Applying Thermodynamic Orientors: The Use of Exergy as an Indicator in Environmental Management

The importance of thermodynamic laws at various hierarchical levels of living systems is discussed, and the same principles appear equally relevant to populations, societies and even ecosystems. Nevertheless, while at lower hierarchical levels interpretation appears relatively clear, it has been very difficult to transport the thermodynamic concepts out of their pure physicochemical context and to make evident their importance at other levels, e. g. ecosystem. Since thermodynamic relationships cannot easily be measured at the ecosystem level, theoretical assumptions have to be made in order to estimate thermodynamic balances of ecosystems. Moreover, an adequate data background must exist to make satisfactory calculations. The application of thermodynamic concepts in the analysis of ecosystems is exemplified, mainly focusing on the use of exergy, a well-known concept from energetic analysis of physical systems. The methodology to perform exergy estimations is described and difficulties discussed. Nevertheless, attempts made from other thermodynamic directions of interpreting ecosystems are also referred. The exergy concept is derived and its meaning in describing the condition of an ecosystem is discussed, comparing two exergy based ecological indicators, the Exergy Index and Specific Exergy, with several ecosystem properties, in the scope of ecosystem biological integrity assessment. Several examples of application of thermodynamic orientors are reported concerning ecological structures and balances of lakes, evaluation of terrestrial systems, and agro-ecosystems. Finally, the advantages of thermodynamically based holistic ecological indicators in comparison to other ecological indicators are discussed.