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

Critical depth concept is critical! Respiration Photosynthesis Understand why R is a straight line Understand why P is an exponential curve Know the difference between: critical depth and compensation depth Depth

Compensation & Critical Depth

Annual cycle in N. Atlantic Nutrients Light Temperature Mixing Mixing Stratified Relative increase Spring bloom Fall mini- Phytoplankton biomass Zooplankton biomass

Latitudinal variation in seasonal cycles driven by variation in irradiance [Also Irradiance] 90oN = N. Pole 60oN ~Anchorage,AK 30oN ~N. Florida 0oN = Equator

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

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.

Phytoplankton Irradiance Physical mixing processes Nutrients Irradiance Zooplankton Higher Trophic Levels Sinking & Senescence Particle Dynamics Particle Flux (Carbon flux)

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 This basking shark can filter ~25,000 L seawater per day! *The bigger you are, the fewer you are

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

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)

Have to add heterotrophic bacteria, heterotrophic protists, and autotrophic bacteria Heterotrophs Autotrophs 20,000 2,000 200 20 2 0.2 Size (μm)

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

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.

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 Sea 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 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

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

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

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 Open Ocean 90.0 39 10% 5 4 Coastal 9.9 8.6 15% 29 Upwelling Zones 0.1 0.23 20% 2 46

=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

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 Bigger plankton sink faster. They also have bigger, faster-sinking fecal pellets. Marine snow Large plankton and their fecal pellets Small plankton

In eutrophic conditions, there are more, larger particles that sink into deep ocean. Temp. Depth Dcr Large fecal pellets Large Marine snow

In oligotrophic conditions, there are fewer, smaller particles that sink more slowly into deep ocean. Temp. Depth Dcr small fecal pellets

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