Use of Thermodynamic Orientors for the Management of Aquatic Systems

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.

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

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

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. Exergy can be written as the weighted sum of three gradients (of intensive variables): thermal, mechanical and chemical, where the weight is the corresponding extensive variable: dEx=SdT-Vdp+GN,dmi (1) where S entropy, V volume and Ni number of molecules of the i-th species are extensive variables and dT and dP are gradients of intensive variables (temperature, pressure and chemical potential of the i-th species) between the system and the environment.

Exergy is calculated from the elements constituting the system, on the assumption that many properties and functions of a substance or chemical compound are the result of the superimposition of the contributions of the atoms and bonds of the molecules. The exergy function is therefore obtained as the sum of the contributions of the various groups. This approach has been applied to many compounds of chemical and biological origin and has given results which are in line with experimental data. This theory cannot, however, distinguish different forms of complexity, because it is based upon elemental composition. Since living systems have an information content which is vastly different from the mere concentrations of chemical substances, it is necessary to adapt the definition of exergy to their basic characteristics. Jergensen [7] proposed classifying ecosystem components according to their complexity, on the basis of the number of genes in the DNA of the species that compose them. His formula is:

Ex = RT Cl [ C, In CJCi,q + (Cl - C1,,J] (2) where R is the gas constant, T absolute temperature, Ci the concentration of component i of the ecosystem and Ci.,, the concentration of the components at thermodynamic equilibrium. If we apply this formula to an ecosystem, for example an aquatic environment containing phytoplankton P, zooplankton 2, fish F and detritus D, we obtain: Ex~RT=P(I.79~10~+~(1.05~108)+~(2.52~108)+(~+~+~+~)7.34~1d[g/l] (3)

The approximations used enable this formula to be used to obtain a relative but not absolute value of exergy. In other words, the values can be used to compare two ecosystems or the same ecosystem at different times.

4 Results

4.1 Emergy and exergy results

The flows of local renewable resources (solar energy, rain, wind, tides and geothermal heat) were calculated for the total area of the Figheri basin. Solar energy was calculated from the insolation of the area minus 20% for albedo. Rain was calculated from mean rainfall data. Assuming that the same volume of water enters and leaves the lagoon in a year, freshwater entering through the Lova and Novissimo canals was estimated as the difference between evaporation and rainfall, since the water level is constant. Each year, the basin is drained to remineralize the nutrients and is flushed several times with water from the lagoon. The quantity of water entering was calculated from the rise in water level during flushing. The maintenance costs of the basin, including fuel, electricity and labour, were obtained and transformed into emergy. The total emergy flow, calculated in this way, made it possible to calculate the transformity of fry and fish, given the quantities produced. Table 1 shows the emergy evaluation. Emergy is given as the total of renewable, local non-renewable and imported non-renewable inputs. Renewable input is calculated as the sum of rain, tides, geothermal heat and freshwater from the lagoon. We only consider the largest (rain) of the three (sun, rain and wind) inputs of Figheri basin in Table 1, because they are all coproducts of the same process, namely the flow of solar energy to the Earth. This avoids counting the same phenomenon twice under different names. Freshwater is regarded as a totally renewable input since at the end of the cycle it goes into the lagoon. It can be regarded as a temporary storage of water from the canal, and is therefore renewable. The renewable fraction of input is high (almost 75%), despite the sum of fry-related emergy flows, which are largely of imported non renewable origin. This is because the fry-related input is up to two orders of magnitude less than the basin-related input. For what concerns exergy analysis, only the interior of the basin was analysed because it was difficult to obtain data, especially for fish, outside the basin (outside the banks and in the open lagoon). When the Jmgensen equation (3) was applied to Figheri basin, the contribution due to cormorants was also considered, since these predators are increasing rapidly and have a heavy impact on the ecosystem. Each term was obviously calculated on the basis of estimates, although many samples (mainly phytoplankton and zooplankton) were taken. The biomass of fish was calculated by cohort analysis, considering the fj introduced and mortality due to natural causes and fishing, since Sparus auratus is generally fished at an age of 2 years and bass at an age of 3 years. The number of cormorants and their biomass were estimated from observations and from the quantity of fish they are estimated to eat in a year. This gave us the number of cormorants supported by the Figheri basin ecosystem. To bring out the impact of cormorants on fish production, we considered the years 1989 and 1999 (Table 2). We made the assumptions (based on information obtained by farm managers) an absence of cormorants in 1989 and a decrease in fish production of 40% in 1999; the other components were taken as constant, for lack of data for 1989.

Table 1 : Emergy evaluation of Figheri basin fish production

Solar Solar Input Quantity Unit Transformity Emergy (sejfunit) (El 6 sejlyear)

Phytoplankton 3.90.10~ £/year 1.1 1.10~ 0.43 Zooplankton 1.25.10~ £/year 1.1 1.10~ 1.39 Yeast for zooplankton 1.60.10~ £/year 1.1 1.10~ 0.18

Food 3.00.10~ £/year 1.1 1.10~ 0.33 Lagoon water 3.23.10'~ g/year 8.99.10~ 0.29 Fuels 1.47.10'~ J/year 6.60.lo4 9.67 Electricity 1.11~10" J/year 2.00.10~ 2.2 1

Labour 3.01.10~ J/year 7.38~10~ 2.22 Solar energy 1.56,10i6 J/year 1 1.56 Rain glyear Wind Jlyear Tides J/year Geothermal heat Jlyear Freshwater g/year

Lagoon water glyear Fuels Jlyear Electricity J/year Labour Jlyear

Services £/year

Local renewable inputs

Local non-renewable inputs

Local inputs (renewable and non-renewable) 163.77

Imported inputs

Total emergy for fish production

PRODUCT: FISH 2.60.10' g 8.43.10'~

8.92.10'~ J 2.46.lo4

Table 2. Calculation of exergy for Figheri basin in 1989 and 1999

Year Phytopl. Zoopl. Fish Cormor. Detritus Exergy (g/l) ( g11 ) (g/]) (gll) (gll) (JII) 1989 0.418 0.159 ~.93.10'~ 0 2.25.10-~ 81.7.10~

Total exergy was calculated using the approximated formula illustrated above. The concentrations of the various species and total exergy of the system in 1989 and 1999 are also reported in Table 2. Exergy turned out to be larger in

1989, showing that the presence of many cormorants does not increase ecosystem complexity, but, if anything, lowers it (Table 2). The result is fairly obvious, since the advent of a new species always causes imbalances in the food chain, modifying the self-organization of the ecosystem. Indeed, a greater distance from thermodynamic equilibrium is only reached after enough time has elapsed for the self-organization process to select the species, and the relations between them, most suited to the environmental conditions, available resources and any fluctuations in the latter.

4.2 Exergylempower ratio

empower) The exergytemergy flow (or ratio, a new parameter introduced by Bastianoni and Marchettini [8], can be calculated from exergy values and the flow of emergy. This ratio indicates how much organization of a complex system is sustained by one unit of solar energy equivalent. The higher the ratio, the greater the efficiency of the ecosystem in transforming available inputs (as emergy) into structure and ecosystem organization (as exergy).

The results for the basin of Figheri can be compared with those of three other aquatic ecosystems. Two of the water bodies used for comparison are in North Carolina, U.S.A., and are part of a group of similar systems, planned and constructed to purify urban waste waters. Of the six ponds near Morehead City, three are "control" ponds that receive a mixture of estuarine waters and purified waters from the local sewage treatment plant and three are "waste" ponds that receive estuarine waters mixed with more polluted, or nutrient-rich, waste waters.

Plants and animals were introduced to the ponds and surrounding land to create new ecosystems by natural selection. The different conditions have produced quite different ecosystems in the two types of pond, with a prevalence of phytoplankton and crustaceans in the waste ponds and a great abundance of aquatic plants in the control ponds. The third water body used for comparison is the lake of Caprolace in Latium, at the edge of the Circeo National Park. This is an ancient natural formation fed mostly by rainwater, plus an input rich in nitrogen, phosphorus and potassium that percolates from nearby agricultural land. Human impact is low. A quantity of fish is taken each year, but is not such that the fish population decreases. Table 3 shows emergy and exergy density values and ratio. Densities were used to enable comparison between ecosystems in different areas. Figheri basin

is an artificial ecosystem, but has many characteristics typical of natural systems. This depends partly on the long tradition of fish farming basins in the Lagoon of Venice, which has "selected" the best management strategies. In general, we had observed that the natural lake (Caprolace) had a higher exergylemergy ratio than the control and waste ponds, due to a higher exergy density and a lower emergy density.

Table 3. Emergy and exergy density and exergylempower ratio

Figheri Lagoon of Control Waste b&in ~a&lace pond pond

empower density 12.2.10' 0.9.10' 20.1.10' 3 1.6.10' (sej1yr.l) exergy density 71.2.10~ 4.1.10~ 1.6.10~ 0.6.10~ (Jm exergylempower 58.5 44.3 0.8 0.2

In natural systems where selection has acted undisturbed for a long time, this ratio is higher, and decreases with the introduction of artificial stress factors.

The human contribution at Figheri basin manifests as a higher emergy density (of the same order as that of artificial systems) than in natural systems. However there is a striking difference in exergy density, with values of a higher order of magnitude than in any of the other systems used for comparison. 'I3e fact that Figheri can be regarded as a stable ecosystem makes this result even more interesting and significant.

5 Conclusions

Certain indications useful for the management of the basin and for understanding the role of structures and ecosystems of this type emerge from the present study. Emergy analysis indicates that fish production at Figheri basin has high overall efficiency and preponderance of renewable over non-renewable resources, a relevant fact because most agricultural production is the other way around. Exergy analysis shows that the ecosystem of Figheri basin is more organized than the other ecosystems used for comparison by at least one order of magnitude; furthermore the presence of cormorants is not a benefit for the overall organization of the ecosystem. The present results indicate a decrease in exergy ofthe ecosystem between 1989 and 1999 due to these birds. A relatively small number of cormorants presumably increases the biodiversity of the ecosystem, but uncontrolled increase transforms Figheri and nearby basins into temporary cormorant farms, constituting a threat to the existence of the ecosystems. The threat also regards other birds that compete for the same ecological niche (e.g. herons). The exergylemergy ratio shows that the ecosystem at Figheri basin has a higher overall efficiency than the others in transforming input into ecosystem components (fish, zooplankton, etc.). Its efficiency is of the same order of magnitude as Lake Caprolace and two orders of magnitude greater than artificial

ecosystems, by virtue of a higher level of organization and less need for external input. On the whole, we can say that Figheri basin production is very sustainable. As an ecosystem, it seems to benefit from human intervention rather than be damaged by it, as is usually the case. Human intervention has greatly promoted ecosystem efficiency. A possible improvement would be to limit the uncontrolled growth of the cormorant population, which exploits the shallow, densely populated waters of the basin.

References

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[4] Daly, H.E. Toward Some Operational Principles of Sustainable Development. Ecological Economics, 2, pp. 1-6, 1990. [5] Tiezzi E., Tempi storici, tempi biologici. Garzanti, pp. 1-224, 1983. [6] Brown M.T. and Herendeen R.A., Embodied energy analysis and EMERGY analysis: a comparative view. Ecological Economics, 19, pp. 219- 235, 1997.

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