Vegetative Growth of Duckweeds
Designed for Rapid Growth
 Overgrowth (Blooms)
Senescence
Environmental Stress
Controlling Duckweed


Designed for Rapid Growth

Duckweeds spread rapidly across quiet bodies of water enriched in nutrients.  Under ideal conditions, the area covered by fronds can double in a few days.  This rapid growth is propelled by the design of the plant.  Each frond contains a central meristem that is the origin of new fronds, as many 10-20 more during the vegetative life cycle, depending on species.  Studies show that duckweed can grow 30 percent faster than water hyacinth.

Below:  Time-lapse video of Spirodela polyrhiza growing, courtesy of Philomena Chu, Rutgers University.
 


Overgrowth (Blooms)

Duckweed
          covering a wastwater treatment lagoon.

As with many other aquatic plants, duckweeds grow best in water with high levels of nitrogen and phosphateEutrophication of a body of water can produce these conditions and encourages the growth of duckweeds.  As they grow, duckweed plants absorb these nutrients from the water.  Floating duckweed plants are relatively easy to remove by skimming or are eaten by herbivorous fish.  These traits make duckweeds useful in nutrient removal and bioremediation schemes.

A cover of duckweed fronds shades the water below and can reduce the growth of algae, which otherwise occurs under eutrophic conditions.  Preventing this growth of algae is beneficial, since an overgrowth of algae will eventually die and decay, reducing the content of oxygen dissolved in the water.  This loss of oxygen is harmful to fish.

Despite their advantages, when they overgrow bodies of water duckweeds may become unwanted.  Read how to keep them under control.


dead and dying duckweeds floatSenescence

When a mature frond has produced its limit of daughter fronds, it ages (becomes senescent) and dies.  Senescent duckweed fronds, like senescent leaves of other plants, appear yellow because they have lost chlorophyll.  The number of daughter fronds that a mature frond can produce depends on the species.

Unlike freshwater algae, which sink and decompose when they die, duckweeds continue to float as they senesce (see photo right).  Their air pockets, aerenchyma, keep them afloat for a considerable period.  In a well-managed pond, the duckweeds can be skimmed of as they begin to senesce. This effectively removes nutrients from the pond and prevents the loss of oxygen that occurs when senescent aquatic plants sink and decay.


Eutrophication

Eutrophication is an increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aquatic ecosystem such as a lake.  Traditionally, eutrophication refereed to the gradual increase in such nutrients that occurs in the aging of lakes and wetlands as their biomass grows.  However, human activities, such as untreated sewage, agricultural runoff, or industrial wastes (including livestock production, slaughter, and processing) can result in large and rapid eutrophication events.

Such excessive concentrations of nutrients in waterways and wetlands have the potential to change healthy waterways and wetlands in drastic ways.. Ecological, recreational, cultural, aesthetic, educational, scientific and human use values held by society for these resources can suffer as a result of these changes.

A US EPA publication summarizes the effects of eutrophication on aquatic plants:

Submersed and floating-leaved or mat-forming species usually respond more strongly to enrichment than do emergents (e.g., Ozimek 1978, Shimoda 1984), because the former obtain nutrients directly from the water column, whereas the latter obtain them from sediments. In many regions, vascular floating-leaved plants such as pondweed (Nuphar), duckweed (Lemna), and water-meal (Wolffia) become more prevalent with increasing enrichment (e.g., Bevis and Kadlec 1990, Burk et al. 1976, Ewel 1979, Kadlec et al. 1980), and in severely eutrophic lakes, emergent species may survive as floating mats (Graneli and Solander 1989). Species shifts may be less immediate or noticeable when moderate amounts of nutrients are added to wetlands that are already eutrophic, because high microbial populations that characterize such environments can be highly effective at first in competing for the nutrients (e.g., Richardson and Marshall 1986). With extreme enrichment, submersed macrophytes can eventually decline, probably as a result of being shaded out by algae (e.g., Mulligan et al. 1976, Phillips et al. 1978), and emergent species may increase.

From  Impacts on Quality of Inland Wetlands of the United States:  A Survey of Indicators, Techniques, and Applications of Community Level Biomonitoring Data, U.S. Environmental Protection Agency, Excerpts from Report #EPA/600/3-90/073, URL: http://www.epa.gov/owow/wetlands/wqual/herb.html, Revised March 23, 1998

When human activities cause waterway eutrophication, human activity is required to correct the problem and restore ecosystems.  Management action is preferred to reduce the nutrient loads entering waterways, rather than expensive technological solutions to the problems after they have occurred.  This management action requires cooperation by landholders and management agencies.

Read more about [ Eutrophication ] or how duckweeds can be used in [ bioremediation ] to remove unwanted nutrients from wastewater.
 
 


Learn more firsthand.  If you are interested in duckweed growth, try some experiments yourself.  They are easily grown in small containers outside or in the laboratory.


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Last revised: June 9, 2013