Die Dynamik des limnischen Stoff- und Energiehaushaltes

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Summary

1.	(p. 304). I have emphasized the principal difference between matter and energy in the dynamics of ecosystems. Inorganic and organic substances circulate more or less, whereas energy transformations involve an irreversible flow with considerable losses as heat. In order to understand the functional aspects of an ecosystem, we have to study the energy flow passing through it. Detailed studies of two communities in running water have made it possible to evaluate their energy budget in agreement with the first two principles of thermodynamics.
2.	(p. 306). A new approach to ecosystem dynamics is afforded by application of information theory. The information content of a community is proportional to its negative entropy, which can be calculated from energy data. The dynamics of an ecosystem through its successional stages seem to be determined by its information balance, in analogy with an organism which during its development shows at first a positive balance (growth), then equilibrium (adolescence), and finally a negative balance (senescence).
3.	(p. 308). The horizontal distribution of phytoplankton is often quite irregular, and the heterogeneity appears to be rather correlated with the seasonal succession. The irregularities become more pronounced when the succession proceeds from the first stage of mainly autotrophic, small forms (as a rule diatoms) to the stages of more specialized and bigger forms (in lakes often Chrysophyceae and, finally, blue-green algae). This pattern seems to be common to many marine and limnetic waters, and its exploration is taken as an example of the importance of closer cooperation between marine biologists and limnologists. It demonstrates, at the same time, the value of the holistic approach, studying successfully the properties of entities like populations, communities, and ecosystems, though our knowledge of all their constituents is still incomplete.
4.	(p. 309). The C14-technique does not give data on the respiration of phytoplankton, unless the assimilation experiments in situ are combined with laboratory experiments at different light intensities. Nevertheless, the field C14-method has proved to be useful for comparative determinations of the primary production in the free water of different lakes. The integral assimilation of the phytoplankton as a whole, in relation to light and temperature, can be described by a mathematical model, except for the light inhibition in the subsurface layer. Irregularities in the vertical distribution of plankton algae do occur, however, rather frequently in the epilimnion and may then cause considerable deviations from the predictable form of the primary production profile.
5.	(p. 310). Despite decades of extensive and intense work, the true relations between primary production of limnetic phytoplankton and its chemical conditions remain rather obscure. The main reason seems to be that the dynamic aspect (rates of regeneration and turnover) is still very little studied, though it may often be more important than the static aspect (concentrations). In the open sea, where e. g. phosphate has a higher concentration but is more slowly regenerated than in lakes of corresponding productivity, a much better correlation has been observed between nutrient concentrations and primary production.
6.	(p. 311). Radioactive tracers have been used to a comparatively little extent in limnology to clarify nutrient metabolism, but they have already demonstrated high rates of turnover for phosphorus, for instance. It is necessary to distinguish, however, the simple exchange of one atom or ion against another of the same kind (maybe radioactive) from active incorporation or liberation of an element. The functions of bacteria in these processes urgently need to be studied more closely, in order to understand the nutrient conditions of primary producers.
7.	(p. 312). The contamination of inland waters by radionucleides will raise a major and vital problem in the near future. To solve it, a thorough knowledge of ecosystem dynamics and community metabolism is fundamental. Thus hitherto mainly or purely academic topics of theoretical limnology can be expected to obtain great practical importance. Limnologists have to realize this challenge before it gets too late.