Application of the exergy method to benthic assemblages of the Valli di Comacchio (Italy): Environmental gradient assessing

Temporal trends of exergy, specific exergy, biodiversity (species richness and heterogeneity) and an indicator of stress of the macrofaunal assemblage (W statistics) were compared along a gradient of disturbance in the Valli di Comacchio. Data were gathered at four reference sites, characterized by different disturbance levels, and spanned to a six year-period (1996-2002). Biodiversity parameters fluctuated through space and time as commonly observed in estuarine and lagoonal habitats; higher biodiversity values and less pronounced fluctuations were detected at the minimally disturbed site, while lower values of these indicators and wider fluctuations were observed at the other sites along the disturbance gradient. Exergy and specific exergy were higher at the minimally disturbed site, but, all temporal trends exhibited a zigzag pattern, being characterized both by very high and very low values. A different result was obtained when comparing the trends of specific exergy with disturbance expressed by the W statistics: at the four sites a close correlation resulted, with exergy and specific exergy well in phase with W statistics trend. While exergy, at least in the studied environment, does not seem comparable with biodiversity, it appeared to be capable to elucidate about the disturbance state of the ecosystem, and therefore they may constitute suitable system-oriented characteristics, and a good indicators of ecosystem integrity