife.

So I was pleased a few days ago to see another example of the work of a creature fairly well-known to make small, garden-pond sized, ponds: the Wild Boar.

This little pond, made in the base of a summer dry clay-lined depression in Northern Italy – just outside Trieste – is a breeding ground for the Yellow-bellied Toad, one of the cutest little amphibians to be found on the continent, but sadly absent from Britain.

The Yellow-bellied Toad is well named - this is one from the muddy pig pool

Yellow-bellied Toads are specialists of the kind of little muddy ponds that the pigs make.

And what is the lesson for garden ponds? Well, this pond is entirely satisfactory as a habitat for the toads – yet it is only a few inches deep, and completely devoid of vegetation. Perhaps the lesson here is that what’s good for wildlife often looks completely awful from a human perspective! Yet this little hole in the ground has clean water – there are no nearby pollutant sources – is very shallow, and also has extremely gently shelving edges, the three key features of many good wildlife garden ponds.

But I don’t suppose we’re going to see too many gardeners opting for a muddy hollow in the middle of the lawn for wildlife just yet.

Inevitably as the weather gets drier those not lucky enough to have a good store of rainwater are wondering how to get hold of clean water.

One possibility we’ve recently been asked about is whether you can use reverse osmosis filters to clean up tapwater.

Reverse osmosis filters – which force tapwater under pressure through a semi-permeable membrane – seem to be sold as domestic appliances for people not happy with their tap water. It looks like they would cost several hundred pounds to get installed, and then have some regular running costs for filters and maintenance.

I haven’t investigated them carefully – just scanned one companies website – but they appear to have a serious flaw as far as ponds are concerned. Although the filters had a very high removal rate for most chemicals – usually in excess of 90% – for nitrate, which will usually be the biggest problem, the removal rate is only 50 – 90%.

So you could fit the filter and then remove only half the nitrate. If your tapwater comes with 25 milligrammes of nitrate in each litre (which it often does) halving this amount would still leave you well above natural levels.

Which brings us to Robert in Lewisham who has a pigeon infested roof with passing traffic depositing all sorts of dirt on the roof as well. And to compound the problem, he has no downpipes to get the water from anyway.

The only practical suggestion I have is to see whether there are any friendly neighbours who might be prepared to share water from their roof. You collect the water from their place and bring it back to your garden pond.

Where did this come from?

Rather to my surprise, the New Pond is getting quite a bit of blanket weed growing on it.

I’d expected not to be suffering from this as I’d assumed that the both the Old and New Ponds had the low nutrient levels you expect in unpolluted waters. This winter phosphorus and nitrogen concentrations in the New Pond were low with just 28  microgrammes of phosphorus in each litre of water – and under one milligramme of nitrogen.

But this week we’ve measured the phosphorus levels in the pond again. Phosphorus is essential for plant growth but too much of it is like living just on burgers. Because of this, phosphorus is both a fertiliser and also one of the most widespread and pernicious pollutants in ponds, lakes and rivers.

And now phosphorus has risen to around 200 microgrammes per litre in the New Pond. What I haven’t worked out is where it has come from - although its possible its simply the result of the low water levels leading to chemicals becoming more concentrated.

Fortunately the blanketweed doesn’t seem to be causing too many problems at present: Lesser Pondweed and Small Pondweed are both growing well, as is the stonewort we added from the local gravel pits. We’ve also still got a little bit of Water-violet as well.

Having four kinds of submerged water plants is quite a bit better than the most ponds in the countryside which, sadly, are too degraded by pollution to support this variety of water plants.

It’s not a drought yet, but the weather is very dry.

The Met Office web site today is headlining the news that ‘The UK has had the driest first six months of any year since 1929.’

So far I’ve nearly used up one of my five 220 litre water butts of rainwater.

And I’m taking an unhealthy interest in when it’s going to rain next!

Watch out for more dry weather advice….

Hi Elizbeth

To try to answer some of the questions – goes on a bit I’m afraid (god, I do seem to talk a lot!).

Large growths of algae are usually the sign of an excess of nutrients – though in your case it’s difficult to know where they could have come from. Are you confident that your rainwater tank only has rainwater in it (sorry – I had to ask!).

And it is worth saying that, although garden ponds seem so familiar, we still know remarkably little about what makes them tick – and as you’ll know if you’ve been reading the blog, much of what is said about them has not much basis in fact. So with that caveat in mind…

It’s a little surprising that algae are growing quickly in your pond given that you’re pulling it out regularly – it does suggest some kind of excess of nutrients.

Are filamentous algae harmful to other plants and animals? Well, they’ll certainly compete with other submerged plants, and could in effect smother them. They probably wouldn’t do much harm to marginal plants.

They’re not what I would call harmful to animals – only in as much as they may limit the range of habitats available. And mats of algae often teem with creatures (and also have a forest of smaller algae growing on them) – so they make a pretty prodcutive kind of habitat for animals.

Is it the high temperatures that are making the algae proliferate? That will play a part – they’ll be able to grow faster – but what makes filamentous algae proliferate in small ponds is most probably a combination of excess nutrients and plenty of light (once again we have no controlled experiments to be sure of the answer – but that’s what experience seems to suggest).

We’ve never had any filamentous algae in my Old Pond – it’s low in nutrients and semi-shaded.

But now in the New Pond we do have filamentous algae – much to my surprise.

But then the other day we found that nutrient concentrations were actually rather high – at a level I would call pollution (I’ve no idea why because I thought we’d taken all necessary measures to keep nutrients low). Added to this the pond is in full sun all day long.

Are snails any use? Probably not. Snails are often hugely abundant in filamentous algae – they love it as a habitat! But the idea that they will control algae is, in my experience, one of those myths put about, I suspect, by people who want to sell you snails. Of course snails do browse algae (we’ve all seen the snail tracks on the side of an aquarium) but I’ve never seen a pond where, when polluting nutrients are in excess, snails can keep the algae down.

And the $64,000 question – What can I do to regulate the algae?

Well, that depends on the cause. If it’s nutrients – reduce nutrient levels.

If you’re really keen to find out, Pond Conservation supporters can get simple water testing to get you started – but to really know the nutrient levels in your pond you need a lab. water test. Contact my colleague Angela Julian for more information.

As for barley straw – it has some effect but it’s hit and miss (it doesn’t kill all algae) and it doesn’t cure the source of the problem – just temporarily kills the algae. It’s not a substitute for clean water.

You could simply do as Martin suggests and just wait and see what happens – but I would always aim for nutrient levels being near natural as the best way of ensuring limited growth of algae and the best range of wildlife in the pond.

In your case the question is – are nutrient levels as low as you hoped from using rainwater?

Female Broad-bodied Chaser

My Oxford colleague Liz Mitchell has just sent me this nice photograph of a recently emerged female Broad-bodied Chaser which emerged from her 1 year old garden pond a few days ago.

The picture prompted a few thoughts about this supposedly common dragonfly.

The Broad-bodied Chaser is usually described as common and widespread: so it’s  easy to think you’d find it in pretty much every pond you looked at, at least inside it’s range. But the evidence suggests it may not be quite as common as this picture implies.

The distribution of the Broad-bodied Chaser: each dot shows the presence of the animal in a 10 x 10 km square of the national grid

In the best conditions – pollution-free, high quality ponds – it should be found in about 1 in 10 ponds. This is how often we found it in the National Pond Survey – our study of what lives in ponds when they’re in good condition. So even where ponds are in good condition, it’s not really all that frequent.

But sadly out in the real world, most ponds are not in good condition: in fact 80% are in poor condition.

And in the ‘typical’ (degraded) ponds of the British countryside, Broad-bodied Chasers turn up in only 3% of ponds – that is, 1 in every 30 ponds.

With a range covering half of Great Britain, where there are roughly 250,000 ponds, this means that this animal probably is found in maybe 7,500 ponds – say 10,000 to be generous.

So this suggests it’s nothing like as widespread as a simple glance at the map might lead one to expect. (Just by way of contrast the Southern Hawker dragonfly is found in about a quarter of all ponds – so perhaps 65-70,000 ponds altogether).

So is the Broad-bodied Chaser under any kind of threat? Well – at the moment, probably not. It’s a good disperser, it colonises new ponds – like Liz’s – and it’s spreading north.

But is it as common as it should be? Equally certainly – no. If ponds were in better shape generally we might expect to see it in 25,000-30,000 ponds.

Well, we’re heading that way.

The National Hydrological Monitoring Programme says:

“For the UK as a whole, the provisional January-May rainfall total is the lowest since 1964 with exceptional deficiencies across parts of western Britain.”

So after some of the wettest summers for years in 2007, 2008 and 2009 now we’re seeing the pendulum swing the other way.

A couple of thoughts about dead tadpoles and topping up.

One thing that might be responsible for tadpoles dyng is heat. The lethal temperature for tadpoles is around 35 – 36 C: for young tadpoles, water at this temperature can kill 80% or more. Older tadpoles are more resilient.

We saw mortalities in our (open to full sun) New Pond in the very hot days we had in May; although we weren’t measuring temperature at the time, measurements we’ve been making during the latest warm weather suggest that the pond was at, or very close to, the tadpole-killing temperature in May. And we certainly had quite a lot of dead tadpoles, although some did survive – an outcome which was pretty consistent with the idea that their deaths were due to the high temperatures. We also had dead water slaters and it seems pretty likely to me that this was also due to heat. There are lab studies showing Daphnia are killed by water temperatures around the mid 30s, so it seems quite possible slaters could go the same way.

The possibility raised by Alison is that her dead tadpoles were due to the chemicals added to tapwater to disinfect it – to kill bacteria and viruses. Traditionally, chlorine was used to do this. Chlorine is still used in water treatment but modern processes also include the addition of ammonia to generate chloramines.

Chloramines are used to treat drinking water because they reduce the risk of what the water industry slightly coyly calls ‘disinfection by-products’. These are the chemicals inadvertently formed when chlorine (amongst other things) is added to the water and reacts with organic matter to form by-products which are implicated with causing cancer - obviously not a good thing to have in drinking water.

Unfortunately chloramines are both toxic to aquatic organisms and don’t simply evapoarate – they have to be positively removed.

I haven’t have a chance to look at what data are available on chloramine toxicity to amphibians – beyond seeing lobby group websites which claim they are toxic. So whether adding tapwater equivalent to 10% of the volume of the pond (which is what Alison did) is enough to kill tadpoles I don’t know yet: my suspicion is that it wouldn’t be, since it’s not something a lot of people have reported. But I will dig a bit deeper here.

What all this does point to is that it seems increasingly important to me that the first thing you have to think about with making a pond is not the size, shape, depth, location or plants. It’s how to locate a clean, unpolluted, water supply which is safe for wildlife and which will be available throughout the drier times of the year.

In a world where a lot of the water that comes out of taps contains contaminants, obtaining this water requires a bit of planning.

About a year ago Joe contacted me wondering whether a Laburnum tree was going to pose any problems for the animals in his pond.

He wrote:

And here is said pond.

Underneath the spreading Laburnum tree.....

Well….Laburnum was, at the time, just another of the many subjects about which I didn’t really know anything, but elsewhere on the web you can find somewhat apocalyptic warnings like:

“All parts of the Laburnum tree are poisonous: the leaves, bark, wood and especially the seeds. They contain the alkaloid cytisene. Just half a dozen of the little round black seeds can kill a small child. There is a variety that has seedless pods call Laburnum vossii but that still does not solve the problem of the poisonous leaves, and even the pollen from the flowers is poisonous. Any of this material allowed to fall into a pond or pool can only have a detrimental effect on the life in the water.”

Sounded pretty dire – as though having said tree would be a bad idea – perhaps only marginally better than setting up a shelter for mad axe murderers in the garden shed.

So I had a little dig around.

It turned out there wasn’t much in the way of facts on this subject – nothing new there then – and some hints that the ‘a-handful-of-seeds-will-kill-a-small-child‘line was a bit of an exaggeration. I’ve copied what I wrote to Joe at the bottom (it goes on a bit – people politely call these answers ‘comprehensive’) but the gist of it was – well, it doesn’t look too bad, why not just see if frogs survive in your pond?

So it was very interesting to get a report back from Joe today that:

“So far the Laburnum is not having any noticeable effect on the wildlife. The pond has dealt with fallen Laburnum leaves last autumn, flowers in the spring and now pods are dropping into the water too. Despite this there are masses of tadpoles (mostly now mini-frogs), four resident adult frogs, and I spotted my first smooth newt yesterday (very excited about that as I haven’t seen newts in the area before!).

So far good! I also noted masses of bees on the Laburnum flowers so it must have some nectar value too…[remember...'even the pollen from the flowers is poisonous'].

So, maybe Laburnum isn’t so bad after all – though probably you would need a PhD to know for sure.

Still, as it’s related to nicotine (and apparently used as a substitute if you’re trying to kick the habit) it’s probably not a good idea to sprinkle the seeds on your toddlers organic muesli to give them a boost in the morning.

THE ORIGINAL REPLY TO JOE

Hi Joe

There is no information out there to answer this question exactly because, as far as I can tell from the scientific literature, there have been no  studies of the effects of this chemical on aquatic organisms.

One thought is, if you get frogs, spawn and tadpoles and they grow up successfully, it’s probably not doing much harm.

The toxic chemical in Laburnum is cytisine which is one of a group of chemicals known as quinolizidine alkaloids. I’m not a biochemist but scanning the scientific papers on the subject this is related to nicotine, and apparently it’s used in medicine as a nicotine substitute for people trying to kick the habit.

Cytisine (there are similar chemicals in lupins and other legumes) is a pretty active chemical and people are experimenting with it as an insecticide for terrestrial insects. If it’s anything like nicotine, which is a pretty good all round killer, it could be quite toxic. There is also some evidence that it can be passed from one trophic level to another, from caterpillars eating the plants to parasitic wasps.

However, and there are quite a few ‘howevers’ coming here, I wouldn’t rush to get the axe out straight away!

How toxic cytisine is in water, there is no information about. There is a bacterium that breaks it down, so it’s quite possible even if it does get into the water it’s quickly broken down. And the gist of some of the older human literature is that its toxicity to vertebrates is exaggerated.

Plant chemicals like cytisine are generally interpreted as a form of chemical defence and, as far as I understand it, they work by making the insect (or whatever is doing the eating) use up energy to de-toxify them. So they don’t necessarily kill things.

As there is no clear answer to this question at present (the suggestions on the Wild About Britain website that it would need a PhD is on the right track!) you might want to make a few observations yourself to assess the scale of problem.

For example you might compare your pond with neighbouring ones: if frogs spawn in your and neighbouring ponds but never succeed in yours this might be a hint of things going wrong. But it could of course be one of many other problems.

You could look at the animals as well: if you have a normal amount of creatures in the pond (work out the Big Pond Score for your pond) then its probably not doing too much harm.

Best wishes

Jeremy

It’s a familiar theme of mine that, when it comes to ponds, you don’t want to believe everything your read.

Here are a few myths and misconceptions about garden ponds which you might notice without digging very deep on the web. These are all packed into one paragraph of text from an apparently reputable source. The quote is in italics.

‘……most pond plants and creatures prefer sun and warmer water; it [YOUR POND] should be away from excessive shade, something which will also help to avoid too many leaves falling in during autumn.’ – Two for the price of one here. As I’ve noted a few times, plenty of freshwater plants and animals are happy in shade. My Old Pond is in the shade for about half the year, is right next to 20 foot tall trees and shrubs and is in excellent condition (if a little over-loved by frogs this year). In mid-summer, around now, it gets the sun for about 4 of 5 hours  a day, on one side. The leaves, it goes without saying, are an important habitat and food resource – though if your pond is completely full of leaves it won’t be such a great place. This problem looks to be especially bad if you’ve got the traditional 2 foot or more deep sump in the centre of the pond which accumulates a lot of rotting leaves. And I’ve just been speaking at an international pond workshop where we’ve been discussing the fact that polluted ponds are affected by shade (it reduces the variety of plants) but unpolluted ponds are much less impacted. Indeed, clean shaded ponds are as likely to support a wide variety of plants and animals, including the most sensitive species, as open ponds.

‘The larger the better but even a small pond is of great value.’ One of the widely repeated older myths is that the ‘bigger the pond, the better’. This is actually a version of the ‘ponds are just inferior versions of  lakes’ idea: a small thing can’t be important so you want to make it as big as possible. We’ve known for a long time – nearly 20 years now – that many factors as well as size influence how many species you’ll find in a pond. And of course, there is some truth that bigger ponds have more things in – however, to roughly double the number of things in a pond you need to increase the area by 10 x and just as important is how polluted the pond is, how much habitat there is, it’s structure, it’s location and a number of other factors.

‘Depth needs to be 2ft (60cm) or more in the middle, giving the pond creatures an ice-free refuge in winter‘. I think we fairly comprehensively dispelled this idea last winter when the coldest winter for some time froze ponds to a depth of 2 or 3 inches. In our garden where the ponds are about 25 cm maximum depth there was no problem with freezing solid and we had no dead amphibians.