PRIMARY SUCCESSION OF AQUATIC MACROPHYTES IN EXPERIMENTAL DITCHES IN RELATION TO NUTRIENT INPUT

In four experimental ditches, receiving four different levels of exter nal nutrient input, ranging from as low as possible to a very high ext ernal loading rate, the development of the species composition of the aquatic macrophyte communities was monitored during a period of 3.5 ye ars (May 1989- October 1992). The rate of primary succession was posit ively related to the level of external loading with nutrients (N and P ). At the start of the nutrient loading programme, 1 year after the co nstruction of the ditches, the vegetation was in a pioneer stadium. It consisted mainly of Characeae, with an abundance, expressed as areal coverage, of over 90% in all four ditches. After the start of the prog ramme the Characeae disappeared and were replaced by Elodea nuttallii (Planch.) St. John in all four ditches. The rate of the transition was positively related to the level of external nutrient loading. In a di tch receiving a very high nutrient input, Elodea nuttallii was in turn replaced by Lemna minor L. At two intermediate levels of nutrient loa ding Elodea has remained stable towards the end of the research period , performing a horizontal growth pattern, and forming a dense near-sur face biomass with an areal coverage of almost 100% during the summer. At the lowest level of nutrient input Elodea nuttallii reached an area l coverage of almost 50% in October 1992, still performing a vertical growth pattern.