Original paper

Experimental analysis of the competition between algae and duckweed

Roijackers, R.; Szabó, S.; Scheffer, M.


We performed indoor competition experiments between algae and Lemna gibba L. in order to unravel mechanisms of competition. To separate effects of shading and physical interference from nutrient competition we grew the two groups physically separated while sharing the same water. A multifactorial design was used with five levels of initial nitrogen concentration (0.1– 50 mg N l–1) and four shade levels mimicking 0 –100 % duckweed shade on the algal compartment. In the experiments in which algae were not shaded, the growth rate of Lemna was reduced strongly (60 – 62 %) at moderate initial nitrogen concentrations (0.1–1 mg N l –1). The impact of algae was less at high N-loading and if algae were shaded. The reduction in the chlorophyll content of the fronds was even more dramatic (72 – 80 %). Analyses of nutrients and pH indicated that algae inhibited the growth of Lemna by the removal of N, P and Fe, but also by their photosynthetic effect on pH. When algae and duckweeds were grown together, floating algae occurring at high nutrient levels partly covered the duckweeds and reduced the growth further than in the absence of physical contact. Since under those conditions Lemna growth was still marked, it seems likely that on the long run Lemna will always expand sufficiently to outcompete the algae at high nutrient levels.