1. Open Oceans: Pelagic Ecosystems II

2. Global scale patterns of pelagic productivity

3. What are controlling processes?
Primary Productivity
Different estimates of productivity
Gross Primary Productivity
Net Primary Productivity
Standing crop
and Grazing Rates

4. What is “productivity”?
primary productivity is defined as the total quantity of carbon fixed by autotrophs
a rate expressed as grams of carbon fixed per square meter of sea-surface per unit of time
gross primary production is the total amount of organic matter produced by autotrophs
net primary production is the energy remaining after respiratory needs have been met
NPP = Gross Primary Production - Respiration

5. Questions to consider
Why should we care about patterns of biological productivity in oceans?
What are the spatial patterns of productivity?
What mechanisms promote or limit productivity?

6. Why should we care about productivity?
Photosynthetic activity in oceans created current O2-rich atmosphere
Plankton form ocean sediments & fossil fuels
Plankton are a critical part of “carbon pump” that influences atmospheric CO2
Phytoplankton form the base of food webs and associated biological diversity
Limits to productivity may limit the amount of harvestable biomass from ocean ecosystems

7. Measuring Primary Productivity: Data
Standing crop methods
Chlorophyll concentration (water extraction, satellite)
Cell counts (flow cytometers)
Rate measurements
Light-dark bottle method
Carbon-14 uptake
Advantages & disadvantages

8. chlorophyll density & temperature

9. Link between producers & grazers

10. Measuring Primary Productivity: Inferences
Each direct measure of GPP has disadvantages
Computer models integrate different parameters to estimate rates of productivity through the water column and over time
Model components (Field et al. Science 281: )
chlorophyll concentration
water depth in photic zone
fraction of water column where photosynthesis is light-saturated
surface temperature

11. Results of productivity model

12. Some patterns
average primary productivity in the oceans is ~50 g C/m2/yr
300 g C/m2/yr considered relatively high rate of primary productivity 
low rates of primary productivity typically 20 to 30 g C/m2/yr 
What accounts for this variation?

13. Comparing the makeup of water and plankton
Mean Elemental Ratios of N, and P
Organisms: 16.0N / 1P
Sea Water: 14.7N / 1P
On average, seawater lacks sufficient organic N to take advantage of the available phosphorus

14. Limited water mixing further limits nutrient availability

15. Testing models: measuring plankton in open oceans
Techniques for studying plankton consumers
Plankton levels may be dropping (NASA study)

16. Changes in Productivity
                      

17. From productivity to food webs: A size-based pelagic food web

18. Grazing pressure

19. Limits & Flows of NPP

20. Marine “Snow”
krill

21. A spatially realistic pelagic food web

22. What are links among trophic levels in pelagic systems?

23. Large phytoplankton vary with depth

24. Nutrient distribution with depth

25. Distribution of food web biomass

26. The global nitrogen cycle

27. Influences on biologically useful Nitrogen

30. Diatom mats can migrate vertically

31. Consequences of active transport of NO3

32. Links to oceanic nekton