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?


2 Responses to “In answer to Elizabeth (in France) – see the comments”

  1. Diana Says:

    Hi Jeremy

    Have just read your reply about Elizabeth’s query re algae – we have the same type of algae in our old pond which has been established for about 15 or 16 years and which used to have fish. It’s only been cleaned out perhaps twice in that time. No fish now and so this year for the last two years wildlife has been increasing. There is quite a lot of vegetation in the pond and the only place the algae proliferates is around the waterlilies which is the only ‘open’ water area. The rest of the pond has plants around the margin, there is a shelf, and the rigid hornwort which you cleverly identified and which I had thought was Canadian pondweed until my other half reminded me we had removed most of that and replaced it and the hornwort seems to have taken over. But there is no or at least not much algae anywhere else. At the moment with the sunny weather it requires removal once a day to keep it at bay. However, I have noticed that the tadpoles quite often bury into it – perhaps they graze on it too but often just seem to be static, I rather thought they might be hiding in it. Our new pond, with no tap water, has no algae at all and no plants to speak of as the weather has been so dry, the margins have rather dried out and I am unwilling to plant anything until there is a better chance of the plants surviving. Rain is promised for tonight though! At the moment the new pond is mainly being used as a bird bath – we have had sparrows, blackbirds, thrushes, goldfinches and pigeons, although I am not encouraging the latter! Even the neighbour’s cat is happy to drink out of it.

  2. pondolive Says:

    Diana, fish can leave a footprint in a pond after they have gone. Most fishponds tend to have quite a lot of fish in them, and because of this the fish need to be fed. All of the nutrients in the food go into the pond, and most of this then ends up in the mud at the bottom. Unless you cleaned out all the mud when you removed the fish, it’s quite likely that there is a layer of nutrient rich mud in the bottom of the pond. It’s interesting that you say the main plant is hornwort as this likes relatively nutrient rich conditions.

    That said, it doesn’t sound like you have too bad a blanket weed problem. Blanket weed is a bit unsightly but in most ponds it’s just a fact of life and so long as it isn’t taking over it’s probably best not to worry too much. Bear in mind also that removing the blanket weed may disturb the sediment, releasing nutrients into the water column and encouraging more growth… I agree with Jeremy that blanket weed does seem a very productive environment.

    I would recommend introducing some stoneworts. These are ‘complex algae’ that form rather attractive, dense branched lawns at the bottom of the pond. They act as ground cover by forming a dense mat and therefore lock nutrients in the pond sediments in, and they don’t form the gassy floating mats you get with blanket weed.


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