Suppression of the Invasive Plant Watermeal (Wolffia columbiana) by Interfering with Floatation
After research which identified waxy cuticular pits as organs of floatation in Watermeal (Wolffia columbiana), this study examined the use of detergents for control by disruption of floatation. In addition to disruption of floatation, treatment with the detergent Tween 20 dramatically increased the infection of plants by natural fungi. Most Wolffia plants grown under conditions similar to natural conditions formed clumps, sank and died within 5 days when exposed to 0.1% Tween 20 by a combined mechanism of chemical and biological control. This study describes a method for control of Watermeal which depends on enabling natural fungi to opportunistically attack these plants, once their floatation is disrupted by low concentrations of relatively non-toxic detergents. This method may be developed to use low toxicity detergents as a method of control for floating nuisance plants in urban environments.
Eutrophication is a destructive process that is often seen in lakes overtaken
by residential development and associated increases in fertilization, erosion
of tributaries and the erection of engineering works that prevent efficient
flushing (Nixon, 1995). Members of the Lemnaceae (duckweeds),
including Watermeal (Wolffia sp.) have long been recognized as important
members of aquatic plant communities and a perennial unsolved problem of water
plant management (Alexander et al., 2008; Les
et al., 2002) and are common members of eutrophic systems (Santamaria,
2002; Frédéric et al., 2006).
Most Watermeal reproduction is vegetative (Frédéric
et al., 2006; Cayuela et al., 2007)
with an average in vitro doubling time of approximately 17 days (Bernard
et al., 1990), allowing for rapid and invasive growth. Therefore,
duckweeds periodically form large continuous mats on the surface of ponds and
lakes which prevent penetration of light to lower levels. This causes death
and decay of plants lower in the water, accelerating eutrophication and production
of nuisance gases. Godfrey Lake in New Jersey is formed from a freshwater stream
and reed bed system that has been separated from salt marshes by two causeways
(Fig. 1). This lake is part of a residential district and
is periodically covered by a mat of waterweeds, predominantly the watermeal
Watermeal are very simple angiosperms with few gross anatomical features. Although,
some varieties have been studied as model plants, few physiological details
relevant to its natural habitat or ecological interactions are known. Distribution
by floating is a key feature of watermeal ecology and White
and Wise (1998) have suggested that flotation is achieved by a mechanism
involving surface tension interactions between cuticle and pond water. However,
this recent study has identified waxy cuticular pits which underlay adaxial
stomata as organs of floatation in Wolffia columbiana.
Control of watermeal may be achieved using herbicides such as Fluridone (Kay,
1991), however, restrictions and strong public opinion in urban environments
may make the use of such chemicals difficult. Observations by
Kay (1991) suggested the surfactant rather than the expected active compound
in herbicide preparations may play a role on suppression of the watermeal
W. columbiana and to confirm this using low toxicity detergents for control
by disruption of floatation. Tween 20 is amenable to remediation and therefore
suitable for use in the inland waterways of New Jersey.
MATERIALS AND METHODS
Chemical and physical parameters in Godfrey Lake, New Jersey, were measured
by wading into the lake and using an automatic chemical analysis device i.e.,
a YSI 6600 EDS data sonde equipped with multiple sensors. Measurements of water
temperature, salinity, dissolved oxygen and pH were made along two transects
(Fig. 1). The sensor was placed at the transect points defined
in Fig. 1, the physical parameters water temperature, salinity,
dissolved oxygen and pH were measured, then the sensor moved to the next transect
point and measurement repeated. Samples of W. columbiana were collected
from Godfrey Lake during May 2008 and used to initiate tank colonies in the
Monmouth University Laboratory. Plants from tanks were then transferred to a
series of 25 mL conical flasks containing Godfrey Lake water under 19 μE
m-2 sec-1 from fluorescent lamps which approximated environmental
conditions. Dilute Tween 20 was added to flasks in order to create the range
of concentrations from 0 to 0.1% v/v Tween 20 in quintuplicate as described
in Fig. 2.
||The location and environmental parameters of Godfrey Lake.
The Lake is well mixed as shown by two transects with measurements for temperature
(°C), pH, dissolved oxygen (mg L-1) and salinity (ppt)
||(a) W. columbiana were incubated in 25 mL Godfrey Lake
water supplemented with small amounts of the detergent Tween 20. Natural
shaded daylight (19 μE m-2 sec-1) was used and
sunk plants counted periodically. Treatments were in quintuplicate, i.e.
a total of thirty flasks were used, then averages calculated. (b) Typical
flask of W. columbiana in plain Godfrey Lake water. Most are healthy
and floating. (c) Typical flask of W. columbiana in Godfrey Lake
water plus 0.1% v/v Tween 20. By day five most are beneath the meniscus
and entangled in hyphae
Volume was made up to 25 mL using Godfrey Lake water and a uniform amount
of W. columbiana added by spooning with a disposable spatula i.e., 1
cm2 of floating plant material. Incubation was continued with periodic
inspection and numbers of W. columbiana were counted as they sank (Fig.
RESULTS AND DISCUSSION
Many herbicide preparations contain detergents to dissolve hydrophobic components
and wet plants to ensure efficient absorption of active compounds from a soapy
solution that contacts all parts of plants (Showell, 2005).
It is assumed that the herbicide is the effective compound and that the water
or detergent plays only the minor role of increasing efficiency of herbicide
delivery. However, aquatic plants occupy a special situation not usually considered
in the formulation of herbicide preparations. Aquatic plants may sink then decay
and are particularly dependent on surface tension to keep in contact with the
atmosphere, making them vulnerable to detergents, which reduce surface tension
Chemical and physical characterization of Godfrey Lake shows that it is a typical
Pine Barrens lake with low salinity and pH suitable for rapid growth of Wolffia
columbiana (Fig. 1). Treatment of Watermeal with Tween
20 confirmed that detergents accelerate watermeal sinking (Fig.
2a). In addition, an unexpected result was seen. Water fungi which were
previously commensal proliferated and watermeal plants were entangled in clumps
as they submerged (Fig. 2b, c). These fungi
were stained with Methylene blue and examined under bright field microscopy.
No spores but septa were seen in hyphae that invaded plant tissue. It is assumed
that detergents compromised the protective cuticle and allowed opportunistic
fungi to flourish at the expense of these plants. This effect was seen for concentrations
of Tween 20 from 0.04 to 0.01% v/v but was marked for 0.08% v/v and 0.10% v/v
Tween 20 where counting of individual plants became difficult after 4 days (Fig.
Many of the herbicides currently used to control duckweed are harmful to people
(Michel et al., 2004). In contrast, some detergents
have comparatively low toxicity (Desalva, 1982). This
phenomenon may be further developed as a method of control for floating nuisance
plants in urban environments; even the United States Food and Drug Administration
has granted generally regarded as safe status to several detergents such as
stearic acid (USFDA, 2009). The procedure depends on altering
the interactions of watermeal with other species, in particular water fungi,
by compromising their defensive cuticle. Wetting and clump formation is promoted
and watermeal are weighted down then sink. Once sunk, watermeal will not shade
other plants and may be subject to the decay processes they would otherwise
inflict on others.
John Tiedemann is an inspiration to us all.
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