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Food Chains and Food Webs

1. The marine food chain

  • Food chain = linear arrangement of energy and organic material transfer 
  • Trophic level = groups of organisms that gain energy in similar ways
  • Phytoplankton = primary producers = first pelagic trophic level
  • Herbivores = primary consumers, feed on phytoplankton, 2nd tr. level
  • Carnivores = secondary consumers, feed on herbivores
  • Tertiary, etc. consumers – feed on smaller animals
  • Secondary Production = sum of all animal (heterotrophic) production
  • Trophodynamic studies address factors that control energy and biomass transfer among trophic levels
  • Essential nutrients are also transported within food chains; part of nutrients are recycled (regenerated) by animals (excretion) or decomposition by bacteria 
  • Energy cannot be recycled – food chain depends on energy harvest by primary producers: plants are primary, all else is secondary
  • 2. Transfer Efficiency of Food Chains
    • Ecological Efficiency = Transfer Efficiency: efficiency of energy transfer between trophic levels:
      ~20% for herbivores
      10-15% for carnivores
      80-90% of energy is lost due to respiration
    • Number of trophic levels will determine how much energy and food is available to top predators (whales, fish, birds) 
    • Number of trophic levels:
      High in open ocean waters
      Low in upwelling regions
    • Phytoplankton size: If phytoplankton is small (pico-, nanoplankton), then more trophic levels (protozoa) because larger animals (copepods) are unable to graze such small food particles; upwelling regions – large phytoplankton!

    3. Closer to Reality: Food Webs
    • Food Web: multiple and shifting interaction among organisms, more real than linear food chains
    • Problems: mostly still too complex to be accurately modelled or budgeted
    • Benthic species can interact with pelagic food webs; e.g. shellfish filter plankton out of the water, and carnivores feed on larvae of benthic species
    • Benefits: Food web analysis can provide understanding on how ecosystem functions, how it might react upon disturbance (pollution, overfishing), can predict maximum sustainable commercial use (e.g. sustainable fish harvest)
    4. The "Microbial Loop"
    • Concept: The „microbial food web“ complements the classical food chain
    • Major effect: returns dissolved organic carbon into particulate food web
    • Sink or Link? High energy loss due to multiple trophic levels (sink), but returns organic carbon and energy back into higher trophic levels that otherwise could not access this carbon/energy (link) 

    5. Bacteria in the Sea
    • Bacteria abundance:
      • generally ~ 5 x 106 ml-1, range 103 to 108 ml-1
      • Increase with increasing chlorophyll a (i.e. with phytoplankton biomass) – exception: low chl.a areas

    • Bacterial production
      • Increase with phytoplankton primary production
      • Can make up to 50% of primary production in oligotrophic, open ocean
      • Depends on algal exudation; exudation increases -- with increasing primary production -- with nutrient limitation (nitrate, phosphate) -- with algal senescence and lysis (end of blooms)
      • Measured by uptake of 3H-thymidine or 3H-leucine as a measure of DNA synthesis( thymidine) or protein synthesis (leucine); Caution! radioactive!
    • Viruses: Have been shown to infect and lyse bacteria, discussed as control of bacteria populations in addition to nutrient (DOC) limitation and grazer control