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Open Oceans: Pelagic Ecosystems II

Open Oceans: Pelagic Ecosystems II. Global scale patterns of pelagic productivity. What are controlling processes?. Primary Productivity Different estimates of productivity Gross Primary Productivity Net Primary Productivity Standing crop and Grazing Rates. What is “productivity”?.

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Open Oceans: Pelagic Ecosystems II

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  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 281:237-240) • 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

  28. Diatom mats can migrate vertically

  29. Consequences of active transport of NO3

  30. Links to oceanic nekton

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