365 Urban Species. #195: Duckweed

[urbpan]

Photos by cottonmanifesto. Location: Ward's Pond, Boston.

Urban species #195: Duckweed Lemna minor

The reaction that most people have when they see duckweed is "what's wrong with that pond?" This miniscule floating plant reproduces vegetatively frequently spreading to cover the entire surface of small bodies of water. Duckweed doesn't necessarily indicate that a pond is polluted, but it does do well in water rich in nutrients, and fertilizer or sewage runoff can create that condition. Duckweed also tolerates a wide range of water pH, including the acid conditions typical of urban environments. The plant can have a beneficial effect on ponds, absorbing ammonia and reducing murk suspended in the water. Currently researchers are studying duckweed's usefulness as a bioremediation agent, to remove certain toxic chemicals from water. Ducks feed on duckweed and transport it from one body of water to another, as the small plant clings to their feathers and feet. Carp and goldfish eat it as well.


This duckweed in Franklin Park fully covers the surface of the pond, except for where the ducks swam through it.

Comments

[cailin-t.livejournal.com]

that picture is a duck's wet dream, lol.

[klandaghicat.livejournal.com]

I used to buy duckweed for my goldfish tanks. I can't get it anymore, because they determined it was too invasive. I'm reduced to feeding them broccoli to keep the fish from eating my plants.

[turil.livejournal.com]

"I can't get it anymore, because they determined it was too invasive."

That's an irony, isn't it?

How about taking some from the places it's already invaded!

[klandaghicat.livejournal.com]

I have actually done that, but then there's the risk of introducing "nasty" things into the tank. I actually no longer have the fabulous goldfish tanks that I used to, just a new 29 with some carnival/feeder goldfish to cycle it. Personally, I'm a pro-duckweed person, when it's contained. It's a great top plant in a tank, where you want the fish to be as comfortable as possible, thereby behaving naturally.

[ex-wellread.livejournal.com]

So that's what that "sludge" is!

[apathy.livejournal.com]

looks like butterflies!

[badnoodles.livejournal.com]

Doesn't a heavy cover of duckweed reduce light to submerged plants (causing die off and drop in oxygen content) and hindering visual predators like Odonata naiads and fish?

[urbpan.livejournal.com]

These seem plausible. I found very little on duckweed that didn't vilify it as an ugly scourge or praise it as a pond messiah.

A sample praise: Role in the habitat: Duckweed is an important food for wild waterfowl and fish both directly and as a source of food for small creature that are in turn eaten by the birds and fish. As it grows, Duckweed absorbs nutrients from the water. Thus it has a useful role in controlling the growth of algae, both by removing nutrients and by shutting out sunlight as the Duckweed covers the water surface. Algae absorbs oxygen and as it decays, it further reduces oxygen levels. Algal blooms can thus severely affect aquatic life. By shading the water, Duckweed also keeps it cool and thus allow for more dissolved oxygen. And by covering the water surface, it minimises water loss through evaporation. http://www.naturia.per.sg/buloh/plants/duckweed.htm

Sample vilification: Duckweed and watermeal are free-floating plants that can totally cover the surface of a pond. In addition to making a pond unsightly and not very appealing for swimming (Fig. 1), thick growths of these plants can harm the fish population. A surface layer of duckweed or watermeal will prevent sunlight from
reaching the deeper parts of the water column. This means that underwater plants and algae can no longer photosynthesize and produce oxygen. The lack of oxygen can greatly stress or even kill fish; typically duckweed or watermeal infested ponds lose good fishing quality over a period of years. www.btny.purdue.edu/pubs/APM/APM-2-W.pdf

[roaming.livejournal.com]

Just to say, this LJ is THE beset, most interesting, most informative. Thank you!

[urbpan.livejournal.com]

Thanks for the kind words!

[turil.livejournal.com]

Gotta say that the anti-duckweed people sound kinda unscientific to me: It's bad 'cause it looks ugly and we don't want to swim in it's ickyness! Oh, and we can't catch as many fish when there's a lot of duckweed (probably because they catch more duckweed than fish as it tangles their fishing line)!

Wouldn't the plant produce oxygen, thus making the water more oxygenated, rather than less?

[drocera.livejournal.com]

I'm always tempted to eat duckweed when I see it. Almost like watercress. But I'm sure it's rather bitter.

[maybethecat.livejournal.com]

have a pond smothered with duck weed at the zoo, it never seemes to end as we try to clear it

and it stings like hell when you get it in your eye

[droserary.livejournal.com]

An amusing simile from David Quammen (in Song of the Dodo) that I read today:
"In the early decades of the twentieth century, Gregor Mendel's classic work on inheritance among pea plants was rediscovered and the science of genetics burgeoned like duckweed on a warm pond."