Wetlands: Duckweed Habitat
|Duckweeds in competition||Organisms that feed
|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
can grow where. Salinity can be high because of the presence of
water, as in tidal marshes and estuaries, or because of leaching from
soils at inland locations. Duckweeds, like many plants can
a small amount of salt. Small amounts are even stimulatory for
(figure). However, above a threshold (>60 mM NaCl), growth is
to be inhibited. At yet higher concentrations, duckweeds cannot
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
full strength sea water is approximately 550 mM NaCl. Barley and
the beet are two exceptions, crop plants with a high salinity
Sugar beet will survive in 2/3-strength sea water. Plants that
tolerate higher levels of salt, up to that in seawater, are called
Halophytes deal with high salinity in a variety of ways:
excretion and osmotic balance. Different plants have evolved
of these mechanisms.
Experiments you can try:
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Revised: December 10, 2005