Duckweeds in competition
Examples of competition with other organisms.
Red algae versus duckweed Other competitors:  green algae and water fern

Algae and duckweeds are natural competitors.  In this sequence of photos (right) a population of duckweeds is progressively reduced by growth of a red alga, species not determined.  The top frame shows the presence of both Lemna and Spirodela, while the lower two frames show no Spirodela.  Red-colored blue-green algae and dynoflagellates are often the source of toxins that kill other organisms, but we do not know if this alga secretes a toxin.  These populations were growing in an experimental water treatment system for animal wastes.

Unanswered Questions:

  • Does the alga kill the duckweed or just grow faster?
  • Is the Spirodela more sensitive to the alga than Lemna, or does the Lemna grow faster than Spirodela in the presence of alga?
  • If the alga kills the duckweed, does it secrete a toxin?
  • Or does the alga deprive the duckweed of nutrients?
Interesting links:

Harmful Algal Blooms: Blue-Green Algae and Human Health, Thomas Morris, M.D., M.P.H., Medical Epidemiologist, North Carolina Harmful Algal Blooms Program, December 2000.

The Rise in Toxic Tides, What's behind the ocean blooms?  Christine Mlot, Science New Online, September, 1997.

Harmful Algae and Phycotoxins Information Links, Stephen S. Bates, The International Society for the Study of Harmful Algae, May 2001.

Algal Toxins, from Field Manual of Wildlife Diseases, National Wildlife Health Center, U.S. Geological Survey, 1999-2001.

Photos "Death by Algae" courtesy of Paul Skillikorn.

Other competitors.

Each photo at right depicts a different competitor for the duckweed:

  • Top frame:  Green algae and Lemna.
  • Bottom frame:  Azolla (water fern) and two duckweeds (Lemna and Wolffia).
 (Click on a frame for a enlarged view.)

Green algae:  Since algae mostly grow submerged, duckweed competes with their growth by shading them and depriving them of the light needed for photosynthesis.  However, some algae will be at the surface and compete with the duckweed for light and nutrients.

Water Fern (Azolla):  The Azolla in the lower frame are hard to see.  Their color is more olive-green than that of the duckweeds, and their fronds are bordered with a pink anthocyanin pigment.   Azolla are known for their ability to harbor nitrogen-fixing bacteria, but this would not be an advantage in nitrogen-rich wastewaters.

Green algae and Lemna
Azolla and Two Duckweeds
Photos courtesy of Paul Skillikorn.



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Revised:  April 25, 2003