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Food web and microbial loop Eutrophic vs. Oligotrophic food webs
Review Seasonal cycle spatial variation Food web and microbial loop Eutrophic vs. Oligotrophic food webs Biological pump
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Annual cycle in N. Atlantic
Nutrients Light Temperature Mixing Mixing Stratified Relative increase Spring bloom Fall mini- Phytoplankton biomass Zooplankton biomass
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Primary production and its seasonal cycle vary greatly in space
Chl a from SeaWIFS satellite Nutrient sources to surface waters are: rivers and land runoff upwelling atmosphere The most productive regions of the oceans are the coastal regions because this is where upwelling is strongest and where river and land runoff meet the sea. Here nutrients result in high productivity rates, which in turn result large fisheries.
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Mixed layer is deeper in Atlantic than in Pacific
Atlantic Ocean Depth (m) South pole Equator North Pole Pacific Ocean Depth (m) South pole Equator North Pole Temperature
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Latitudinal variation in seasonal cycles driven by variation in irradiance
[Also Irradiance] 90oN = N. Pole 60oN ~Anchorage,AK 30oN ~N. Florida 0oN = Equator
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Annual cycles in other regions
Phytoplankton biomass Zooplankton biomass Try this on your own: Draw the vertical profiles of temperature and light and the critical depth for each region as we did in class for the North Atlantic.
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Biological Pump Photosynthesis Respiration Sinking Remineralization
Chisholm, 2000
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On average, predators are ~10x bigger than prey
ESD = Equivalent Spherical Diameter Hansen et al. 1994
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What’s in a liter of seawater?
This basking shark can filter ~25,000 L seawater per day! 1 Liter of seawater contains: 1-10 trillion viruses 1-10 billion bacteria ~0.5-1 million phytoplankton ~1,000 zooplankton ~1-10 small fish or jellyfish Maybe some shark, sea lion, otter, or whale poop *The bigger you are, the fewer you are
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Assume a trophic transfer efficiency of 10%
Biomass 10 100 1000 Efficiency 0.1 fish zooplankton phytoplankton Trophic transfer efficiency = fraction of biomass consumed that is converted into new biomass of the consumer
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Traditional view of simple food web: Small things are eaten by (~10x) bigger things
Heterotrophs Autotrophs 20,000 2,000 200 20 2 0.2 Size (μm)
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Have to add heterotrophic bacteria, heterotrophic protists, and autotrophic bacteria
Heterotrophs Autotrophs 20,000 2,000 200 20 2 0.2 Size (μm)
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Dissolved organic matter
Bacteria absorb organic molecules leaked by microbes and phytoplankton. This creates a microbial “loop.” Heterotrophs Autotrophs 20,000 2,000 200 20 2 0.2 Microbial Loop Size (μm) Dissolved organic matter
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Zoom in on food web Photosynthesis respiration Chisholm, 2000
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Phytoplankton are eaten by zooplankton
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Plankton size structure is important
Diatoms, dinoflagellates Coccolithophores, cyanobacteria
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Importance of microbial loop depends on environmental conditions.
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Definitions Eutrophic environments have high nutrient concentrations and high productivity. Coastal upwelling regions and estuaries are Eutrophic. Oligotrophic environments have low nutrients and low productivity. Subtropical gyres (open ocean) are Oligotrophic. It takes a lot of mixing or a big nutrient influx to make an environment eutrophic. Stratified systems eventually must become oligotrophic.
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Diatom bloom in Barents Sea
Eutrophic -coastal -estuaries -upwelling -high latitudes Oligotrophic -open ocean -central gyres Clear water over Great Barrier Reef Diatom bloom in Barents Sea
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Microbial loop is less important
In eutrophic systems, large phytoplankton (diatoms) dominate and more biomass goes directly to large plankton and fish. Temp. Depth Dcr Microbial loop is less important
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In oligotrophic systems, small phytoplankton (e. g
In oligotrophic systems, small phytoplankton (e.g. cyanobacteria) dominate and biomass goes through more levels of plankton to get to fish. Temp. Depth Dcr Microbial loop is key
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Oligotrophic Eutrophic
Open Ocean Tuna Carniv. Fish Carniv. Plankton Herbiv. Plankton Phytoplankton 5 Levels 10% Efficiency Coastal Ocean Carniv. Fish Carniv. Plankton Herbiv. Plankton Phytoplankton 4 Levels 15% Efficiency Upwelling Zone Anchovies Phytoplankton 2 Levels 20% Efficiency
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Draw biomass spectrum here
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Area % of ocean area Total Plant Production Transfer Efficiency Trophic Levels Estimated Fish Production (x109 metric tons carbon per year) (x106 metric tons Open Ocean 90.0 39 10% 5 4 Coastal 9.9 8.6 15% 29 Upwelling Zones 0.1 0.23 20% 2 46
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=106 metric tons fish per year
=109 metric tons C per year =106 metric tons fish per year Open ocean (90%) Coastal ocean (9.9%) Upwelling zones (0.1%) 5 Trophic levels 10% Efficiency 4 Trophic levels 15% Efficiency 2 Trophic levels 20% Efficiency
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Food-web structure affects the export of carbon to deep ocean
Photosynthesis respiration Chisholm, 2000
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How does organic matter get to the bottom of the ocean?
Dead cells and fecal pellets (plankton poop) sink. Big ones sink faster. Dissolved organic matter, pieces of gelatinous animals etc. stick together and form bigger “marine snow” that sinks. Organic debris is collectively known as Detritus.
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Bigger plankton sink faster
Bigger plankton sink faster. They also have bigger, faster-sinking fecal pellets. Marine snow Large plankton and their fecal pellets Small plankton
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In eutrophic conditions, there are more, larger particles that sink into deep ocean.
Temp. Depth Dcr Large fecal pellets Large Marine snow
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In oligotrophic conditions, there are fewer, smaller particles that sink more slowly into deep ocean. Temp. Depth Dcr small fecal pellets
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Eutrophic vs. Oligotrophic summary
Mixed layer More mixing Cooler More stratified Warmer Nutrients High concentration Newer Low concentration More recycled Plankton Larger Smaller Particles Faster-sinking Slower-sinking Carbon Export More Less
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