From a theory of local processes in aquatic ecosystems to a theory at the ecosystem scale

In aquatic ecology, a deterministic theory has been developed of the complex physical, chemical and biological processes at the scale of limnological measurements (i.e. 10−2–102 m, 10−3–102 days). The theory results from non-linear thermodynamics of irreversible processes by introducing biological state characteristics and processes into the balances of mass, energy, momentum and entropy. Additional principles are assumed to govern the biological phenomena at this scale. Basic hypotheses and some of the conclusions are discussed by the present paper, since this ‘local theory’ offers the basis for the derivation of theoretical tools characterizing the ‘ecosystem scale’ (⩾ 100 m, > 10 days). Looking for key variables and organizational principles at the ecosystem scale, local quantities (e.g. energy, entropy production, etc.) are integrated over the ecosystem during a finite time interval in order to aggregate the physical, chemical, and biological processes observed below the ecosystem level. Dissipation of energy (or, more precisely, entropy production) and entropy export are considered to belong to the most important controlling factors of ecosystems. By mathematical methods used also in statistical thermodynamics, the most probable state of the ecosystem can be determined with regard to entropy export. Conclusions can be drawn about equations between key variables which may become the basis for aggregated ecosystem models.