Examination and optimization of different exergy forms in macrophyte societies

The replacement of rooted macrophytes, phanerogames, by non-adhered, drifting macroalgae have been reported from estuaries and lagoons at various geographic positions all over the earth. The observed shift in composition in the macrophyte society thus seems to be a general problem. This paper presents the results of an attempt to simulate the structural dynamics of the macrophyte societies in the Danish estuary, Roskilde Fjord. The modelling efforts concentrates on the shift between Z. marina and U. lactuca, that has been observed during the recent decades. The study is a part of a larger cooperation founded by the EU involving Italy, study area Lagoons of Venice, and Portugal, study area the estuary at Figueira da Foz, where other equivalent species and evolution of the ecosystems are studied. The results show that it is possible to simulate the competition between the species and allow spatial coexistence through time in a very simple model based on growth capacity, light and temperature characteristics alone. Four types of exergy have been proposed in literature to act as goal function to ecosystem development. In the model the traditional exergy, internal exergy, structural or modern exergy, together with the normalized exergy are examined and optimization are carried out. The behaviour of the various exergy forms differ considerably in time, which should lead to the hypothesis that they would act quite different as goal functions. Meanwhile the different exergies behave quite alike when used as goal functions—normalized exergy being the only exception. The consequence of these results for optimization strategies in the future is discussed.