1. Biology of mixed layer Primary production by Phytoplankton - small drifting organisms that photosynthesize Competition and limits on production Critical and compensation depths Irradiance Phytoplankton Physical mixing processes Nutrients 1

2. Photosynthesis (P) Carbon dioxide (C,O) + Water (H,O) + Nutrients (N,P) + Light energy Oxygen (O) + Organic matter (C,H,O,N,P) proteins fats carbohydrates nucleic acids Requires chloroplasts Plants, algae

3. Respiration (R) Carbon dioxide (C,O) + Water (H,O) + Nutrients (N,P) Oxygen (O) + Organic matter (C,H,O,N,P) proteins fats carbohydrates nucleic acids Every living thing respires (plants too!) 3

4. Primary Production - Definitions Gross Primary Production (GPP) = rate of carbon fixation by photosynthesis units = [Mass / Area / Time], e.g. [g C m -2 y -1 ] Respiration (R) = rate of carbon (CO 2 ) loss through metabolism Net Primary Production (NPP) = GPP - ΣR Need GPP>ΣR for net growth!

5. Production ≠ Biomass Production is a rate e.g. [g C m -2 y -1 ] Biomass is a concentration e.g. [g C m -2 ]

6. “Paradox of the Plankton” There are many species of phytoplankton, despite few limiting resources and lots of mixing. Phytoplankton (single-celled primary producers) have various competitive strategies that enable coexistence.

7. Large (5-2000 μm) Have unique life cycle & blooms Small (2-25 μm) Have CaCO 3 tests Large (2-200 μm) Have silica frustules Small (<1 μm) or Large (0.5-4 mm) Nitrogen fixers Four major players CyanobacteriaDiatoms CoccolithophoresDinoflagellates

8. What limits production? Nutrients Light –Intensity –Spectrum Temperature Grazing by zooplankton

9. The environment varies in space and time. Different phytoplankton grow well under different conditions. High Low Nutrients Low High Light Intensity Narrow Broad Light Spectrum Low High Temperature Deep water / Winter Shallow water / Summer

10. Nutrients N, P, Si, Fe Nitrogen is most often limiting in ocean Bioavailable forms of inorganic N: –Nitrate (NO 3 - ) –Ammonium (NH 4 + ) –Nitrite (NO 2 - )

11. At low nutrient concentrations, smaller phytoplankton tend to grow faster Assume cell is a sphere. Surface area: Volume: Surface area to volume ratio: Smaller cells have relatively more surface area for taking up nutrients. r r

12. N = [Nutrient] μ = Specific growth rate (d -1 ) Growth rate varies with [nutrient] Curve “saturates”

13. KsKs μ max /2 μ max N = [Nutrient] μ = Specific growth rate (d -1 ) Growth rate varies with [nutrient] “half-saturation constant”

14. KsKs μ max /2 μ max N = [Nutrient] μ = Specific growth rate (d -1 ) Growth rate varies with [nutrient] Michaelis-Menten Kinetics “half-saturation constant”

15. Diatoms High μ max High Ks Coccolithophores Low μ max Low Ks High or variable nutrients High mixing, upwelling Low average irradiance High turbulence Chronically low nutrients Stratified conditions High average irradiance Low turbulence Different strategies of nutrient use

16. Larger plankton (diatoms and dinoflagellates) most adapted to high-nutrient conditions. High Low Nutrients Low High Light Intensity / Irradiance Narrow Broad Light Spectrum Low High Temperature Deep water / Winter Shallow water / Summer

17. PAR = photosynthetically active radiation (visible light wavelengths) Irradiance = power of electromagnetic radiation per unit area of ocean’s surface (e.g. W m -2 ) - or - energy per area per time (e.g. mol photons m -2 s -1 ).

18. Light (PAR) attenuates with depth I z = irradiance at depth z Units of [Watts m -2 ] or [mol photons m -2 s -1 ] Light attenuates as it is absorbed and scattered by particles in the water. Z IzIz Z0Z0

19. Hiscock et al. 2008 Average Primary Production saturates at high PAR (photosynthetically active radiation) Inside Fe patch Outside Fe patch

20. Species adapt to different light levels Irradiance 1010 Ryther 1956 Photo-inhibition at high light levels Too much light damages cells and reduces photosynthesis (photo-inhibition)

21. Diatoms most adapted to low-light conditions High Low Nutrients Low High Light Intensity / Irradiance Narrow Broad Light Spectrum Low High Temperature Deep water / Winter Shallow water / Summer

22. Attenuation varies with wavelength. More wavelengths are available near the surface. Plankton use colored pigments to harvest light at different wavelengths. violet red

23. Different color pigments absorb different wavelengths of light Pigments (colored molecules) Phytoplankton with different pigments Chlorophyll*

24. Phytoplankton with multiple pigments capture more wavelengths All phytoplankton have chlorophyll Coccolithophores and diatoms have carotenoids Cyanobacteria have phycoerythrin, phycocyanin

25. Coccolithophores and Cyanobacteria most adapted to broad spectrum of light found in shallower mixed layer High Low Nutrients Low High Light Intensity / Irradiance Narrow Broad Light Spectrum Low High Temperature Deep water / Winter Shallow water / Summer

26. Growth-temperature curves vary among species but share upper limit Temperature o C Growth rate Upper limit (Eppley 1972)

27. Diatoms grow fastest at low temperatures Temperature o C Divisions per day DiatomsFlagellates

28. Diatoms most adapted to colder temperatures High Low Nutrients Low High Light Intensity / Irradiance Narrow Broad Light Spectrum Low High Temperature Deep water / Winter Shallow water / Summer

29. Keep in mind the physics & chemistry of the mixed layer Nutrients Light & HeatWind

30. Draw the mixed layer here

31. Primary production varies with depth Respiration Depth Photosynthesis Depth + biomass - biomass (Requires light) (Independent of light)

32. Compensation Depth Depth P>R Biomass increases P = R Compensation depth P<R Biomass decreases R P

33. Critical Depth 0 GPP=ΣR Net Primary Production (NPP) = 0 Critical depth Depth R P Gross Primary Production (GPP) Sum of Respiration (ΣR)

34. Depth R P Bottom of mixed layer Critical depth If critical depth > mixed layer depth, GPP>ΣR, NPP >0 Gross Primary Production (GPP) Sum of Respiration (ΣR)

35. Depth R P Critical depth Bottom of mixed layer If critical depth < mixed layer depth, GPP<ΣR, NPP<0 Gross Primary Production (GPP) Sum of Respiration (ΣR)

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