Return of the duckweed...!

After an absence of nearly five years, duckweed is being re-established at Ireland’s only large-scale duckweed cultivation site, the Mt Lucas fish farm. The Mt Lucas site integrates fish cultivation with duckweed wastewater valorisation (see Mount Lucas Circular Bioeconomy Aquaculture Initiative) according to the principles of Integrated Multitrophic Aquaculture (IMTA) (Stejskal et al., 2022). Previously, duckweed was successfully cultivated in 2019, 2020 and 2021, with yields of nearly 40 tonnes dry duckweed per year (Paolacci et al., 2022). 

The re-established Mt Lucas site, generously funded by the Department of Agriculture, Food and the Marine (Ireland), has a broad remit to demonstrate generation of high-value commercial bio-products at scale, utilising local agri-food waste streams in the Irish Midlands Just Transition Territory. This means that apart from cultivation of rainbow trout and duckweed, there is a strong focus on production of freshwater macroalgae, development of innovative products through biorefinery activities, digital monitoring of water quality as well as  fish and duckweed growth for performance modelling, pioneering of open data portal and content sites and especially, engaging and strengthening of the local bioeconomy.

Figure 1. Duckweed pool (6 m²) in a polytunnel as duckweed nursery. 

Figure 2. One of the three large (10 m²) duckweed growth tanks for upscaling and trials

Margot Clair, a talented UCC research assistant based at Mt Lucas, has kick-started Lemna minor production by establishing a 6 m² duckweed nursery in a polytunnel (figure 1) as well as three large (10 m²) tanks for further upscaling and trials (figure 2). Produced duckweed biomass (figure 3) is used to establish a population in the one-hectare system of canals at the fish farm.

Establishing duckweed at the fish farm is a serious challenge, clearly demonstrating the substantial and unexpected environmental impediments that occur when upscaling to outdoor conditions. These impediments comprise vigorous growth of various submerged macrophytes that compete for plant nutrients, as well as the presence of a large family of duckweed-consuming swans. Competition by submerged macrophytes is expected to lessen considerably once a “light-blocking” duckweed mat has formed.

Figure 3. Duckweed biomass

Consumption of duckweed biomass by swans is a substantial challenge especially in the early stages of duckweed establishment, when the ratio of “plant-to-swan” is low, but is expected to cease to be an issue once duckweed biomass increases and daily duckweed growth exceeds daily consumption by swans. For now, trials of swan-proof barriers are taking place (figure 4) while duckweed gets established. In the meantime, we are proudly appreciating the biodiversity present at the organically farmed Mt Lucas site.