Help: attack of the killer filamentous algae….or maybe not

There be life in that there slime......oh, aaarrrr

Some questions from Karen, which I’ve tried to answer below.

‘……….I am told that if I don’t remove the algae, the oxygen levels will decrease to the detriment of the wildlife.’

‘However, it is clear to me that a certain amount of algae is important to the aquatic animals – as is evidenced from my trays full of lovely little creatures.’

‘Should I remove some of the algae from the pond, or should I leave it in the pond as shelter and food for the wildlife?’

The first thing to say is that filamentous algae are a perfectly normal part of the life of ponds: the highest quality ponds will have small quantities – maybe covering 10% of the surface area of the pond at the maximum.

But in ponds in the countryside where you see a lot of filamentous algae, especially when there are no other submerged water plants…..these ponds are usually suffering from unnaturally high levels of nutrients in the water, something which affects a very large proportion of ponds in the UK (as you can see by reading the Countryside Survey Ponds Report).

Many garden ponds have large quantities of filamentous algae as well. Some people think this inevitable – but I think it’s only inevitable when the water is polluted. I think this is almost certainly the result of excess nutrients, caused by using polluted tap-water or adding nutrients in soil or turves or planting compost. But as is so often the case at present actual measurements to prove this unequivocally are, at the moment, non-existent, although the results from the Abingdon garden pond survey will shed some light on this.

In my old pond – which has very low nutrient levels, I have had no filamentous algae at all. In my new pond, which has equally low levels of nutrients I have a little filamentous algal growth – possibly because the new pond is in the full sun, and the old pond is not. But still, even in the new pond, algal growth is modest.

As for Karen’s specific questions: first, will oxygen levels will go down if you remove algae?

This is a tricky one to answer – of course we have no actual observations to go on because I’m not aware that anyone has actually measured oxygen levels before and after removing dense growths of filamentous algae.

So here’s my speculation: algae produce oxygen, and if there’s a lot of algae, they’ll produce a lot of oxygen. Equally at night they’ll use a lot of oxygen too. So you should see the classic very high oxygen levels in the afternoon, followed by a big drop in the night and early morning before sunrise.

But does pulling out the algae reduce oxygen levels: well, not necessarily. For one thing, if you take away filamentous algae there’s a good chance that single-celled planktonic algae will replace them, and they produce (and use) oxygen just like the filamentous kind.

I don’t know which would produce more. In my old pond, where I have had abundant planktonic algae at times, the planktonic algae produced plenty of oxygen. My suspicion is it wouldn’t make much difference, except in the very short-term.

So should you remove algae or not? If you remove it, as Karen has said, you are taking away a habitat that often seethes with life. If the algae isn’t replaced by another kind of water plant, then there will be less habitat in the pond for animals.

But an abundance of algae is not what I would ideally aim for because it’s not the way ponds would naturally be – in a pond with the normal variety of water plants, algal growth would usually be modest, and there would be submerged plants under the water too to provide habitat.

But you might remove it in order to try to reduce the nutrient levels in the pond – but once again we are in the realms of guesswork here because nobody could tell you quite how much you need to remove to get nutrient levels under control; my guess would be as much as possible, but that is a guess.

All in all, if you can start with clean water first, and keep the problems to a minimum, and get hold of good submerged plants, that’s probably the best option.

But…here we are into a whole new set of problems! Getting native, wild, submerged water plants to grow because they seem to be some of the hardest plants to get started, and the most fussy.

It’s one thing to get the non-native Curly Waterweed (Lagarosiphon major) or Canadian Pondweed (Elodea canadensis) that you pick up in garden centres to grow: it’s something else to get any native stoneworts, true pondweeds (a wide variety of species in the genus Potamogeton), water-crowfoots, water-milfoils, bladderworts or floating club-rushes to grow.

In fact, the only thing that we can be be sure will work in the countryside is to make ponds as clean and unpolluted as possible – and then wait for the plants to arrive.

This is more difficult to arrange in gardens: for one thing, we don’t know whether submerged water plants can get to garden ponds. Maybe the things that spread them (wind, water, birds, other animals) don’t work so well in garden; certainly it doesn’t look as though too many garden ponds have been naturally colonised by submerged plants. And if it takes two or three years (or longer) many people won’t want to wait that long.

In my new pond I’m adding local, wild collected, submerged water plants to see what takes.

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2 Responses to “Help: attack of the killer filamentous algae….or maybe not”

  1. Sally Says:

    Would it be feasible to add beneficial bacteria to the pond? This should eat up excess nutrients that the algae feed on. I don’t know how large your pond is. In my garden pond, I find that algae is only a problem in the spring when there is not an adequate balance of bacteria, plant life, and nutrients.

  2. Sally Says:

    To add to my last comment, it comes back to me that the only effective way to get rid of string algae is to put Barley Straw extract in the pond. Or let bales of Barley decompose in the pond–as slower but less expensive method.

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