A comparative study of the nutrient responses of the invasive duckweed Lemna minuta, and the native, co-generic species Lemna minor

• L. minuta outgrows L. minor in all treatments investigated, except in those with a low amount of P-PO43−.
• L. minuta shows tolerance to imbalances in Ca-Mg ratio in surface waters.
• Possible key factors responsible for the invasibility of freshwater ecosystems were identified providing a focus for further studies on aquatic plants invasion.

Invasive alien plant species are usually characterized by a growth rate higher than their native competitors, and this higher rate can be achieved through the opportunistic use of plant nutrients. The growth of the invasive alien duckweed Lemna minuta and the co-generic native Lemna minor were compared under different conditions of nutrient availability. The two species were grown for one week under fully controlled conditions on medium containing increasing concentrations of NO3−-N, NH4+-N and PO43—P. The effects of different concentrations and ratios of Ca and Mg on growth were also determined. At the end of the experiment the Relative Growth Rate (RGR) of the plants was calculated on the basis of the biomass and number of fronds. The data highlighted that Lemna minuta outgrew L. minor under conditions of high phosphate supply while, when phosphate concentrations in the medium were low, it grew less than L. minor. However, L. minuta is not simply an opportunistic species as we observed relative tolerance to imbalances in Ca-Mg ratio in the growth medium. The different responses observed when the L. minor and L. minuta were grown at specific concentrations of P, Ca and Mg suggest that these elements are potential determinants of the invasibility of freshwaters and that they should be considered in future field studies.