Emergy/exergy ratio as a measure of the level of organization of systems

Emergy and exergy have been developed as complementary goal functions. By definition, emergy is the solar emergy directly and indirectly required to generate a flow or a storage. Exergy is a property of a system, measuring the maximum work that can be extracted from st system when it goes towards the thermodynamic equilibrium with a reference state. The concept of emergy contains the history, the time and all the different processes involved up to the present state of the system, while exergy is a measure of the actual state, of the level of organization and of the information content. These two approaches are very suitable for describing self-organizing systems such as ecosystems. The ratio of the emergy flow to the exergy can give further information on the state of a system, showing what concentration of solar emergy equivalent, space and time (emergy) is required to maintain or create a unit of organization (exergy). The proposed index is related to the efficiency with which a system organizes itself or, if steady, maintains its complexity. To test this approach we considered three coastal lagoons. Two of them are artificial, built by man to purify sewage. One is a control pond fed with estuarine water and 'clean' water from the local sewage treatment plant, and one is a 'waste pond' fed with estuarine water mixed with more 'polluted' (i.e. richer in nutrients) effluent. The third system is the lagoon of Caprolace (Italy) a 'natural' system, placed in a national park. Experimental results show that the emergy/exergy ratio has the lowest value for the ecosystem of Caprolace. The waste pond has the highest environmental cost for the production of a unit of organization. Both the control and the waste ponds show a decreasing value of the emergy/exergy ratio with time, meaning that natural selection is organizing the systems. (C) 1997 Elsevier Science B.V.