Specific Dissipation in Lake Ecosystems

Ludovisi presented a series of case studies for estimating the specific dissipation in lake ecosystems. He tested the adequacy of this indicator to assess ecosystem maturity along two different ecological series: the seasonal progression of phytoplankton and the trophic gradient. Specific dissipation relative to phytoplankton was calculated on a monthly scale starting from the monthly values of S bprod and the exergy (Exb ) embodied into the phytoplankton community, putting Te equal to the monthly average temperature of the water. He investigated specific case studies (e.g., lake Batorin, Myastro, and Naroch in the basin of river Neman, Byelorussia; lake Trasimeno in Italy) that clearly belong to three categories according to a trophic classification of lakes: (1) oligotrophic, (2) eutrophic, and (3) hypereutrophic (Figure 1).

Results obtained by Ludovisi, even from specific case studies, show that the general behavior of lake ecosystems respects the following trends. The progression of the monthly biological entropy production for the three classes of lakes approximately shows a regular increasing trend from January to June and a subsequent decrease until December. The estimated values of Sbprod are highest for the eutrophic lakes and lowest for the oligotrophic lakes, the annual level of the former being almost twice that of the latter. The progression of the monthly exergy stored (in unit MJm~2) by the phytoplankton communities presents a peak in August due to the growth of biomass, with a magnitude coherent with the trophic state, from the oligothrophic to the hypereutrophic. In all the lakes detected by Ludovisi, generally one relevant bloom of algae occurred during the warm season, but its inception significantly differs, being early in the case of the euthrophic lakes.

The seasonal progression of specific dissipation of phytoplankton reveals that all lakes are significantly different during spring (from March to June), after which they tend to converge in summer and maintain low values in autumn and throughout winter. Generally, specific dissipation increases abruptly in March, after which its behavior is different. After the divergence of the spring evolution, a common minimum value of specific dissipation is reached in summer and maintained until autumn, in spite of the very different monthly values of entropy production

J FMAMJ JASON D J FMAMJ JASOND J FMAMJ JASOND

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Figure 1 Progression of the monthly (a) biological entropy production (Sbpr0d in unit MJ m"2 month"1 K"1), (b) ecological exergy (left axis) and chemical exergy (right axis) stored by the phytoplankton community (in unit MJ m"2), (c) specific dissipation (in unit month"1) of the phytoplankton community (left axis: calculated from exergy expressed as ecological exergy; right axis: calculated from chemical exergy). Reproduced from Ludovisi A (2006) Use of the thermodynamic indices as ecological indicators of the development state of lake ecosystems: Specific dissipation. Ecological Indicators 6 (1): 30-42.

J FMAMJ JASON D J FMAMJ JASOND J FMAMJ JASOND

Month Month Month

Figure 1 Progression of the monthly (a) biological entropy production (Sbpr0d in unit MJ m"2 month"1 K"1), (b) ecological exergy (left axis) and chemical exergy (right axis) stored by the phytoplankton community (in unit MJ m"2), (c) specific dissipation (in unit month"1) of the phytoplankton community (left axis: calculated from exergy expressed as ecological exergy; right axis: calculated from chemical exergy). Reproduced from Ludovisi A (2006) Use of the thermodynamic indices as ecological indicators of the development state of lake ecosystems: Specific dissipation. Ecological Indicators 6 (1): 30-42.

and exergy storage exhibited by the three lakes. Although very different in magnitude, the values of specific dissipation, calculated using the chemical or the ecological exergy, show the same trend along the trophic gradient, but the latter is more capable to enhance the relative distance among the three ecosystems investigated.

As discussed by Ludovisi, in thermodynamic terms, during the spring phase, the high level of specific dissipation represents the dissipation of the incoming solar radiation per unit of exergy stored into the phytoplankton biomass that is higher with respect to the rest of the year. The subsequent convergence of the ratio to a common minimum value in summer shows a sort of thermody-namic 'goal' of the community evolution during the warm season. In ecological terms this means that the summer communities of the lakes appear as the final stage of an autogenic process of evolution, emanating from the adaptabilities and responses of the species themselves. This sequence is accompanied by an increasing complexity. The trophic state appears to play a significant role in determining the rate of evolution: the higher the trophic potentiality of the system, the earlier the attainment of a mature community in which a more complex structure is maintained with respect to the amount of entropy produced.

See also: Biomass, Gross Production, and Net Production; Entropy; Exergy.

Further Reading

Aoki I (1995) Entropy production in living systems: From organisms to ecosystems. Thermochimica Acta 250: 359-370.

Jorgensen SE and Svirezhev YM (2004) Towards a Thermodynamic Theory for Ecological Systems. Oxford: Elsevier.

Ludovisi A (2006) Use of the thermodynamic indices as ecological indicators of the development state of lake ecosystems: Specific dissipation. Ecological Indicators 6(1): 30-42.

Ludovisi A and Poletti A (2003) Use of the thermodynamic indices as ecological indicators of the development state of lake ecosystems, 1: Entropy production indices. Ecological Modelling 159: 203-222.

Ludovisi A and Poletti A (2003) Use of the thermodynamic indices as ecological indicators of the development state of lake ecosystems, 2: Exergy and specific exergy indices. Ecological Modelling 159: 223-238.

Ludovisi A, Pandolfi P, and Taticchi MI (2005) The strategy of ecosystem development: The specific dissipation as an indicator of ecosystem maturity. Journal of Theoretical Biology 235: 33-43.

Nicolis G and Prigogine I (1977) Self-Organization in Non-Equilibrium Systems: From Dissipative Structures to Order through Fluctuations. New York: Wiley Interscience.

Odum EP (1969) The strategy of ecosystems development. Science 164: 262-270.

Odum HT (1968) Work circuits and systems stress. In: Young H (ed.) Mineral Cycling and Productivity of Forests, pp. 81-146. Bangor: University of Maine.

Odum HT (1983) System Ecology. New York: Wiley Interscience.

Schrodinger E (1944) What is Life. Cambridge: Cambridge University Press.

Tiezzi E (2003) The Essence of Time. Southampton: WIT Press.

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