**3.2.3 Eco-exergy as an ecosystem health indicator**

The idea to use exergy as an indicator of ecosystem health was proposed by S.E. Jørgensen (1992, 1999, 2002, 2006a,b), but the first applications of exergy as an ecosystem health indicator were fulfilled with mathematical models. Mathematical modelling with the use of exergy was shown to be applicable to explain ecosystem reactions (Jørgensen & Padisak, 1996), and to facilitate the estimation of parameters of models. The first pioneer papers describing the application of exergy indicators for natural aquatic ecosystems were published in 1997 (Xu et al., 1997; Marques et al., 1997). In 1998 the first application of exergy analysis to the results of field and laboratory experiments with real aquatic ecosystem was published (Silow, 1998). This work was continued by few more publications (Xu et al., 1999a, 1999b; Silow & Oh, 2004). The possibility to use such parameters as structural exergy and exergy for estimation of ecosystem state and its changes under various external influences was demonstrated for real natural and experimental ecosystems. These parameters were shown to reflect the state of ecosystem and they can indicate the degree of ecosystem adaptation, decreasing when important for ecosystem functioning components were depressed or eliminated. S.E. Jørgensen (2006a) proposed to use Eco-Exergy, specific Eco-Exergy and ecological buffer capacities as Ecological indicators for ecosystem development and health assessment.

Exergy is now often used for eutrophication assessment (Xu et al., 1999, 2001, 2011a; Fonseca et al., 2002; Marques et al., 2003; Ye et al., 2007), for ecological engineering purposes (Nunneri et al., 2008), for ecosystem health assessment (Vassallo et al., 2006; Libralato et al., 2006; Xu et al., 2011b). Exergy and specific exergy indices as Ecological indicators of the trophic state of lake ecosystems were tested on a set of lakes (Ludovisi & Poletti, 2003). The ecosystem maturity was estimated for Lake Trasimeno (Ludovisi et al., 2005).
