AbstractThe extent of decline of reed belts (Phragmites australis) during last decades has been established by using aerial photographs of the mesotrophic lake Parsteiner See and the eutrophic lake Müggelsee. Final standing crop and nitrogen accumulation within the above‐ground plant biomass of 15 different sites of 8 lakes correlate with nitrogen availability indicated by both dissolved nutrient content of the sediment and nitrogen content of biomass during the main growth period. Reed stands well supplied with nitrogen show high shoot loss and compensatory tiller growth till late in the season. Some results demonstrate a delayed switching from the vegetative phase to the generative one and lowered allocation of reserve material to the rhizome as at least one of the factors responsible for lowering stability of the reed be