Plant Biology and Issues

Identification

Duckweeds are tiny free-floating aquatic plants and are the smallest angiosperms [1], although they are often mistaken for algae. They float on or just beneath the surface of still or slowly moving bodies of fresh water and wetlands. The common term duckweeds refers to species belonging to the Lemnoideae subfamily, which includes five genera: Lemna, Spirodela, Wolffia, Wolffiella and Landoltia. These plants have a simple structure, lacking an obvious stem or leaves. Depending on the species, each plant may have no roots (Wolffia, Wolfiella) or may have one (Lemna) or more simple roots (Spirodela, Landoltia)[1] (Figure 1).

Diversity of duckweedsjpg

Figure 1 – Diversity of duckweeds species of Louisiana:
(a) W. columbiana (b) W. brasiliensis; (c) W. gladiata; (d) L. minuta; (e) L. valdiviana;
(f) L. minor; (g) L. turionifera; (h) L. aequinoctialis; (i) L. gibba; (j) L. punctata; (k) S. polyrrhiza
(Pictures by Flaminia Mariani, 2021).

Reproduction

Reproduction is mostly vegetative, but occasionally, simple flowers are produced, by which sexual reproduction occurs (Figure 2a). These consist of one or two staminate flowers and one pistillate flower (Figure 2b). The flower of the genus Wolffia is the smallest flower ever known, measuring only 0.3 mm in length [1].

Landoltia punctata plant in flowerjpg

Figure 2 - (a) Landoltia punctata plant in flower; (b) detail of the flower showing
two stamens (white arrow) and a pistil (red arrow) (Pictures by Flaminia Mariani, 2021).

Habitat and Issues

Duckweeds can proliferate rapidly in fertile and eutrophic waters [2], where they form extensive mats covering the surface of the water body (Figure 3). These large floating multilayer mats of duckweed limit light penetration to the underlying water column and reduce air-water gas exchange thereby decreasing dissolved oxygen in the water. The resulting negative impacts on other aquatic plants, macroinvertebrates and fish can disrupt ecosystem function and warrants the control of invasive duckweed species [3, 4].

Duckweed infestationjpg

Figure 3 - Heavy duckweed infestation of small waterbody in St. Francisville, LA (Pictures by Carlos Wiggins, 2021).

Distribution

Duckweeds are widely distributed throughout the United States. In Louisiana there are twelve species of native duckweeds: six in the genus Lemna (L. minor, L. minuta, L. perpusilla, L. valdiviana, L. obscura, L. aequinoctialis), one in the genus Spirodela (S. polyrrhiza), two in the genus Wolffia (W. brasiliensis, W. columbiana), and three in the genus Wollfiella (W. gladiata, W. lingulata, W. oblonga) [5]. The only non-native duckweed species present in Louisiana is Landoltia punctata [5].

Economic Value

Duckweeds grow rapidly, quickly producing new offspring. In Louisiana dense duckweed populations are an important food source for many animals that share the same habitat such as fish and ducks. Some duck species, especially surface-feeders like mallards, teals and wood ducks, consume large quantities of duckweed [6]. This explains the origin of the common name duckweed by which Lemnoideae species are usually known.

Duckweed populations are a valuable resource for some animals, but can become a nuisance to humans. In recent years researchers have been harnessing the high growth rate and nutrient uptake of duckweeds for bioremediation of streams with high phosphorus and nitrogen loads from agricultural runoff. Harvesting duckweeds from these waters allows the removal of pollutants from the water while providing a valuable new livestock feed or fertilizer resource [7, 8].

Chemical and Manual Control

Duckweeds species are very sensitive to herbicides, such as diquat, bispyribac, flumioxazin, fluridone, and penoxsulam. [9, 10]; however, herbicides could be also detrimental to some local aquatic flora and fauna, directly or through bioaccumulation [10]. Therefore, the use of herbicides should follow the recommendations by the manufacturer.

Physical control by removing the duckweed biomass with handheld fine mesh nets can only temporally mitigate the problem because of the very rapid re-colonization ability of these plants, even starting from few fronds that could remain in the site. Under optimal growing conditions, some species of duckweed can duplicate their biomass in two to three days [1], and thus re-colonize an entire waterbody quickly [11]. Moreover, the small size of these plants makes it quite difficult to remove [11].

References

[1] Landolt, E., 1986. Biosystematic investigations in the family of duckweeds (Lemnaceae) Vol. 2: The family of Lemnaceae: a monographic study. – Morphology, karyology, ecology, geographic distribution, nomenclature, descriptions. Zürich: Eidgenössische Technische Hochschule Zürich.

[2] Keddy, P.A., 2010. Fertility. Wetland Ecology: Principles and Conservation (2nd ed.). Cambridge: Cambridge University Press.

[3] Dussart, G., Robertson, J., and Bramley, J., 1993. Death of a lake. Biololgical Sciences Review 5 (5), 8–10.

[4] Janes, R.A., Eaton, J.W., and Hardwick, K., 1996. The effects of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth on the growth of submerged macrophytes. Hydrobiologia 340 (1-3), 23–26.

[5] Weakley, A.S., 2020. Flora of the southeastern United States. University of North Carolina Herbarium, North Carolina Botanical Garden, Chapel Hill, NC.

[6] Men, B., Ogle, B., and Lindberg, J., 2002. Use of Duckweed as a protein supplement for breeding ducks. Animal Bioscience; 15(6), 866–871.

[7] Gupta, C., and Prakash, D., 2013. Duckweed: an effective tool for phyto-remediation. Toxicological & Environmental Chemistry, 95(8), 1256–1266.

[8] Ceschin, S., Crescenzi, M., and Iannelli, M.A., 2020. Phytoremediation potential of the duckweeds Lemna minuta and Lemna minor to remove nutrients from treated waters. nvironmental Science and Pollution Research 27, 15806–15814

[9] Dial, N.A., Bauer, C.A., 1984. Teratogenic and lethal effects of paraquat on developing frog embryos (Rana pipiens). Bull. Environ. Contam. Toxicol. 33, 592-597.

[10] Rzymski, P., Klimaszyk, P., Kubacki, T., Poniedziałek, B., 2013. The effect of glyphosate-based herbicide on aquatic organisms – a case study. Limnol. Review 13 (4), 215-220.

[11] Ceschin, S., Della Bella, V., Piccari, F., Abati, S., 2016. Colonization dynamics of the alien macrophyte Lemna minuta Kunth: a case study from a semi-natural pond in Appia Antica Regional Park (Rome, Italy). Fundam. Appl. Limnol. 188 (2), 93-101.

[12] Center, T.D., Dray, F.A.Jr., Jubinsky, G.P., and Grodowitz, M.J., 2002. Insects and other arthropods that feed on aquatic and wetland plants. USDA/ARS Technical Bulletin 1870.

[13] Kipp, R.M., J. Larson, and A. Fusaro, 2021. Tanysphyrus lemnae Paykull/Fabricius, 1792: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI, https://nas.er.usgs.gov/queries/greatLakes/FactSheet.aspx?Species_ID=2363&Potential=N&Type=0&HUCNumber=DGreatLakes, Revision Date: 12/6/2019, Access Date: 8/4/2021

[14] Mills, E.L., Leach, J.H., Carlton, J.T., and Secor, C.L., 1993. Exotic species in the Great Lakes: a history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19(1):1–54.

[15] Thorpe, W.H., and Crisp, D.J., 1949. Studies on plastron respiration. IV. Plastron respiration in the Coleoptera. Journal of Experimental Biology 26:219-260.

[16] Buckingham, G., 1989. Lemnaphila scotlandae (Diptera: Ephydridae) and three of its parasites discovered in Florida. The Florida Entomologist, 72(1), 219–221.

[17] Scotland, M.B., 1934. The animals of the Lemna association. Ecology 15 (3).

[18] Scotland, M.B., 1940. Review and summary of studies of insects associated with Lemna minor. Journal of the New York Entomological Society, 48(4), 319–333.

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