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11/28/2007
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Food WebsOCN201 Fall 2007
Zackary JohnsonDepartment of OceanographyDepartment of Oceanography
http://www.soest.hawaii.edu/oceanography/zij/education.html
Major Concepts
Food webs describe how organisms are linked in the environment
Energy is lost (~90%) when ‘transferred’ between different organisms, mass is re‐used
Microbial food webs (loop) “recycles” nutrients and energy
All organisms are linked, directly or indirectly
How are the organisms of the ocean linked? Food webs: describe a pathway for energy or elemental flow
A simplified open‐ocean food webHow many kilograms of food does each person eat per day?
About 1kg!
Then why don’t we gain 1kg per day?
Because of the second law of thermodynamics:The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.
What?
Because energy is required for maintenance and we are inefficient.
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Trophic Levels – the facts
Each level of transfer is called a trophic leveltrophic – trophikos, Greek word for nourishment
Only ~10% of energy is transferred‐energy ‘lost’ to respiration, heat, physiological maintenance, etc.,
Because of inefficient transfers, higher trophic levels require more energy than lower trophic levels to produce the same amount of biomass
All mass is transferred (mass is not destroyed)‐ not all is necessarily used
A simplified marine food web
Trophic LevelsPhytoplankton – T1 Microzoo‐
plankton ‐T2
Zooplankton / Small Fish –T3
Tuna –T4
Trophic Levels
About how many kilograms of phytoplankton are you ‘eating’ when you eat 200g Ahi?
Phytoplankton = T1Ahi = T4
200g Ahi 0 2kg Ahi (Tuna)200g Ahi 0.2kg Ahi (Tuna)
0.2kg T4 / 10% T3 (Small Fish) = 2 kgT3 / 10% T2 = 20 kg (Zooplankton)T2 / 10% T1 = 200 kg (Phytoplankton)
a 20kg Ahi ≈ 5km x 5km of oceanHigher trophic organisms require a lot of resources!
A simplified marine food web ‐ what’s missing?
Microbes! (or at least some of them)
A New and Improved Food WebDOM (dissolved organic matter) links classic food web with microbes called the Microbial Loop
Higher Trophic LevelsProtozoa
Zooplankton
Phytoplankton
DissolvedOrganicMatter
Heterotrophic Bacteria
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Importance of the Microbial Food Web
• Dissolved organic matter (ex. sugars) is the result of dead/lysed cells or release of materials from living cells/organisms
• Prokaryotes are the dominant consumers of DOM. Most Bacteria and Archaea likely subsist on DOM.y
• Microbial communities in the sea include a full suite of ecological interactions among organisms less than 5μm. Primary producers, herbivores, carnivores, etc.
• The principal effect of microbial food webs is nutrient regeneration. Prokaryotes incorporate organic matter, then serve as food for higher organisms.
A simplified open‐ocean food web
Cod Fisheries Food Web
• Everything is connected to nearly everything else
• ‘Real’ food webs are incredibly complex
• Food webs as used by scientists serve as a modelscientists serve as a model – a simplified starting point for understanding the system
Southern Ocean Food Web
MiniWorld: a lesson in food webs
What creatures are in Miniworld?Can we draw a food web for Miniworld?What is providing the energy for Miniworld?Why aren’t there more shrimp (Opae’ ula) in MiniWorld?
http://www.fukubonsai.com/
MiniWorld: a lesson in food webs
PhytoplanktonOpae’ ula
Bacteria
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Major Concepts (Redux)
Food webs describe how organisms are linked in the environment
Energy is lost (~90%) when ‘transferred’ between different organisms, mass is re‐used
Microbial food webs (loop) “recycles” nutrients and energy
All organisms are linked, directly or indirectly