Effects of the allelopathically active macrophyte Myriophyllum spicatum on a natural phytoplankton community: a mesocosm study

We investigated the effects of Eurasian watermilfoil (Myriophyllum spicatum) on a natural phytoplankton community in 85 l mesocosms. Changes in phytoplankton community composition, biomass, primary productivity, nitrogenase activity and the abundance of toxic and non-toxic Microcystis aeruginosa subpopulations were investigated during 13 days of exposure. We aimed to test whether the known allelopathic activity of M. spicatum towards certain algae and cyanobacteria could be proven under field-like conditions with potential interference of resource competition and zooplankton grazing. The presence of M. spicatum had only short-term inhibitory patterns on total phytoplankton and green algae, whereas consistent negative effects were observed on cyanobacteria biomass. Quantitative PCR analyses revealed that toxic and non-toxic M. aeruginosa were equally inhibited. Total phytoplankton gross primary production was significantly suppressed by the presence of M. spicatum. Despite the partial interference by nutrient limitation, there are indications for a significant impact of macrophyte-excreted allelo-chemicals on phytoplankton. We argue that more mesocosm experiments with complex natural phytoplankton communities are needed to unravel the ecological relevance of macrophyte allelopathy.