Transactions on Ecology and the Environment vol 46, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 Use of thermodynamic orientors for the management of aquatic systems S. ~astianoni',A.G. erns stein^, M. ~anfredi~,L. ~ontobbio~ & E. ~iezzi' '~e~artmentof Chemical and Biosystems Sciences, University of Siena, Italy. 2~nvironmentalEngineering Department, Consorzio Venezia Nuova, Italy. Abstract The Lagoon of Venice is a very complex ecosystem, in which the action of humans has played a fundamental role for centuries. The effects of this action are very different: the construction of a uniquely charming city, the industrial zone of Porto Marghera, the development of fishery farms, the continuous and huge inflow of tourists. The Lagoon of Venice consequently needs a very careful management, in order to maintain, or better to improve, the equilibrium among the various aspects that constitute this unique "whole": social functions (productive systems, jobs, population, historical heritage) and ecosystems health. To have a better understanding of such a system we have used a holistic approach, using exergy and emergy in order to select the best alternative policies for the overall organization of the lagoon. Exergy is a thermodynamic potential that is strictly connected to the degree of organization of a system: the higher the exergy value, the farther is the system from thermodynamic equilibrium. Emergy is the solar energy directly and indirectly required to generate a flow or storage. Emergy contains the history of the energy and matter involved until the present state of the system, while exergy is a measure of the actual state, of the level of organization, of the information content. This paper considers the relation between the exergy stored in a system and the emergy flow necessary for its maintenance. These two functions are complementary and the ratio of the exergy stored to the emergy flow reflects the efficiency with which a system organizes itself and, if steady, maintains its complexity. As a test case we have selected a particular area in the lagoon of Venice to compare the performances of these approaches. Transactions on Ecology and the Environment vol 46, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 1 Introduction The lagoon of Venice is a very complex system. In the past territorial interventions were entrusted to the experience and judgement of the hydraulic engineers of that time. Under the guidance of empirical criteria, the Republic of Venice carried out exceptional public works such as the diversion of three large rivers, moving their mouths out of the lagoon [l]. Problems nowadays concern the rising number of high waters, which endanger the city of Venice. Several solutions to this problem are proposed; among them, at least in part, the re- opening of the fish farms present inside the lagoon. The system analyzed in this study is a fish farming basin in the southern part of the lagoon of Venice; in the same part, there are other such basins that occupy a total of 9000 hectares. Fish farming basins consist of peripheral areas of lagoon surrounded by banks in which local species of fish and crustaceans are raised. Salt water from the sea and freshwater from canals and rivers are regulated by locks and drains. Control of water levels, salt content and drainage towards the sea are part of an ancient tradition which is an economic and cultural heritage. Man learned how to exploit the instinct of certain species of fish that enter the lagoons, delta and coastal ponds, attracted by available food and calm waters. In spring, young specimens and adult fish come from the sea; traditionally, they were herded into the basin, where they grew quickly in the shallow nutrient-rich waters. The same principles are exploited today, however the basins are stocked with artificially raised juveniles or fry. In autumn, the fish would normally return to the sea, attracted by the warmer water and for reproductive reasons. Instead they are directed into special structures which act like traps by regulating the flow of water, where they are selected on the basis of size and type. The fish of highest demand raised in basins are Dicentrarchus labrax (bass) and Sparus auratus. Various types of mullet are also raised, as well as eels and molluscs. The presence of so many fish in such a small area attracts predatory birds. Besides the usual herons, many cormorants have appeared in the last 10 years. These heavy consumers have made it difficult to continue fish farming in the traditional way. The Figheri basin is in the central lagoon and was chosen as an example in which to check the environmental and productive effects of regulated opening of the basin. This practice was begun in 1997. The area was divided into two parts: one that remains closed and the other that is opened to the tides by means of gates. The parts are separated by an earth bank with two locks that regulate water exchange between the open and closed parts. The effects of opening were monitored continuously. In the open basin, fish farming is able to proceed due to a freshwater input and systems for catching the fish. Communication with the lagoon is only interrupted when fry is introduced (three months for acclimatization and capture), or during very high tides and certain weather conditions. The closed basin, where fish farming continues as before, is also monitored for comparative purposes. Environmental monitoring provides data on the biotic (macrophytes, macrozoobenthos, phytoplankton and zooplankton) and abiotic (sediment and water column) components. Fish production was also monitored where changes had been noticed in the last 10 years, namely an increase in the fraction of Transactions on Ecology and the Environment vol 46, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 Sparus auratus and a decrease in mullets and eels, due to deterioration of water quality in the open lagoon. Although captured by cormorants, Sparus auratus avoids the predators better than the other species, especially in winter. The ecosystem of Figheri basin was analysed using thermodynamic parameters for open systems. Specifically we used emergy, which expresses the history of the flows that went into creating the present state of the system, and exergy, which is a measure of the present state, organization and information content of the system, or how far the system is from thermodynamic equilibrium. 2 Emergy analysis as an environmental accounting tool Solar Emergy is a concept developed by H.T. Odum in the early nineteen-eighties in order to account for the basic energy requirements in obtaining a product (Odum [2]). it has been defined as "the available solar energy used up directly and indirectly to make a service or product" (Odum [3]). Solar energy is the fundamental unit since it is the basis of all other types of energy in the biosphere. Emergy and the "intensive" quantity, transformity (defined as the solar energy required, in direct and indirect ways, to obtain a joule of product), can be seen as the expressions, in measurable terms, of the first principle of sustainable development stated by H. Daly: "harvest rates should equal regeneration rates (sustained yield)" (Daly [4]). Emergy expresses all the energy in space and time going into a product. The concept of time and its different scales are fundamental for environmental accounting and sustainability as pointed out by Tiezzi [5]. Since the definitions of emergy and transformity are based more on a logic of 'Lmem~ri~ation'',than of conservation, an algebra of emergy has been introduced (Brown and Herendeen [6]). The rules of emergy analysis are: 1) all source emergy to a process is assigned to the processes' output; 2) by-products from a process have the total emergy assigned to each pathway; 3) when a pathway splits, the emergy is assigned to each 'leg' of the split based on its percentage of the total energy flow on the pathway; 4) emergy cannot be counted twice within a system: a) emergy in feedbacks cannot be double counted; b) by-products, when reunited, cannot be added to equal a sum greater than the source emergy from which they were derived. The main indices provided by emergy analysis cover practically all the aspects of the sustainability issues, even though pollution and wastes problem are treated in a quite indirect way, not focusing on the role of the particular wastelpollutant. In our opinion they are the transformity, the environmental loading ratio (ELR) and the emergy yield ratio (EYR). When comparing two or more processes with the same output, transformity is a measure of efficiency: more product obtained with a given quantity of emergy, or less emergy needed to produce a given amount of product (Odum [3]). EYR is the ratio of total emergy to the emergy purchased on the market, including fbels, goods and services. It is "a measure of its (the system's) net contribution to the economy beyond its own operation" (Odum [3]). Considering that the total emergy is the sum of all the local and external emergy inputs, the higher the ratio, the higher is the relative contribution of the local (renewable and Transactions on Ecology and the Environment vol 46, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 non-renewable) sources of emergy to the system. This index therefore shows how efficaciously the system uses available local resources. ELR (Odum [3]) is the ratio of all non-renewable emergy (from inside and outside the system) to the renewable emergy. This index is high for systems with high technological level andlor with high environmental stress. This stress is not necessarily local, but it is mostly located at the emergy source. 3 Exergy and ecosystem organization Exergy is the maximum work that can be obtained from a system when the system is brought from its present state to the state of thermal, mechanical and chemical equilibrium with the surrounding environment.