Chapters 21 & 22 Energy & Nutrient Flows
Harned Hall 301 (615) 963 - 5782

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Sections

• Some Physical Constraints on Flow:
• Primary Production
  • Limits to Primary Production
• Secondary Production
  • Control of Secondary Production
• Nutrient Cycling
  • Global Carbon Cycle
  • Global Phosphorous Cycle
  • Global Nitrogen Cycle
• Terms

Some Physical Constraints on Flow

First Law of Thermodynamics

• Constrains the system to a complete accounting of all material and energy that enter or leave the system
  • If the system is in equilibrium, then net input = net output
  • Compounds that enter the system remain in the system until they or compounds made from them leave the system
  • This rule applies to those materials we intentionally or inadvertently add to systems, such as nutrients in our household waste or biocides used to control target organisms

Second Law of Thermodynamics

• Constrains the system to a net loss (of heat energy) when energy is transferred between various components of the system
  • 10% Rule of thumb is that only 10% of the energy that leaves one trophic level is transferred to the next level and 90% is lost as heat
  • Constrains the system to a net increase in entropy unless sufficient energy enters the system to counter the tendency for entropy to increase

Primary Production

Primary production is often measured in weight of producer (usually plants or algae, but can be bacteria in some systems) tissue added to the system

• Problems inherent in this is that the energy content of different tissues is not constant
• Better expressed in terms of energy (calories or joules are most usual)
  • One can convert between biomass and energy through calorimetry
    • Calculates energy in a material by burning it and measuring the temperature change in the apparatus (usually one called a bomb calorimeter)
  • Of course, this does not measure total energy, but it does measure total potentially useful energy, since respiration will oxidize the tissue in any case

  Gross Primary Production (GPP) is the amount of energy fixed by photosynthesis and chemosynthesis in an ecosystem

  Net Primary Production (NPP) is GPP minus the energy lost to respiration by the producers

  Net Change in Biomass (B) is NPP minus the energy lost to herbivory (G) and that lost to plant (algal) mortality (L)

DB = NPP - G - L

There are some generalities that can be made about the relationship between productivity and standing biomass

• Primary Productivity and standing crop (total biomass) are not always positively correlated
  • Reefs - most productive but have a tiny standing crop because turnover is so high
• Total continental versus total marine
  • Standing crop of marine producers is much smaller than terrestrial system producers with similar productivity, although the marine system supports lots of herbivores and carnivores
  • This means that turnover is much greater in marine systems

Production Efficiency is the proportion of energy fixed to that available (sunlight for photosynthesis)

• GPP efficiency is never greater than 4%

  NPP efficiency is usually 1% or less

Limits to Primary Production

Terrestrial ecosystems

• Soil Moisture and Evapotranspiration rate
• Length of Growing Season (depends on average temperature)
• Nutrient Limitation
  • Must be in correct chemical form and must be soluble
  • Nitrogen (as NO₃, NH₄)
  • Phosphorous (as PO₄)

Aquatic Ecosystems

• Light
  • Penetrates less in turbid waters
• Temperature
  • Mostly important to freshwater systems (turnover leads to recharging the epilimnion with nutrients)
• Nutrients
  • Nitrogen
    • Very important in open ocean
  • Phosphorous
    • Important in freshwater systems
    • Eutrophication of lakes occurred by the addition of phosphate-detergents
      • Process of increasing the concentration of nutrients until unusual photosynthesizers (Cyanobacteria often) turn the water turbid and often decrease oxygen concentration
  • Iron is often limiting
    • Needed to make many of the electron-accepting members of electron-transport chains

Question: As CO2 builds up in the atmosphere, some predict that some dry terrestrial ecosystems will become more productive and that there will be little gains in aquatic systems. What is responsible for this? I'll give you a hint - it's not the CO2 concentration, but something affected by it.

Secondary Production

Biomass produced by producers (primary production) can only move into two other components

• Detritus feeders
  • Majority of plant/algal biomass dies and is eaten by detritivores
• Herbivores
  • Important to plants and algae, as they eat living material

Measuring secondary production

Gross assimilation (amount of plant/algal material eaten) is reduced by the amount of energy (and the compounds needed to supply it) used to maintain the herbivore/detritivore

Gross Assimilation - Maintenance = Net Secondary Production

• The book refers to maintenance as respiration, but this is not correct, as respiration provides energy for both production and maintenance
• In endothermic vertebrates, maintenance is easiest to measure as the basal metabolic rate

Net production is measured as biomass increase

• Growth of individuals
• New individuals

Secondary Production Efficiency of secondary producers is the ratio of Net Secondary Production/Assimilation

Lindeman Efficiency is different from the efficiencies discussed so far

• the ratio of assimilation between trophic levels
• Maintenance is included in both the denominator and numerator, and will cancel out if it is the same proportion for each trophic level

Terrestrial ecosystems

• Often near 10% for all but decomposers
• Decomposers get what the herbivores and carnivores do not
• Total energy flow not much affected by carnivores (who produce 10% of 10%, or only 1% of primary production)

Aquatic Ecosystems

• Higher than for terrestrial systems, leading to rapid turnover in biomass
• Often as high as 40 % for aquatic herbivores
• Often as high as 80% for aquatic carnivores

Control of Secondary Production

Increase in primary production often leads to an increase in secondary production

This leads to the phenomenon that well-fertilized fields are attacked by herbivorous insects more than unfertilized fields (remember the ecosystem exploitation hypothesis)

• Sometimes the effect is not felt by the herbivores, but is seen in increased productivity of carnivores
  • More herbivore productivity is eaten by the next level (and so on up the line)
• Productivity tied to maturity of system
  • In late succession communities
    • Biomass stays constant
    • Less assimilation goes to production
    • more to maintenance
  • early succession communities
    • biomass in accumulating
    • more production, less maintenance (less to maintain)

Nutrient Cycling

Follows nutrients as they flow through the system

• Remember that the input of material to a system will equal output if there is no net loss or gain of that material in the system
  • Immature, early succession systems may accumulate materials
  • Agricultural systems lose more that annual natural inputs can replace, and may lose materials (when no fertilization takes place)
• Remember that ecosystems are always open
  • New materials arrive
  • System always leaks, so materials leave the system (no matter how slowly)

Nutrient cycles are the most thoroughly modeled ecological processes, also called  Biogeochemical cycles,  and are the paths of materials as they enter an ecosystem, pass through various componenets of the ecosystem, and finally exit the ecosystem

• Local models - apply to one ecosystem
• Global models - apply to the biosphere
  • Biosphere -- all earth is a single ecosystem

  Nutrient pools - nutrients in a species, a trophic level, or in some abiotic portion of the system (such as the soil or the atmosphere)

  Flux rates are the rates of flow from pool to pool

  Sources - a pool that is outside of the system and is often considered to be inexhaustible, although the flow from the source may be very slow

  • Weathering of rock
  • Rainfall (local systems)
  • Groundwater flow (local systems)
  • Atmosphere (local systems)

  Sinks - where materials go when they leave the system

  • Can often be sources too
  • Notice that atmosphere can be a pool in a global cycle, but a source in a local cycle

General model

• Materials enter the system from a source, are passed from pool to pool, and leave the system to enter a sink
• Cycles arise then one can reside in the system by moving from pool to pool and return to the first pool

Global Carbon Cycle

Sources

Volcanism

Ocean is generally considered to be in equilibrium with atmosphere, such that an alteration in one will inevitably lead to an alteration in the other

Global Phosphorous Cycle

this needs some work

Global Nitrogen Cycle

note that the role of microorganisms is key

• Fixation
• Denitrification

Terms

biocides, target organisms, Primary production, Gross Primary Production, Net Primary Production, Terrestrial ecosystems, Soil Moisture, Evapotranspiration rate, Length of Growing Season, Nutrient Limitation, Eutrophication, secondary production, Gross Assimilation, net secondary production, the basal metabolic rate, Efficiency, Production Efficiency, Lindeman efficiency, Biomass, assimilation, maintenance, early and late succession communities, Nutrient Cycling, Local models, Global models, Biosphere, Biogeochemical cycles, Nutrient pools, Flux rates, Sources, Sinks, Global Carbon Cycle, Volcanism, Global Phosphorous Cycle, Global Nitrogen Cycle, Fixation, Denitrification

Last updated October 26, 2006