Long-Term ecological research in freshwater ecosystems

Abstract

Long-term changes of freshwater ecosystems are mainly caused by immissions from drainage basin and atmosphere (nutrients, acid substances, etc.) and by changing climatic conditions. Freshwater ecosystems often react in non-linear ways to these external forces. Beyond a certain threshold, gradual shifts may cause catastrophic switches to another state. The way back to the previous state rarely corresponds to the past changes because of memory effects of the system. Long-term studies are necessary, but they do not allow for a simple extrapolation of past observations into the future. Freshwater systems are also influenced by rare events like invasion of new species, spates or droughts. Effects of perturbations should be studied until the system establishes a new equilibrium. The analysis of long-term processes needs sound knowledge about natural oscillations or gradual changes of the baseline. Monitoring programmes of German lakes and reservoirs rarely last longer than 30 years. They usually started after serious environmental problems had emerged; they do not cover periods without human impacts (baseline conditions). Therefore, long-term monitoring should be accompanied and extended by palaeolimnological approaches. © 2010 Springer Science+Business Media B.V.