Thermodynamic Orientors: Exergy as a Holistic Ecosystem Indicator: A Case Study

Abstract

Benthic eutrophication may cause qualitative changes in marine and estuarine ecosystems, for example the shift in primary producers. Subsequently, changes in species composition and trophic structure at other levels may often occur, and through time a new trophic structure might be selected. In structurally dynamic models such changes may be simulated using goal functions to guide ecosystem behavior and development. The selection of other species and other food webs may then be accounted by a continuous stepwise optimization of model parameters according to an ecological goal function.

Exergy has been applied as goal function in structurally dynamic models of shallow lakes. Hypothetically, exergy is assumed to become optimized during ecosystems development. Therefore, ecosystems are supposed to self-organize towards a state of an optimal exergy configuration. Exergy may then constitute a suitable system-oriented characteristic to express natural tendencies of ecosystems development, and simultaneously a good ecological indicator of ecosystems health.

Biodiversity, a powerful and traditional concept, is also an important characteristic of ecosystems structure. We found it suitable to test the intrinsic ecological significance of exergy. Therefore, we examined the properties of exergy (exergy and specific exergy) and biodiversity (species richness and heterogeneity) along an estuarine gradient of eutrophication, testing the hypothesis that they would follow the same trends in space and time. This hypothesis was only partially validated, since exergy, specific exergy and species richness decreased as a function of increasing eutrophication, but heterogeneity behaved mostly in the opposite way. Nevertheless, exergy and specific exergy behaved as hypothesized, providing useful information regarding the studied communities. They appeared therefore suitable to be used as goal functions in ecological models and as holistic ecological indicators of ecosystem integrity. Moreover, since exergy and specific exergy showed to respond differently to ecosystem seasonal dynamics, we recommend using both as complementary parameters.The method proposed by Jørgensen et al. (1995) to estimate exergy,which takes into account the biomass of organisms and the thermodynamic information due to genes, appeared to be operational, but more accurate (discrete) weighting factors to estimate exergy from organisms biomass need to be estimated. We propose to explore the assumption that the dimensions of active genomes, which are primarily a function of the required genetic information to build up an organism, are proportional to the relative contents of DNA in different organisms.