‘Driest start for more than 80 years…’

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….

In answer to Elizabeth (in France) – see the comments

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?

How common is a common dragonfly?

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.