Wetlands:  Duckweed Habitat
Where to find duckweeds
 Surviving Harsh Conditions
Duckweeds in competition Herbivores and Pathogens

Where to find duckweeds

Wetlands and ponds are the most common sites to find duckweeds, but other quiet bodies of water may harbor them.  Duckweeds may also be found on the fringes of larger lakes and in quiet backwaters and sloughs cut off from mighty rivers.  Since duckweeds may be brought in by migratory birds, there is no necessity that a site communicate with another body of water having duckweeds.

Duckweeds of every genus, Spirodela, Landoltia, Lemna, Wolffia and Wolffiella are found world-wide, although each species has certain areas and climates where it is particularly well-adapted.

Environmental Conditions for Growth:  Like other plants, duckweeds have specific mineral requirements.  Each species and clone grows best under specific environmental conditions (day and night temperature, day-length, geographic latitude and elevation etc.).  However, these optimal conditions are broad generalities -- duckweeds aren't fussy. 



Above:  Huntley Meadows, a natural wetland in Fairfax County, Virginia.
Below:  Duckweed populations often grow among the reeds and other emergent plants in shallow, slow-moving water.

Surviving Harsh Conditions:  Environmental Stress

Actively growing duckweed plants are sensitive to freezing and to drought.  Duckweeds can tolerate a degree of salinity, but are unable to grow in salt marshes.

Cold and Drought

Certain duckweeds can prosper and grow in chilly (but not freezing) conditions, particularly at night.  However, during vegetative growth all duckweeds are sensitive to frost.  Duckweeds survive freezing as turions or seeds, which sink to the bottom after they are released from the mother fronds.

Dormant buds and seeds allow many species of duckweeds to survive periods that would otherwise be lethal.  Some species can form dormant buds (turions), which are resistant to frost (but not to drought).  Many species can flower and set seed.  Duckweed seeds are resistant to cold and drought.  Not all species of duckweed are known to flower.  Those species may have lost the ability to flower, or perhaps we do not yet know the conditions which will cause them to flower.

Mud and silt can protect duckweeds:  When their water dries up, duckweeds may become stranded on mud.  Duckweed fronds are drought sensitive, but they will survive on a surface of wet mud.  The small immature fronds and buds hidden inside the mother frond dry up last and have greater resistance to drought.  These fronds and buds will grow out and re-establish duckweed plants if they are rewatered before it is too late.  Winter-dormant seeds and turions resting on the bottom are insulated from the cold and drying conditions on the surface.  Silt and sediments on the bottom may cover these dormant strucures and help protect them until good growing conditions return.

Salinity (Salt) Stress.  Another stress factor is salinity, the concentration of sodium chloride in the water or soil.  Worldwide, salinity is a major factor in what plants can grow where.  Salinity can be high because of the presence of sea water, as in tidal marshes and estuaries, or because of leaching from saline soils at inland locations.  Duckweeds, like many plants can tolerate a small amount of salt.  Small amounts are even stimulatory for growth (figure).  However, above a threshold (>60 mM NaCl, equivalent to a 1/7 dilution of seawater), growth is begins to be inhibited.  At yet higher concentrations, duckweeds cannot survive.

Figure:  Growth of Lemna minor in diluted sewage with added amounts of salt.  Error bars are +/- standard error.
Growth conditions starting with three replicates of 10 plants/each were:  seven days at 20 C, 2700 lux from CW fluorescent lamps, in 100 mL 2% swine waste in 125 mL bottles with loose-fitting clear plastic lids.  Regraphed from Stanley, RA and Madewell, CE (1976) "Chemical tolerance of Lemna minor L." Tenn. Valley Authority, Muscle Shoals, Al. Circular Z-72, 17 pp. (Polycopy).

Almost all of the major food crops grown on land are salt sensitive and will not complete a full life cycle above 100 mM NaCl.  For comparison, full strength sea water is approximately 550 mM NaCl.  Barley and the beet are two exceptions, crop plants with a high salinity tolerance.  Sugar beet will survive in 2/3-strength sea water.  Plants that can tolerate higher levels of salt, up to that in seawater, are called halophytes.  Halophytes deal with high salinity in a variety of ways:  exclusion, excretion and osmotic balance.  Different plants have evolved combinations of these mechanisms.

Links for more information on salt stress:

Experiments you can try:

  1. Compare the growth of different duckweed species under specific environmental conditions.
  2. Study the effect of drought on duckweeds:  What happens when they are stranded on mud?  How dry can they get before they die?
  3. How long can duckweeds survive in the cold?  Which species can survive cold best?


 
 
 
 
 
 

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Revised:  August 27, 2013