D****d Duckweed

The Old Pond today

I hadn’t expected to have a lot of duckweed on my ponds – given their low nutrient status –  so it’s interesting that after Common Duckweed colonised the Old Pond this spring, the pond is now pretty much covered.

The traditional interpretation of this colonisation would be that the pond is the worse for it; certainly it doesn’t look as nice, and out in the big wide world, many places which are badly damaged by pollution have abundant growths of duckweed.

So does it mean that my Old Pond is now in serious trouble?

Well, so far the duckweed has made no difference to the variety of plants in the pond – other than to add another native species (itself) to my list of naturally colonising species.

It hasn’t by itself reduced the abundance of the String Moss that was previously the dominant water plant (that job was done by this spring’s tadpoles!). If anything, it was the other way round: with the String Moss severely knocked back by tadpoles, the duckweed has had little competition.

It’s harder to tell what effect the duckweed has had on animals: certainly the animals haven’t changed too much this year, and most of the species that were present this time last year are still present this year (see this recent post).

And it’s complicated because as the duckweed became abundant we also had very low water levels for most of the summer – a big change compared to the last two years, and probably as big a change as the spread of the duckweed. We need to be careful of falling into the trap of coming to general conclusions from observations on just one pond – something many freshwater biologists did in the past. The result of that was that they didn’t realise how different ponds were, one from another!

Recent careful studies of what affects the abundance of duckweeds suggest that, when nutrient levels are low, duckweeds are out-competed by submerged aquatic plants. The detailed science is here, for those specially interested (though one word of caution: the study used Fat Duckweed, Lemna gibba – native to the UK but not a duckweed I’ve so far seen in a garden pond – and a species which is particularly nutrient hungry, so it might also be expected to be particularly sensitive to competition for nutrients).

There’s some support for the idea that duckweeds can be out-competed by submerged plants, when nutrients are low, in the tubs and ponds in our garden

We’ve got one tub that’s had duckweeds on it all year – but it’s also stiff with Nuttall’s Waterweed (incidentally an alien plant!), and the duckweed certainly isn’t thriving.

Lots of Nuttall's Waterweed (Elodea nuttallii) and very little duckweed: could this be a sign that the Elodea is out-competing the duckweed?

We’ve another tub with stoneworts – and again a few fronds of duckweed, but it’s not taking over – indeed, the duckweed’s barely growing at all.

Are stoneworts outcompeting Elodea in this tub? It's possible.

Katy’s pond, our newest and smallest, will be an interesting test. It’s got good moss growth and a bit of duckweed – will the duckweeds spread in the spring? We’ll have to wait.

Our newest and smallest pond: plenty of String Moss and only a little duckweed so far.

So maybe the secret of low duckweed is both unpolluted water with just natural levels of nutrients and plenty of competing water plants too. It’s worth remembering that many ponds, in both town and country, have far fewer submerged aquatic plants than they should because of pollution.


3 Responses to “D****d Duckweed”

  1. PondGuy55 Says:

    Did you know that the Community Sequencing Program funded the sequencing of the genome of the giant duckweed? 🙂

  2. Janet/Plantaliscious Says:

    Drat, wish I’d come across your blog earlier! I’ve had terrible duckweed this year, though the water quality underneath is best ever, and no shortage of wildlife (frogs, newts, pond skaters, numerous snails, lots of things I haven’t a clue about). I found it impossible to net off the duckweed because it was tangled in the oxygenators, which had multiplied hugely. So I removed a lot of the oxygenators and hence a lot of the duckweed. Sounds like the wrong thing to do. Guess I will have to see what happens next year…

  3. Ferny Says:

    What species of stonewort do you have in the tub? My understanding was that generally they’re pretty poor competitors…but then the tub isn’t a ‘normal’ pond and I’ve had my perception biased by reading mostly about Chara canescens.

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