• The matter is living and dead, the living being represented best by the trophic levels of organisms, from algae to fourth-level carnivores.

• Insects, fungi and bacteria decompose organic matter.

• The stream's energy source can be:
  • autochthonous, produced from within the ecosystem.

  • allochthonous, produced outside of the ecosystem.

• The relative proportion of autochthonous to allochthonous energy sources is useful for management decisions affecting aquatic ecosystems.

• Figure 1.1 shows that:
  
  
  • Energy flows through the system and does not return.

  • Loss of energy as heat occurs at each step.

  • Allochthonous energy moves through heterotrophic microorganisms (who cannot create their own food) or decomposers.

  • The ultimate source of energy must be either photosynthesis, or to a lesser extent, chemosynthesis.

• Photosynthesis releases oxygen (O₂) and fixes carbon dioxide (CO₂) into reduced organic matter (glucose) (C₆H₁₂O₆).

  6CO2 + 12H2O + light + (chlorophyll a and accesory pigments)  -->  C6H12O6 + 6O2 + 6H2O

• The process is performed by green plants and a few pigmented bacteria.

• Chemosynthesis is the other process through which organisms are totally self-sufficient in trapping energy and building cell material.

• The process yields energy through the oxidation of reduced inorganic compounds; thus, it requires no biological mediation.

• The primitive Earth was rich in inorganic compounds.

• Nitrification is an example:

  2NH3 + 3O2  -->  2HNO2 + 2H2O + energy

• Chemosynthetic processes by bacteria are involved in the cycling of nitrogen and sulphur.

• Nutrients such as N, C, P, and S are consumed by green plants and chemosynthetic bacteria from inorganic pools and fixed into organic compounds.

• The reduced organic compounds are the carriers of the entrapped chemical energy.

• One mole of glucose (C₆H₁₂O₆) contains 674 kilocalories of energy releasable through respiration by plants, animals, or decomposers (macro or micro).

• The chemical compounds are recycled through the inorganic pools and are almost totally reusable by the community.

• A certain fraction may be temporarily lost to the sediments and require tectonic uplift.

• The recycling process is shown in Fig. 1.2.

1.3  EFFICIENCY OF ENERGY AND NUTRIENT USE

• Efficiency of energy is the ratio of net productivity of a trophic level to the net productivity available for its consumption.

• This value is between 10% and 20%.

• Populations organize in such a way that energy usage is optimized.

• Stable ecosystems (for instance, tropical rainforests and coral reefs) maximize organization and complexity, and remain constant in biomass, productivity, species diversity, and efficiency.

• When systems approach steady state, nutrient recycling tends to be tighter and more complete.

• Diversity tends to be low in immature systems and high in mature systems.

• Stability is often suggested to increase with diversity.

• In time, ecosystems approach steady state, with little net productivity and maximized structure.

• In a system with 34% efficiency, 66% of the energy leaves the system downstream, unused.

• For example, in a rainforest, a mature ecosystem, efficiency is 100% and net production is zero; the biomass is constant.

1.4  MANAGEMENT OF ECOSYSTEMS

• In an aquatic ecosystem, the uses may be conflicting.

• In an aquatic ecosystem, to produce bigger and more sport fish would require stimulation of primary production.

• This decreases the structure and possibly the stability.

• There may be a large biomass of inedible algae.

• Fish may be large, but subject to times of high mortality.

• Ecosystems cannot assimilate waste without some cost to their structure and stability, with reduced efficiency in nutrient cycling.

• Stable communities are probably not more able to resist change from waste input that unstable ones.

• Highly diverse communities may produce stable behavior in natural conditions, but such a state is sensitive to changes.

• Changes tend to lower the stability and decrease the efficiency of ecosystem functioning.

• Highly structured and efficient ecosystems are probably not highly resistant to disturbance.