CIVE 633 - ENVIRONMENTAL HYDROLOGY

ECOSYSTEM FUNCTION AND MANAGEMENT

  • An ecosystem is some unit of the biosphere.

  • Chemical substances are cycled and recycled while the energy transported as part of those substances continually passes through the system.

  • A stream and its immediate watershed, as well as a lake and its watershed inputs, may be considered an ecosystem.

  • The system is open with respect to energy (energy flow).
1.1  ECOSYSTEM COMPOSITION AND ENERGY SOURCES

  • 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.

1.2  ENERGY FLOW AND NUTRIENT CYCLING

  • 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, 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.
 
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