LAUSR

Abstract Stratiotes aloides L. (water soldier) is a large representative of Hydrocharitaceae family, which in summer produces a large biomass floating on the water and is known for high demands for space or nutrients and for its allelopathic activity. We examined the effects of this macrophyte on the water in its stands, specifically N and P concentrations, and on resulting effects on the internal N and P concentrations, the relative growth rates (RGR), colony disintegration and turion formation of the free-floating Spirodela polyrhiza in outdoor ex-situ experiments. Natural populations of duckweeds accompanying dense, extensive Stratiotes stands or growing in water bodies free of Stratiotes were cultivated for short periods in original water or in this water enriched with ammonium and phosphates. Waters from Stratiotes stands were poorer in dissolved inorganic nitrogen concentrations but did not decrease tissue N concentrations, RGR or colony sizes of Spirodela. Nutrient enrichment and Spirodela origin neither affected RGR nor colony disintegration of Spirodela. In a long-term experiment, Stratiotes stand water caused a shift in duckweed developmental processes from vegetative frond production to turion formation, manifested mainly by high total number of turions and high number of turions per frond at the end of the long-term experiment and by the fast rate of turion formation. Further, it seems that this water exerted a long-lasting effect on neighbouring populations of Spirodela and shortened their lifespan.