Duckweed species native to Cache Valley (northern Utah)

Lemnacea (common name duckweed) grows naturally in almost every region with a growing season of at least five months. Most studies involving duckweed take place in climates with 9-10 month growing seasons. More rare are the duckweed studies taking place in regions like Cache Valley with only 5-7 month growing seasons [Culley, 1981]. Duckweed is a C3 plant—which helps grow in colder climates with shorter growing seasons. Since water freezes in the winter and duckweed floats on water, it does best in warmer climates. Nonetheless, of the four principal duckweed genera three are still found in Cache Valley. There are three principal species from this genera in Cache Valley, all of which are reported to be cold tolerant. The duckweed plants growing in Cache Valley Utah include Lemna turionifera (or L. minor), Wolffia Borrealis, and Spirodela Polyrhizza. Duckweed’s native presence in Cache Valley, its resilience to temperate climates, and its fast growth rates make it promising for nutrient removal.

The location of the Cache Valley duckweed varies depending on the species. L. minor/turionifera and W. borealis grow in a mixed culture on the Wellsville Municipal Sewage lagoons (56 acres). L. minor is more predominant in the field than Wolffia. The Wellsville lagoons receive some sheltering from wind due to their location in a recessed area bordered by trees along the Little Bear River (Fig. 1, left). These two species are only found in the parts of the Logan Wastewater lagoons that are protected from wind (i.e. culverts, chlorination basin, and wetlands). The majority of the 460 acre Logan lagoons are not protected from wind and do not contain duckweed (Fig. 1, right). The third Cache Valley species, S. Polyrhizza, can be found up Logan Canyon near third dam on the north side of the highway. These duckweed species, especially L. minor have adapted to temperate climates like that of northern Utah [Culley; Landolt 1986 p.421]. During the three years this study took place, full-duckweed coverage on the Wellsville lagoons occurred at the first part of May and continued until about the third week in November. The wastewater lagoons freeze during the winter forcing the duckweed plants into dormancy; however, duckweed fronds appear in the water at the first sign of ice melting off the lagoons in the Spring and occasionally on top of the ice in small puddles of water during particularly warmer periods during winter (Fig. 2).

The duckweed species native to Cache Valley need to tolerate temperate climates with cold winters. Spirodela polyrhizza, Wolffia borealis, and Lemna turionifera all develop special fronds called "turions" when the weather starts to cool. Turions are overwintering buds rich in starch. In the case of L. turionifera they look like single dark green fronds. In the case of W. borealis, they look like tiny spherical balls. Turions have a higher specific gravity than water. During winter, turions sink down into the sediments and then emerge under warmer conditions thus enabling them to survive freezing weather (Landolt, 1986, pp.421). Unlike the other three species, Lemna minor produces "resting fronds." Resting fronds look like the turions from L. turionifera but they do not sink to the sediment. The resting fronds also have a specific gravity > 1 which enables them to sink below the water surface and avoid being frozen at the surface by ice.

Figure1 : Duckweed growing atop 56 acres of Wellsville Municipal Sewage Lagoons which sits in a basin, but not growing atop 460 acres of Logan City lagoons.

Figure 2: (Left) L. minor and Wolffia duckweed species appearing immediately after the ice melts ca. March and April. (Right) L. Turionifera turions or L. minor "resting fronds" floating on 3 cm. water puddle above the ice on Feb. 4, 2011, following unusual rainy winter weather.

Lemnaceae species increase in size from Wolffia, L. minor, and S. polyrhizza species, respectively. Several factors contribute to the size of the fronds. Daughter fronds in lab studies are often smaller than the mother fronds (Al-Nozaily) which needs to be considered when basing growth rate on frond count. As plant density increases frond size often decreases. Landolt listed other factors contributing to an increase in frond size, including: increased light intensity; increased light duration; addition of sugar; increased nitrogen, phosphorus, potassium, calcium, and magnesium concentrations (which can also decrease frond size if too high); and increased temperature (Landolt, 1986, pp.36-37). Typically, full-size fronds for Wolffia, L. minor, and S. Polyrhizza range from 0.5-1mm, 3-5mm, and 1-1.5cm, respectively. L. minor fronds ranged in sizes depending on the development stage of the frond (i.e. whether a bud to a frond or fully separated frond w/ or w/o buds). Figure 3 show how L. minor fronds were characterized depending on development. The Lemnaceae species used in these laboratory experiments had a density of 815ug/ml (s.g. 0.82).

Figure 3: Characterization and digital imaging of L. minor and Wolffia fronds

Table 1: Characterization of L. minor fronds by area, mass, and dimensions

Table 2: Characterization of Wellsville duckweed by elemental composition

Some aquatic species tolerate cold climates as well or better than duckweed but unlike duckweed they do not currently grow on Wellsville nor Logan wastewater lagoons in Cache Valley. Watercress (Nasturtium officinale), have the ability to grow during the harsh Cache Valley winters provided that they are near flowing water like springs [Michaelis, 1976; personal observation]. Watercress grows during the winter in a canal running adjacent and north of Canyon Road in Logan, Utah, near the Utah Water Research Laboratory. Duckweed favors calm water, unfortunately, this turns to ice and limits the duckweed growing season to approximately six months in Cache Valley.

Azolla and pennywort have also been recommended as frost tolerant aquatic plants; unfortunately, their growth rates are lower than duckweed’s and they are not yet established on Cache Valley lagoons [personal correspondence with Louis Landesman, 2/12/09]. Duckweed species already grow on wastewater in Cache Valley and the literature about them is prolific. It is doubtful any other floating aquatic plant could outperform the duckweed since ice formation, not frost resistance, is the limiting factor of winter growth on wastewater lagoons in Cache Valley.