Submersed macrophytes have to compete with epiphytes and phytoplankton for light and nutrient resources. Frequent observations show alternating stable states of either submersed macrophyte or phytoplankton dominance in shallow eutrophic lakes (Scheffer et al. 1993). The production and excretion of allelochemicals may enable submersed macrophytes to dominate over phytoplankton despite abundant nutrient availability in the open water.

The allelopathic potential to suppress phytoplankton of 19 macrophytes was determined. Special emphasis was placed on MYRIOPHYLLUM spp. and related species. Nine of the tested species exhibited a strong algicidal and cyanobactericidal activity (CERATOPHYLLUM DEMERSUM, C. SUBMERSUM, FONTINALIS ANTIPYRETICA, HOTTONIA PALUSTRIS, MYRIOPHYLLUM spp., and PROSERPINACA PALUSTRIS). MYRIOPHYLLUM and PROSERPINACA belong to the Haloragaceae. Members of this family showed the highest inhibitory activity which is based on hydrolyzable polyphenols as active compounds. All tested Haloragaceae contain very high concentrations of phenolic compounds (6 - 15 % of dry weight). These concentrations are about one order of magnitude higher than that measured in other submersed macrophytes.

The main allelochemical from M. SPICATUM (milfoil) was isolated and structurally identified. This hydrolyzable polyphenol, Tellimagrandin II, is present at concentrations of about 1 - 1.5% of the dry weight. Different, yet unidentified, hydrolyzable polyphenols account for the algicidal activity of other tested Haloragaceae. An axenic culture of milfoil was established. Axenic and field-sampled milfoil did not exhibit different patterns of polyphenols and Tellimagrandin II, suggesting that these defensive compounds are not induced by epiphytes. This allowed the controlled study of released compounds into the surrounding medium with axenic plant material. Milfoil exudes several polyphenolic compounds, among them traces of Tellimagrandin II, ellagic acid and three as yet unidentified hydrolyzable polyphenols. Signals for these exuded compounds were also found in the culture filtrate of non-axenic plants, but due to fast bacterial degradation these signals faded out with increasing incubation time. Both plant-extracted, among them Tellimagrandin II, and exuded polyphenols exhibited a strong inhibition of algal extracellular alkaline phosphatase activity (APA) of various algae and cyanobacteria as well as a natural sample of epiphytes.

Thus, M. SPICATUM as well as other Haloragaceae and submersed macrophytes may interfere with vital functions of epiphytes and phytoplankton. These interactions may help maintain dominance of submersed macrophytes, such as M. SPICATUM, in shallow eutrophic lakes.