Testing the effectiveness of exergy-based tools on a seasonal succession in a coastal lagoon by using a size distribution approach

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 formulation 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 structure 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 equilibrium 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 contributed 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.