Ecological Modelling 245 (2012) 125–135 Contents lists available at SciVerse ScienceDirect Ecological Modelling jo ur n al homep ag e: www.elsevier.com/locate/ecolmodel Testing the effectiveness of exergy-based tools on a seasonal succession in a coastal lagoon by using a size distribution approach Alessandro Ludovisi a,∗ , Leonilde Roselli b , Alberto Basset b a Dipartimento di Biologia Cellulare e Ambientale, Università degli Studi di Perugia, Perugia, Italy b Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Università del Salento, Lecce, Italy a r t i c l e i n f o Article history: Available online 15 March 2012 Keywords: Thermodynamic orientors Structural information Ecological succession Transitional waters Size spectrum a b s t r a c t Although several exergy-based indicators of ecosystem state have been put forward during the last decades, their effectiveness still expect a full confirmation. Recently, Ludovisi has proposed a formu- lation of exergy based on classical thermodynamics, which includes three terms – size (C), concentration (X) and structural information (I) – accounting for the contribution to exergy due to the abiotic and biotic components of an ecosystem, with the latter being expressed as a function of (sub)community struc- ture and diversity. In the present study, the response of the different exergy terms is analysed along the seasonal progression of environmental conditions and phytoplankton in the Lagoon of Lesina (Puglia, Italy). According to the proposed methodology, the equilibrium concentration of the main hydrochemical variables has been calculated on the basis of a suitable hydrochemical model. The estimate of the equilib- rium concentration of the phytoplankton (the VECE values) has been performed for logarithmic body-size classes by applying the equilibrium condition to the complete oxidation of the biomass. The results show that: (i) hydrochemical and phytoplankton exergy, as well as exergy terms (size C, concentration X and structural information I) are positively correlated one another, thus suggesting that phytoplankton con- tributed to enhance the total exergy of the water column of the lagoon also affecting the hydrochemistry of the system, especially during massive blooms; (ii) exergy and structural information of phytoplankton increase with increasing phytoplankton biomass, thus indicating that the size spectrum of phytoplankton diverges from the equilibrium distribution as the biomass stored increases (or vice versa); (iii) the size- fractioned Shannon information of phytoplankton increases with increasing biomass density as well as structural information, before reaching a superior limit. On the whole, the observed trends are consistent with theoretical expectations based on thermodynamics and classical ecological theories of succession, thus confirming the effectiveness of exergy-based tools as indicators of ecosystem development, as well as ecological orientors. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Exergy is a thermodynamic measure of the distance covered by a system away from equilibrium. Since ecosystems, like all biological entities, are self-organising far-from-equilibrium systems, exergy has been suggested as an indicator of ecosystem state (Jørgensen and Svirezhev, 2004), as well as an ecological orientor (a function possessing specific realisations towards which ecosystem devel- opment is supposed to be oriented): in particular, Jørgensen and co-workers (Jørgensen, 1992; Jørgensen and Svirezhev, 2004) put forward the hypothesis that “If there are offered more than one ∗ Corresponding author. Tel.: +39 075 585 5707; fax: +39 075 5855733. E-mail address: alessandro.ludovisi@unipg.it (A. Ludovisi). pathway to move away from thermodynamic equilibrium, the one yielding the most stored exergy will be selected”. Although several expression of exergy for ecosystems have been proposed and applied during the last decades (Herenden, 1990; Aoki, 1993; Svirezhev, 2000; Jørgensen et al., 2007; Ludovisi, 2009), their soundness and effectiveness still remains debated. Starting from the basic formulation of exergy for ecosystems (Mejer and Jørgensen, 1979): Ex = RT n  i=1 c i ln c i c i(e) - (c i - c i(e) ) (1) where R is the gas constant, T the absolute temperature and c i the concentration of the i-th ecosystem component in suitable units, with the subscript e indicating the reference equilibrium state, 0304-3800/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2012.02.009