• Review– Seasonal cycle – spatial variation

• Food web and microbial loop

• Eutrophic vs. Oligotrophic food webs

• Biological pump

Phytoplankton biomassZooplankton biomass

Nutrients

Rel

ativ

e in

crea

se Mixing Mixing

Stratified

Light Temperature

Annual cycle in N. Atlantic

Spring bloom

Fall mini-bloom

Primary production and its seasonal cycle vary greatly in space

Chl a from SeaWIFS satellite

Atlantic Ocean

Pacific Ocean

Temperature

South pole Equator North Pole

South pole Equator North Pole

Mixed layer is deeper in Atlantic than in Pacific

Depth

(m)

Depth

(m)

Video of mixed layer with wind mixing

(go to 8:21)

Latitudinal variation in seasonal cycles driven by variation in irradiance

90o

N = N. Pole

60o

N ~Anchorage,AK

30o

N ~N. Florida

0o

N = Equator

[Also Irradiance]

Annual cycles in other regions

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.

Phytoplankton biomassZooplankton biomass

Chisholm, 2000

Biological Pump

Photosynthesis Respiration

SinkingRemineralization

On average, predators are ~10x bigger than prey

Hansen et al. 1994

ESD = Equivalent Spherical Diameter

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!

phytoplankton

zooplankton

fish

Assume a trophic transfer efficiency of 10%

Biomass

10

100

1000

Efficiency

0.1

0.1

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

Siz

e (μ

m)

20,000

2,000

200

20

2

0.2

Heterotrophs Autotrophs

Have to add heterotrophic bacteria, heterotrophic protists, and autotrophic bacteria

Siz

e (μ

m)

20,000

2,000

200

20

2

0.2

Heterotrophs Autotrophs

Bacteria absorb organic molecules leaked by microbes and phytoplankton. This creates a microbial “loop.”

20,000

2,000

200

20

2

0.2

Siz

e (μ

m)

Heterotrophs Autotrophs

Dissolved organic matter

Microbial Loop

Chisholm, 2000

Zoom in on food web

Photosynthesis respiration

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.

Diatom bloom in Barents SeaClear water over Great Barrier Reef

Eutrophic-coastal-estuaries-upwelling-high latitudes

Oligotrophic-open ocean-central gyres

In eutrophic systems, large phytoplankton (diatoms) dominate and more biomass goes directly to large plankton and fish.

Temp.

Dep

th

Dcr

Microbial loop is less important

Temp.

Dep

th

Dcr

In oligotrophic systems, small phytoplankton (e.g. cyanobacteria) dominate and biomass goes through more levels of plankton to get to fish.

Microbial loop is key

Open Ocean

Tuna

Carniv. Fish

Carniv. Plankton

Herbiv. Plankton

Phytoplankton

5 Levels10% Efficiency

Coastal Ocean

Carniv. Fish

Carniv. Plankton

Herbiv. Plankton

Phytoplankton

4 Levels15% Efficiency

Upwelling Zone

Anchovies

Phytoplankton

2 Levels20% Efficiency

Oligotrophic Eutrophic

Draw biomass spectrum here

Area % of ocean area

Total Plant

Production

Transfer

Efficiency

Trophic

Levels

Estimated

Fish Production

(x109 metric tons carbon

per year)

(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

Open ocean Coastal ocean Upwelling zones

=109 metric tons Cper year

=106 metric tons fish per year

5 Trophic levels10% Efficiency 4 Trophic levels

15% Efficiency2 Trophic levels20% Efficiency

Chisholm, 2000

Photosynthesis respiration

Food-web structure affects the export of carbon to deep ocean

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.

Marine snow

Large plankton and their fecal pellets

Small plankton and their fecal pellets

In eutrophic conditions, there are more, larger particles that sink into deep ocean.

Temp.

Dep

th

Large fecal pellets

Large Marine snow

Dcr

In oligotrophic conditions, there are fewer, smaller particles that sink more slowly into deep ocean.

Temp.

Dep

th

Dcr

small fecal pellets

Eutrophic vs. Oligotrophic summary

Eutrophic Oligotrophic

Mixed layer More mixing Cooler

More stratified Warmer

Nutrients High concentration

Newer

Low concentration

More recycled

Plankton Larger Smaller

Particles Larger

Faster-sinking

Smaller

Slower-sinking

Carbon Export More Less