**2. Ecosystem as thermodynamic system**

Ecosystem is an open system. It supports structure and functioning due to external energy input. Usually ecosystem consume solar energy in the form of relatively short-wave radiation (visible light), though we know some ecosystems (e.g., at great depth in ocean)

Some Applications of Thermodynamics for Ecological Systems 321

10. If the ecosystem is offered more pathways or combinations of pathways to move away from thermodynamic equilibrium, then the combinations of pathways that move the system most away from thermodynamic equilibrium (=yield the highest Eco-Exergy of

The exergy of a system is a measure of its deviation from thermodynamic equilibrium with the environment, and represents the maximum capacity of energy to perform useful work as the system proceeds to equilibrium, with irreversibility increasing its entropy at the expense of exergy (Ludovisi, 2009). Taken by itself, the total exergy of an ecosystem is a measure of the change in entropy content from the equilibrium and the actual state (Svirezhev, 2000). We may distinguish between technological exergy and Eco-Exergy: technological exergy uses the environment as reference state and is useful to find the first class energy (work) that a power plant can produce, Eco-Exergy uses as reference state the same ecosystem with the same temperature and pressure but at thermodynamic - chemical equilibrium (Fig. 1). Below

The development and maintenance of the far-from-equilibrium condition of ecosystems is due to the steady storage of free energy into complex organic structures, biosynthesized from simple inorganic compounds. Accordingly, the total exergy of an ecosystem actually reflects the accumulation of biomass into the system, irrespective of the distribution of biogenic matter among ecosystem components. Exergy is a measure of the free energy of a system with contributions from all components including the energy of organisms. The measure for exergy in ecology also includes a factor to weigh the "complexity" of the ecological species. Moving from macroscopic to microscopic information storage, the exergetic contribution due to information grows and becomes even three orders of magnitude higher than the physical one in the more complex living systems. The capacity of packaging information at the molecular level (DNA) that differs from one organism to

Fig. 1. Exergy is calculated for the system relatively to reference environment, Eco-Exergy relatively to the same system at the same temperature and pressure, but as inorganic

**Work (Eco-Exergy)**

**System at T, p**

**Displace Ment work, not useful**

> **Inorganic soup at T, p**

**Work (exergy)**

the ecosystem) will win (**The Second Ecological Law of Thermodynamics**).

we use the terms exergy and Eco-Exergy as synonymic.

another can be taken into account using Eco-Exergy function.

solution without life and even organic molecules.

**Reference environment at T, p**

**System at T, p**

**Displace Ment work, not useful**

**Thermodynamics**).

**3. Eco-exergy** 

surrounding and to thermodynamic equilibrium (**The First Ecological Law of** 

which use chemical energy. Nevertheless general rule is the reception of solar energy by green plants via photosynthesis. They assimilate approximately 0,01 – 3 % of energy of falling radiation and, using this energy, create organic matter (*primary production*) from inorganic compounds (water, carbon dioxide, nitrates, phosphates and a lot of minor substances). The by-product of photosynthesis is oxygen. Organisms, creating primary production are called *producers*. Energy, stored in organic matter is used by producers themselves and is dissipated during the processes of plants respiration, growth and reproduction in the form of heat. The remaining energy, accumulated in plant biomass is used by animals to support their structure and functioning. These processes are balanced at global level, as well as in healthy mature ecosystems. The rate of total organic matter production is called *gross primary production*. The difference between *gross primary production* and the rate of decomposition of this substance by plants themselves is called *net primary production*. Organisms, consuming plants (*consumers*) can utilize not more than 10 % of net primary production consumed, the rest being dissipated in the form of heat. Predators (*secondary consumers*) can use not more than 10 % of primary consumers production. The dead bodies of plants and animals, organic wastes, produced by the last etc. are decomposed and reduced to primary inorganic compounds, available for the new cycle of production/destruction by *decomposers* or *reducers* (bacteria and other microorganisms).

The rate of total increase of ecosystem biomass (yield) is known as *productivity*. In healthy, mature, balanced ecosystem it is equal to zero (or relation of production to respiration, P/R, is equal to one). So, ecosystem consumes high quality energy of solar radiation, uses part of it to support itself and dissipates the rest in the form of heat, increasing the total entropy of whole system Sun – ecosystem – environment. We have seen the ecosystem is functioning according to both the first and the second laws of thermodynamics.
