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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

• 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!

• 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)

• 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) 

• 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