Holistic research approaches to the understanding of lake metabolism created useful tools for both limnological research and lake management. VOLLENWEIDER (1968) and DILLON and RIGLER (1974) have presented models which have been applied world-wide to describe relations between loading variables, especially phosphorus, and lake-internal water quality. The advances in preventing further lake eutrophication and in the developing measures to turn lakes to a lower trophic state were widely promoted by these scientific tools. The validity of models is commonly limited to the investigated systems like lake types, climate, and hydrology, respectively, but in some cases they could successfully applied also to other systems. Broad applicability and predictability of models lead in general to lower accuracy. Nevertheless, deviations from the mean are of scientific and practical interest, because the understanding of the underlying mechanisms for deviating behaviour may be the prerequisite for improving models and extending their validity limits (VOLLENWEIDER and DILLON 1974, BENNDORF et al. 1982, STRAS KRABA and GNAUCK 1983, JORCENSEN 1990). Some hardwater lakes of the baltic lake district show such deviations from the phosphorus/chlorophyll relation, what shows that calcit precipitation may function as lake internal buffer mechanism against phosphorus load (KOSCHEL et al. 1983).
While phosphorus was considered as main eutrophication factor, much less attention was paid to the nitrogen metabolism of lakes and its relation to water quality, although nitrogen plays an important role in controlling water quality but in very different ways.
In shallow lakes, high seasonal dynamics in the nitrogen content are common and the nitrogen/phosphorus ratio shifts in general after the spring development of diatoms to very low values, creating the conditions for dominance of nitrogen-fixing blue-green algae ( KLAPPER 1968, FORSBERG and RYDING 1980, RIPL 1985, DUDEL 1989). The role of nitrogen as lake eutrophication factor has been widely overestimated because primary production is in lakes very seldom really nitrogen limited. If conditions are favourable for the development of blue-green algae at least some planktic species can stabilize primary production also under combined nitrogen shortage, even under additional light limitation (KOHL et al. 1989).
Nitrate plays indeed an important role, especially in shallow lakes, by stabilizing the redox status of the sediment surface well above the redox potential of the Fe3+/Fe2+system. Therefore, nitrate can prevent the release of phosphorus bound in the femc phosphorus fraction, which is under reducing conditions mainly responsible for the internal phosphorus load (RYDING and FORSBERG 1977). This mechanism has already been used successfully for lake restoration (RIPL 1978(RIPL , 1985)).