The Application Of Ecoexergy As Ecological Indicator For Assessment Of Ecosystem Health

Reference from which these applications of eco-exergy used as ecosystem health indicator are extracted:

Zaldivar JM, Austoni M, Plus M, De Leo GA, Giordani G, Viaroli P. 2005. Ecosystem Health Assessment and Bioeconomic Analysis in Coastal Lagoon. Handbook of Ecological Indicator for Assessment of Ecosystem Health. CRC Press, pp. 163-184.

In this paragraph an application of Eco-Exergy is reported (see Chapters 2 and 7) to assess the ecosystem health of a coastal lagoon.

Coastal lagoons are subjected to strong anthropogenic pressure. This is partly due to freshwater input rich in organic and mineral nutrients derived from urban, agricultural, or industrial effluent and domestic sewage, but also due to the intensive shellfish farming.

The Sacca di Goro is a shallow water embayment of the Po Delta. The surface area is 26 km2 and the total water volume is approximately 40 X 106 m3. The catchment basin is heavily exploited for agriculture, while the lagoon is one of the most important clam (Tapes philippinarum) aquaculture systems in Italy. The combination of all these anthropogenic pressures call for an integrated management that considers all different aspects, from lagoon fluid dynamics, ecology, nutrient cycles, river runoff influence, shellfish farming, macro-algal blooms, and sediments, as well as the socio-economical implication of different possible management strategies. All these factors are responsible for important disruptions in ecosystem functioning characterized by eutrophic and dystrophic conditions in summer (Viaroli et al., 2001), algal blooms, oxygen depletion, and sulfide production (Chapelle et al., 2000). Water quality is the major problem. In fact from 1987 to 1992 the Sacca di Goro experienced an abnormal proliferation of macroalga Ulva sp. This species has become an important component of the ecosystem in Sacca di Goro. The massive presence of this macroalga has heavily affected the lagoon ecosystem and has prompted several interventions aimed at removing its biomass in order to avoid anoxic crises, especially during the summer, when the Ulva biomass starts decomposing. Such crises are responsible for considerable damage to the aquaculture industry and to the ecosystem functioning.

To carry out such an integrated approach a biogeochemical model, partially validated with field data from 1989 to 1998, has been developed (Zaldivar et al., 2003). To analyze its results it is necessary to utilize ecological indicators, using not only indicators based on particular species or component (macrophytes or zooplankton) but also indicators able to include structural, functional, and system-level aspects. Eco-exergy and specific eco-exergy are used to assess the ecosystem health of this coastal lagoon. Effects of Ulva's mechanical removal on the lagoon's eutrophication level are also studied with specific exergy (Jorgensen, 1997) and cost-benefit analysis (De Leo et al., 2002). Three scenarios are analyzed (for a system with clam production and eutrophication by Ulva) using a lagoon model: (a) present situation, (b) optimal strategy based on cost-benefit for removal of Ulva, and (c) a significant nutrient loading reduction from watershed. The cost-benefit model evaluates the direct cost of Ulva harvesting including vessel cost for day and damage to shellfish production and the subsequent mortality increase in the clam population. To take into account this factor, the total benefit obtained from simulating the biomass increase was evaluated using the averaged prices for clam in northern Adriatic; therefore, an increase in clam biomass harvested from the lagoon will result in an increase of benefit.

The Sacca di Goro model has several state variables for which the exergy was computed: organic matter (detritus), phytoplankton (diatoms and flagellates), zooplankton (micro- and meso-), bacteria, macroalgae (Ulva sp.), and shellfish (Tapes philippinarum). The exergy and the specific eco-exergy are calculated using the data from Table 9.6 on genetic information content and all biomasses were reduced to gdwl"1 (grams of dry weight per liter) .

Figures 9.7 and 9.8 present the evolution of exergy and specific exergy under the two proposed scenarios: Ulva removal and nutrient load reduction, in comparison with the "do nothing" alternative. As it can be seen the eco-exergy and specific eco-exergy of both increase, due to the fact that in our model both functions are dominated by clam biomass.

However, the optimal result from the cost/benefit analysis will considerably improve the ecological status of the lagoon in term of specific exergy.

Table 9.6

Parameters used to evaluate the genetic information content

Ecosystem component

Number of information genes

Conversion factor

Detritus

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