Graphic presentation of weekly rates of change of algal biomass (expressed as chlorophyll a) and nutrient and dissolved oxygen concentrations can be regarded as harmonic oscillation motion. Maximum amplitudes of these oscillations provide a useful tool to assess the degree of stability of aquatic ecosystems in relation to their trophic state. Data sets from seven different lakes ranging from hypereutrophic to oligo-mesotrophic were processed using a computerized method. The high values of oscillation amplitudes of approximately 150 μg l−1 wk−1 chlorophyll a, 500 μg l−1 wk−1 ammonia nitrogen, 50 μg l−1 wk−1 soluble reactive phosphorus and 10 mg l−1 wk−1 dissolved oxygen, indicated strong ecosystem instability, while low values of less than 10 μg l−1 wk−1 of chlorophyll a, 20 μg l−1 wk−1 ammonia nitrogen, 2 μg l−1 wk−1 soluble reactive phosphorus, and 3 mg l−1 wk−1 dissolved oxygen represented a stable system. Oscillation amplitudes of the chlorophyll a values were found to be the most representative indicator of ecosystem stability.