Eutrophication of shallow waters has often been associated with the decline of submerged macrophytes. In this study, a conceptual model as a working hypothesis for the mechanism responsible for this decline was used to generate the research topics. Aims of the study were to better understand the mechanism by testing parts of the hypothesis, and to compilate the results in a simulation model, a.o. for management purposes.The model hypothesis used suggests that periphyton development acted as a trigger for macrophyte decline, with a postponed phytoplankton reaction due to allelopathic growth limitation by substances excreted by the macrophytes.We selected one abundant macrophyte species, Potamogeton pectinatus L., and studied growth, photosynthesis and plasticity. Tuber size appeared to be an important factor for the plant that developed from it. Temperature and light influenced growth nonlinearly and interactively. With increasing age, maximal photosynthetic rate and respiration decreased. Light level during gro wth also affected photosynthesis. With respect to photosynthetic performance and stem elongation capacity, P. pectinatus can be characterized as 'average' when compared to other macrophyte species.Periphyton development on glass slides under eutrophic conditions in the laboratory was comparable to that in the field as found by other authors. Temperature distinctly affected the growth curves, whilst light did so for the attenuance-density curves of the experimental communities. In the studied light and temperature range, periphyton attenuated about 50% of the light within 3-4 weeks.All four tested freshwater snail species significantly removed periphyton from glass slides, whilst the two tested crustaceans did not. Differences in removal between and within species could be explained largely by differences in snail activity, snail size and taxonomic composition of the periphyton. Temperature effects were rarely found. From the plant's point of view, a population of Lymnaea peregra (Müll.) appears to be the ...