Archive for the ‘Oxygen in ponds’ Category

Ice and oxygen: the end of the story (for now)

January 20, 2009

oxygengraph19jan081

After the ice melted on the pond I carried on with the oxygen measurements to see if levels went down to the pre-ice levels.

And they did.

So now we are back to the oxygen level that’s about right for the present temperature – 5 degrees C – around 12 or 13 milligrams of oxygen in each litre of water (just a little higher than in November probably because the water is a bit colder now, and oxygen dissolves better in cold water than warm water).

When something new and unexpected happens contrary to what most people would have expected, its interesting to see whether it has been reported before.  If it has, you can be much more confident that this is something real, and not some kind of fluke or even a simple mistake.

And there is one example reported by American lake ecologists a couple of years ago of the phenomenon – not in a garden pond, but at least we know that it has been seen somewhere else before, which suggests its a real phenomenon.

You can read a summary of the scientific paper describing this work here, though to read the whole thing you will need access the archive of the journal which requires a subscription.

Ice, oxygen and the cold weather

December 6, 2008

92-saturated

Not a Health & Safety warning about the dangers of eating butter but the amount of oxygen in the water today.

So why the interest in oxygen? Well, in advice about garden ponds I read on a web site:

‘Winter freezing is a problem for many small pond creatures. Holes should be opened in the ice to allow oxygen into the pond, and for toxic respiratory gases to escape.’

This is a common piece of advice. Is there any truth in it?

Well, there’s not much hard information to go on. But you can get a hint from today’s measurement of the amount of oxygen dissolved in my pond.

Today after pretty much continuous ice cover for the last five days there was 11.8 milligrammes of oxygen in each litre of water in the pond. This is close to as much oxygen as the water can physically hold: its 92% saturated at today’s water temperature of 4 C and there’s no sign of any decline as a result of ice cover.

So it looks like the ice hasn’t made much – if any – difference to the amount of oxygen in the water. In reality, a shortage of oxygen under ice is only likely to be a problem if you’ve got a lot of oxygen consuming fish. And even then, it will depend a lot on the size of pond and the amount of light getting through the ice to allow plants and algae to photosynthesise.

How much oxygen do I need in my pond?

November 26, 2008
unless your studying ponds in great detail its not worth buying one because it won't tell you anything you really need to know! Instead - check out what wildlife is living in the pond. It's a better guide to the pond's condition.

My dissolved oxygen meter: unless your studying ponds in great detail its not worth buying one because it won't tell you anything you really need to know! Instead - check out what wildlife is living in the pond. It's a better guide to the pond's condition.

Much is written about the oxygen levels in ponds and, although its been known for years that oxygen levels go up and down naturally in ponds, many people still worry about not having enough oxygen in their ponds.

I mentioned the other day that in my pond the water is about 90% saturated at the moment – enough for even the most sensitive of animals.

At least, it would be for a short time – put a big salmon in a small pond and it would soon be using up oxygen faster than the oxygen could be replenished. That’s why salmon don’t naturally live in ponds!

So how much is the right amount of oxygen?

The graph gives us a clue.

In May, dissolved oxygen concentrations in the early morning in my pond were a quarter to a third of what they were in the middle of the afternoon. I made these measurements over the course of a weekend. Did these big differences make any difference to the pond's wildlife? Probably not.

In May, dissolved oxygen concentrations in the early morning in my pond were a quarter to a third of what they were in the middle of the afternoon. I made these measurements over the course of a weekend. Did these big differences make any difference to the pond's wildlife? Not really.

My garden pond is pretty much like a wild natural pond: water more or less as clean as it could be, naturally colonised by plants and animals, and a good size and depth for wildlife.

In May there were big swings in the dissolved oxygen levels – because there were a lot of algae growing in the water. But there was nothing wrong with the pond. That’s what ponds are like.

Oxygen levels vary less in November - but the daily swing (low in the morning, higher after lunch) is still there

Oxygen levels vary less in November - but the daily swing (low in the morning, higher after lunch) is still there

Now, in November, the daily variation is damped down to just 3 or 4 degrees a day – but it still varies.

Fluctuating dissolved oxygen levels in ponds are essentially harmless. Unless you’re trying to keep sensitive fish in the pond, which wouldn’t naturally live in these conditions, they probably don’t do any harm.

Do leaves use up oxygen in garden ponds?

November 23, 2008
I like leaves in my pond

I like leaves in my pond

Many guides tell people to remove leaves from their ponds. Here’s a typical piece of advice:

‘When leaves fall in the autumn, try to keep them out of the pool if you can and rake out those that do get in.’

And a typical reason for doing it:

If vegetation falls into the pond water and decays the process of decay uses the oxygen in the water and releases carbon dioxide. This means that if large quantities of vegetation enter your pond then enough oxygen can be removed to cause the pond’s wildlife to suffocate.

Is any of this right?

I can only go from my own experience. In our garden the pond is now going through its second autumn: as you can see, there are quite a lot of trees and shrubs nearby, and they drop leaves into the pond.

Despite this, there is no shortage of oxygen in the pond. Todays there’s just over 11 milligrammes of oxygen dissolved in every litre of water in the pond – which is almost as much as it is physically possible to get in water at todays temperature (the techy term is that the water is 90% saturated).

This is enough for salmon or trout, the most oxygen demanding of fish.

So shortage of oxygen is not a problem at the moment. The leaves seem to be haivng little effect on the dissolved oxygen at present.

What will happen as the pond ages and more leaves fall? That will be interesting to find out. Will the pond get ‘worse’. Will it become a poorer wildlife habitat? I suspect not – but we’ll need to wait to find out.

More pond weather

November 20, 2008
Pond daily temperatures Nov 17 - Nov 20 2008

Pond daily temperatures Nov 17 - Nov 20 2008

Summary of the last four days temperatures in the pond. After a warm day the water’s got a bit chillier again.

So why am I interested in the water temperature?

Mainly I’m doing it just to understand how the temperature changes – I don’t think measurements like these have ever been made before in a garden pond.

It also affects the amount of oxygen in the water – the colder it is, the more oxygen dissolves in the water – though as I’ve noted before this isn’t such a big deal as pond dwellers are adapted to natural variations in oxygen concentration – so they’ll be used to the increase in the dissolved oxygen as the water cools down in autumn and winter.

We’re always told it ought to be slowing things down too – though so far there’s not much sign of this.

Oxygen in ponds

November 4, 2008
but you don't need them in wildlife ponds

Fountains add oxygen to the water: but you don't need to add oxygen to wildlife ponds. Instead make sure the pond isn't polluted

Most advice about looking after ponds repeats the old idea that well-oxygenated water is vital for a healthy pond.

But is this true?

Oxygen levels naturally vary in ponds – higher in cold weather, lower in warm, higher during the day, lower at night, higher in ponds with a sandy bottom, lower in ponds with lots of oxygen absorbing plant debris. Water plants, including microscopic algae, also cause big fluctuations: in the light they produce oxygen while at night they use it up.

Rivers and streams, on the other hand, have pretty constant and high disssolved oxygen levels, mainly because their flow constantly mixes oxygen from the air into the water.

Animals living in freshwater have adapted to these natural differences over the millenia. Pond animals often get their oxygen from the air, coming to the surface to breathe. If they live permanently under water they usually tolerate occasional low oxygen levels, like pond olive mayflies.

So does adding ‘oxygenating’ plants make any difference? Not really, because although they produce oxygen during the day, they use it up when the lights go out. They might just as well be called deoxygenators.

Unless you are keeping a lot of oxygen demanding fish, you don’t need to add oxygen to your pond. And if you do, its probably better to get a fountain.  Plants are just as likely to kill your fish by using up all their oxygen in the night – something a fountain won’t do.

But plants are beneficial: they provide vital cover, habitat and food. And they look good too.


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