Lake Zwemlust (area 1.5 ha, Z m 1.5 m) has been the object of an extensive limnological study since its biomanipulation involving removal of planktivorous fish (bream) in March 1987 and emptying of the lake. In the subsequent summer period of 1987 the Secchi depth increased to the lake bottom (2.5 m), compared with ca 30 cm in the earlier summers. The reaction of submerged macrophytes to improving under-water light climate was rapid. In summer 1987, besides the introduced Chara globularis, 5 species of submerged macrophytes occurred and colonized 10~o of the lake area. In 1988 and 1989 only quantitative changes were observed; new species did not appear, but the area colonized by macrophytes increased by 7 and 10 times, respectively. Elodea nuttallii was dominant among the macrophytes and Mougeotia sp. among the filamentous green algae. Their abundance, contributed to transient N-limination of phytoplankton causing a persistent clear water phase in 1988 and 1989, unlike in 1987 when zooplankton grazing contributed chiefly to the water clarity. Laboratory bioassays on macrophytes confirmed nitrogen limitation.

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The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH,-N l-' and 2 mg NH,-N l-' plus 2 mg N03-N l-'. During a 14 day experiment, plants absorbed from 75 % to 90% of nitrogen. Higher nitrogen concentration than 4 mg l-' had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NO,+ over NO, when both ions were present in water in equal concentrations.

Lake Zwemlust (area 1.5 ha, Z m 1.5 m) has been the object of an extensive limnological study since its biomanipulation involving removal of planktivorous fish (bream) in March 1987 and emptying of the lake. In the subsequent summer period of 1987 the Secchi depth increased to the lake bottom (2.5 m), compared with ca 30 cm in the earlier summers. The reaction of submerged macrophytes to improving under-water light climate was rapid. In summer 1987, besides the introduced Chara globularis, 5 species of submerged macrophytes occurred and colonized 10~o of the lake area. In 1988 and 1989 only quantitative changes were observed; new species did not appear, but the area colonized by macrophytes increased by 7 and 10 times, respectively. Elodea nuttallii was dominant among the macrophytes and Mougeotia sp. among the filamentous green algae. Their abundance, contributed to transient N-limination of phytoplankton causing a persistent clear water phase in 1988 and 1989, unlike in 1987 when zooplankton grazing contributed chiefly to the water clarity. Laboratory bioassays on macrophytes confirmed nitrogen limitation.