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Erratum: It Appears to be Duckweed

November 9, 2015

I don’t have too much to say about this yet, because I have more research to do on the matter, but felt anxious about correcting comments I recently made on the subject of the thick green stuff that covers the surface of Putah Creek down in the UC Davis arboretum.

Here’s a picture:

IMG_0355

Apparently, that is not algae…it’s duckweed. Not only that, duckweed may not even be a bad thing. In fact, it may very possibly be good. It may not cause fish to suffocate, it may not be a result of drought, it may not be the result of poorly aerated waterways. All claims I brazenly made over the weekend, not really having any idea, apparently, what I was talking about.

Sometimes, apparently, this does not stop me.

It’s true that in all my years of taking regular walks around the 3 1/2 mile loop, I’ve never seen the green stuff quite like this, and it’s true we are in a four–and perhaps counting–year drought, and there would certainly seem to be a relationship… but maybe not.

Anyway… I’m embarrassed to have been so histrionic… so I’m coming clean and saying I may have been wrong and I need to do a little more research.

A teeny bit of research on the duckweed vs. algae issue turned up this:

Duckweed: Not just for ducks by Suzanne Kollar, PFRA, Beausejour and Darrell R. Corkal, PFRA, Saskatoon

A much maligned plant, duckweed has often been viewed as a nuisance, commonly mistaken for algae and associated with water quality problems in ponds, dugouts and stagnant water bodies.

But the facts are that duckweed will remove plant nutrients from water, block sunlight and out compete algae. It can even reduce evaporation loss [..]

While duckweed is an indicator that excessive nutrients exist in the water, it doesn’t contribute to water quality problems. In fact, this macrophyte plant improves water quality by removing phosphorus and nitrogen from the water and by naturally filtering unwanted matter in the water.

With optimum conditions–food, sunlight and shelter from wind–duckweed can grow exponentially by consuming phosphorus and out compete algae (phytoplankton), which is lower on the food chain.

Duckweed growth, instead of algae, is very desirable since algae pose more problems for water use [..] almost all algae will cause taste and odor problems in water.

Duckweed is an oval shaped plant that floats on the surface of water. It is the smallest flowering plant. When mature, the smallest species is two mm or less in diameter, and the largest species is about 20 mm in diameter, roughly the size of a fingernail or thumbnail. Duckweed looks like tiny floating leaves on the water surface. [..]

Often spread by aquatic birds and floods, duckweed grows in clusters and can grow rapidly with adequate food (phosphorus and nitrogen), sunlight and shelter from wind. Commonplace worldwide and quite hardy, it will even tolerate brackish water. [..]

Often considered unsightly, duckweed blooms can cover an entire water body with a “green blanket” or “mat” containing millions of the small plants. It won’t thrive on sites exposed to wind or where flowing water occurs.

When blanket of duckweed covers a [water way] it limits growing conditions for algae.

This means the plant food for algae will have been reduced as the duckweed blanket blocks sunlight to the water column, limiting photosynthesis and preventing algae growth.

2 Responses to “Erratum: It Appears to be Duckweed”

  1. blk4609 Says:

    But wasn’t someone netting dead fish out of that water?

    • Kari Says:

      Yes, so I’m confused about this… but at least wanted to clarify it’s duckweed and that it is not *necessarily* a horrible thing.. maybe duckweed is its own horrible thing, perhaps in quantities, but it’s not algae. Necessarily. I just don’t really know.


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