Why use emergy flow (empower) and eco-exergy together on the same systems? Emergy and eco-exergy are complementary concepts, the former based on the history of the system (Odum, 1988, 1996) and the latter examining the actual state (Jorgensen, 1992a, 2006). When systems follows a process of selection and organization, we can use the ratio of eco-exergy to emergy flow in order to assess the efficiency of an ecosystem in transforming available inputs in actual information and organization. The higher the ratio, the greater is the efficiency of the ecosystem in transforming available inputs (as emergy flow) into structure and ecosystem organization (as eco-exergy). Its units are J yr sej"1. Since dimensions are those of time, it cannot be regarded as a real efficiency (which is dimensionless), but more as an index of efficiency.
According to Svirezhev (2000), this fact is normal, since this concept resembles that of a relaxation time, i.e., the time necessary to recover from disturbances, so that the exergy to empower ratio should be related with concepts like resilience and resistance of an ecosystem.
The eco-exergy/empower ratio indicates the quantity of external input necessary to maintain a structure far from equilibrium: if the eco-exergy/empower ratio tends to increase (apart from oscillations due to normal biological cycles), it means that natural selection is making the system follow a thermodynamic path that will bring the system to a higher organizational level.
As an efficiency indicator the eco-exergy to empower ratio enlarges the viewpoint of a pure exergetic approach as described in Fath et al. (2004), where the exergy degraded and the eco-exergy stored for various ecosystems are compared: using emergy there is a recognition of the fact that solar radiation is the driving force of all the energy (and exergy) flows on the biosphere, important when important "indirect" inputs (of solar energy) are also present in a process.
To compare ecosystems different in size empower and eco-exergy densities were used. Table 9.11 shows empower and eco-exergy density values and the ratio of eco-exergy to empower. The emergy flow to Iberá Lagoon has been underestimated due to lack of data about the release of nutrients from the surrounding rice farms. In a sense this explains the highest value for eco-exergy to empower ratio, while the ecosystem does not seems to be in ideal conditions (Bastianoni et al., 2006). Nonetheless, the important fact is that all the natural systems that are better protected from human influence show very close figures. It seems that there is a tendency common to different ecosystems in different areas and of different characteristics to evolve toward similar thermodynamic efficiencies.
Figheri basin is an artificial ecosystem, but has many characteristics typical of natural systems. This depends partly on the long tradition of fish farming basins in the Lagoon of Venice, which has "selected" the best management strategies (Bastianoni, 2002). The human contribution at Figheri basin manifests as a higher emergy density (of the same order of magnitude as that of artificial systems) than in natural systems. However, there is a striking difference in eco-exergy density, with values of a higher order of magnitude than in any of the other systems used for comparison: Man and Nature are acting in synergy to enhance the performance of the ecosystem. The fact that Figheri can be regarded as a rather stable ecosystem (i.e., quite regular in its behavior) makes this result even more interesting and significant.
It was observed that the "natural" lagoon of Caprolace had a higher eco-exergy/ emergy ratio than the control and waste ponds, due to a higher eco-exergy density and a lower emergy density (Bastianoni and Marchettini, 1997). These observations were confirmed by the study of Lake Trasimeno (Ludovisi and Poletti, 2003).
Also, the results on the entire lagoon of Venice confirm the general trend, showing figures in the range of Trasimeno and Caprolace, albeit the differences in the structure of the ecosystems and the huge inputs from the watershed.
Table 9.11 Empower density, eco-exergy density, and eco-exergy to empower ratio for eight different ecosystems
Control Waste Caprolace Trasimeno Venice Figheri Ibera Galarza pond pond Lagoon Lake Lagoon Basin Lagoon Lagoon
Eco-exergy/ empower (10 5 J
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