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Marked differences in physiological and morphological traits have been found between different species of Lemnaceae, and between different clones of species. Traits like relative growth rates, salt tolerance, and starch content can vary a lot. This makes different clones and/or species more suitable for some applications than others. This also triggers the question, how to prevent the mixing of “undesirable†species or clones with selected Lemnaceae when these are grown under outdoor conditions for applications such water remediation. Perhaps more fundamentally it triggers the question, how do duckweeds disperse?
Dispersal
In the case of Lemnaceae, it has been argued that rapid drying out of fronds will limit the distance of dispersal, and that the frequency of transport will be low. However, the reality appears different. Neil Coughlan developed a simple system to quantify dispersal of
L. minor. Quite surprisingly, Neil observed a total of 67 separate dispersal events (transfer of at least one frond) over a period of 20 weeks, and across 6 replicate stake and bowl structures. In total 156 colonies comprising 317 fronds were found to be transferred to receiving bowls in a relatively short period (full details see Coughlan et al., 2017), and this was attributed to birds. The question remains, however, over what distances Lemnaceae can be dispersed, a question that focusses heavily on desiccation tolerance of the plants.
Dessication tolerance
Lemna minuta taken out of the aquatic medium was found to have lost viability after just 90 minutes at a Relative Humidity (RH) of 44% and a temperature of 21ËšC (Coughlan et al., 2015). At a slightly higher RH of 58% (T = 23ËšC)
Lemna minuta still displayed some viability after 4 hours out of the aquatic medium (Coughlan et al., 2015). Neil Coughlan’s research showed that between the feathers near the posterior neck of a mallard duck, the RH is around 65% and the temperature 23˚C. Near the inner crural (upper part of the leg), the RH is even higher at around 77% with a temperature of 24˚C. Interestingly, the downy feathers of the inner crural were also found to retain entangled Lemnaceae fronds more effectively than areas of less downy plumage, such as the back of the neck. All in all, we reckon that
Lemna minuta can be entangled between feathers, and survive flights of up to four hour’s duration. Given an average speed for mallards of 65 km/h
-1, we argue that duckweed dispersal over distances of up to 250km is realistic, although much shorter distances (< 50km) are likely more common. This underlines the mobility of Lemnaceae.
Practical considerations
So where does that leave the duckweed industry? There are two practical considerations for Lemnaceae cultivation systems: (1) preventive steps need to be taken if one wants to avoid bird-mediated contamination of an outdoor Lemnaceae culture (e.g. dilution of a selected clone by non-selected, native clones) (2) preventive steps need to be taken to avoid introduction of selected alien species or clones into the local environment. At present, substantial efforts are involved in control of
Landoltia punctata in Florida USA, where this is an alien, invasive species. Similarly,
Lemna minuta is the focus of management efforts in various European countries. There is absolutely no evidence that the introduction of
L. punctata in Florida, or
L. minuta in Europe is associated with cultivation of these species by the Lemnaceae industry. Nevertheless, the industry needs to adopt a responsible approach when cultivating alien species of Lemnaceae, and prevent their spread in to the surrounding environment in order to maintain the positive public perception of duckweed applications as being eco-friendly and sustainable.
Citations
Coughlan N.E., Kelly T.C., Jansen M.A.K., 2015. Mallard duck (
Anas platyrhynchos)-mediated dispersal of Lemnaceae: a contributing factor in the spread of invasive
Lemna minuta?
Plant Biology17, 108–114. Coughlan, N.E., Kelly, T.C. and Jansen, M.A.K., 2017. “Step by stepâ€: high frequency short-distance epizoochorous dispersal of aquatic macrophytes.
Biological Invasions19, 625-634.
Figure1: A male mallard duck (Anas platyrhynchos), which has just left a Lemna-covered pond, takes some “hitch-hiking†duckweed across land on a foraging trip (image © Roy Battell).[/caption]