Modeling matter and energy flows in marine ecosystems using emergy and eco-exergy methods to account for natural capital value

Abstract The identification of principles explaining the development of ecosystems has been a major target of systems ecology and ecological modelling. Among these principles, the thermodynamic goal functions (GFs) named emergy and eco-exergy are recognised as methods unfolding the role of matter and energy flows in the functioning of ecological systems. The biophysical assessment of natural capital stocks is based on the evaluation of matter and energy flows invested by nature for their generation. In this paper, the emergy and eco-exergy methods were used to account for the natural capital value of two Mediterranean Marine Protected Areas (MPAs) located in Southern Italy. In particular, the assessment focused on four main habitats: 1) sciaphilic hard bottom (coralligenous bioconstruction), 2) photophilic hard bottom, 3) soft bottom, and 4) Posidonia oceanica seagrass bed. The emergy method allowed the assessment of natural capital in terms of direct and indirect solar energy flows invested for its generation, while the eco-exergy method accounted for the chemical energy stored in organic matter and the genetic information embodied in biomass stocks. The sciaphilic hard bottom showed a high emergy density value in the two MPAs (2.06∙10 12 and 7.22∙10 11 sej m −2 ), confirming the high convergence of input resource flows in the formation of this habitat. This high value reflects the effort of nature in the formation of this habitat and therefore it suggests the need for its consideration in conservation strategies. Instead, Posidonia oceanica seagrass bed showed the highest value of eco-exergy density in both MPAs (3.56∙10 6 and 3.54∙10 6 kJ m −2 ). The eco-exergy results were influenced by two main factors: 1) the large biomass density of Posidonia oceanica , and 2) the high s value of Posidonia oceanica reflecting its evolutionary history that involved the acquisition of key adaptations for the successful colonization of marine environments. The integration between emergy and eco-exergy methods resulted in a useful approach to account for different nature values based on a biophysical perspective. Such a perspective could complement economic evaluations based on market criteria, thus supporting decision making processes oriented to the sustainable management of marine resources.