1. It is well known that submerged macrophytes can suppress phytoplankton blooms in lakes and thus promote water quality and biodiversity. One of the possible mechanisms through which submerged macrophytes control phytoplankton is by producing allelochemicals that suppress phytoplankton growth rates. The in situ importance of allelopathy, however, is often questioned because it is assumed that phytoplankton communities can rapidly evolve resistance to allelochemicals.
2. Here, we present the results of two mesocosm experiments in which we evaluated whether the submerged macrophyte Elodea nuttallii is capable of controlling phytoplankton biomass over periods of 4 to 8 weeks. Such a time scale is long relative to the generation time of phytoplankton and is therefore expected to allow the development of resistance through compositional shifts at both population and community levels.
3. Although the mesocosms were inoculated with a diverse phytoplankton inoculum including species that had previously been exposed to Elodea, phytoplankton biomass remained consistently low during the course of the experiments in the treatments with Elodea. As zooplankton grazing and competition for nutrients and light by macrophytes were excluded in our experiments, this suggests that phytoplankton was controlled by allelopathy.
4. Dialysis bag assays, performed at the end of each mesocosm experiment, showed that phytoplankton communities from mesocosms with Elodea were equally sensitive to exudates from Elodea than phytoplankton communities from mesocosms without Elodea.