Transcript Pelagic food webs and the microbial loop

• Review
– Seasonal cycle
– spatial variation
• Food web and microbial loop
• Eutrophic vs. Oligotrophic food webs
• Biological pump
Annual cycle in N. Atlantic
Relative increase
Nutrients
Light
Temperature
Mixing
Mixing
Stratified
Spring
bloom
Phytoplankton biomass
Zooplankton biomass
Fall minibloom
Primary production and its seasonal cycle
vary greatly in space
Chl a from SeaWIFS satellite
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
Temperature
North Pole
Video of mixed layer with wind mixing
(go to 8:21)
Latitudinal variation in seasonal cycles
driven by variation in irradiance
[Also Irradiance]
o
90 N = N. Pole
o
60 N ~Anchorage,AK
o
30 N ~N. Florida
o
0 N = Equator
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.
Biological Pump
Photosynthesis
Respiration
Sinking
Remineralization
Chisholm, 2000
On average, predators are ~10x bigger than
prey
ESD = Equivalent Spherical Diameter
Hansen et al. 1994
What’s in a liter of seawater?
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
This basking shark can filter
~25,000 L seawater per day!
Assume a trophic transfer efficiency of 10%
Biomass
10
Efficiency
fish
0.1
100
1000
zooplankton
0.1
phytoplankton
Trophic transfer efficiency = fraction of biomass consumed
that is converted into new biomass of the consumer
Traditional view of simple food web:
Small things are eaten by (~10x) bigger things
Heterotrophs
20,000
Size (μm)
2,000
200
20
2
0.2
Autotrophs
Have to add heterotrophic bacteria, heterotrophic
protists, and autotrophic bacteria
Heterotrophs
20,000
Size (μm)
2,000
200
20
2
0.2
Autotrophs
Bacteria absorb organic molecules leaked by microbes
and phytoplankton. This creates a microbial “loop.”
Heterotrophs
Autotrophs
20,000
Size (μm)
2,000
Microbial Loop
200
20
2
0.2
Dissolved organic matter
Zoom in on food web
Photosynthesis
respiration
Chisholm, 2000
Phytoplankton are eaten by zooplankton
Plankton size structure is important
Diatoms, dinoflagellates
Coccolithophores, cyanobacteria
Importance of microbial loop depends on
environmental conditions.
Microbial loop
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.
Eutrophic
-coastal
-estuaries
-upwelling
-high latitudes
Oligotrophic
-open ocean
-central gyres
Clear water over Great Barrier Reef
Diatom bloom in Barents Sea
Microbial loop is less important
Temp.
Depth
In eutrophic systems, large
phytoplankton (diatoms) dominate
and more biomass goes directly to
large plankton and fish.
Dcr
Microbial loop is key
Temp.
Depth
In oligotrophic systems, small
phytoplankton (e.g. cyanobacteria)
dominate and biomass goes through
more levels of plankton to get to fish.
Dcr
Oligotrophic
Eutrophic
Open Ocean
Coastal Ocean
Upwelling Zone
Tuna
Carniv. Fish
Anchovies
Carniv. Fish
Carniv. Plankton
Phytoplankton
Carniv. Plankton
Herbiv. Plankton
Herbiv. Plankton
Phytoplankton
Phytoplankton
5 Levels
10% Efficiency
4 Levels
15% Efficiency
2 Levels
20% Efficiency
Draw biomass spectrum here
Area
% of
ocean
area
Total Plant
Production
Transfer
Efficiency
Trophi
c
Levels
(x109 metric
tons carbon
per year)
Estimated
Fish
Production
(x106 metric
tons
per year)
Open
Ocean
90.0
39
10%
5
4
Coastal
Ocean
9.9
8.6
15%
4
29
Upwelling
Zones
0.1
0.23
20%
2
46
=109 metric tons C
per year
=106 metric tons
fish per year
Open ocean
5 Trophic levels
10% Efficiency
Coastal ocean
4 Trophic levels
15% Efficiency
Upwelling zones
2 Trophic levels
20% Efficiency
Food-web structure affects the export of
carbon to deep ocean
Photosynthesis
respiration
Chisholm, 2000
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.
Bigger plankton sink faster. They also have
bigger, faster-sinking fecal pellets.
Large plankton
and their
fecal pellets
Small plankton
and their
fecal pellets
Marine
snow
Large
Marine snow
Temp.
Depth
In eutrophic conditions, there are
more, larger particles that sink into
deep ocean.
Dcr
Large
fecal
pellets
Temp.
Depth
In oligotrophic conditions, there
are fewer, smaller particles that
sink more slowly into deep ocean.
Dcr
small
fecal
pellets
Eutrophic vs. Oligotrophic summary
Eutrophic
Oligotrophic
Mixed layer
More mixing
Cooler
More stratified
Warmer
Nutrients
High concentration Low concentration
Plankton
Particles
Newer
More recycled
Larger
Smaller
Larger
Faster-sinking
Carbon Export More
Smaller
Slower-sinking
Less