OC211(OA211) Phytoplankton & Primary Production

1. OC211(OA211) Phytoplankton & Primary Production LECTURE 5 Week 5 (i) Measuring Primary Production: The oxygen light and dark method and the 14C method Dr Purdie SOC (566/18) email: DAP1@soc.soton.ac.uk

2. Scales of Measurement of Primary Production

3. Phytoplankton Production: Possible Methods ..Any method must be both: • accurate & • precise (or sensitive) Photosynthesis: CO2 + H2O ------> CH2O + O2 In principle photosynthesis can be measured by measuring a) increase in organic carbon b) decrease in TCO2 c) increase in O2

4. Phytoplankton Production: Possible Methods 1)Gas Flux Measurements a)CO2 changes: TCO2, pCO2, pH b)O2 Changes 2)Isotopes 14CO2 3H2O 18O2 15NO3 or 15NH4 33PO4 or 32PO4 3) Biomass changes: organic carbon or chlorophyll 4) Fluorescence Kinetics

5. Phytoplankton Production: Possible Methods Comparing Measurements of Primary Production: Plastic sphere experiments:

6. Phytoplankton Production: Gas Flux Measurements: Rates of photosynthesis from changes in O2 or CO2. Measurements complicated by simultaneous O2 consumption and CO2 evolution by respiration. i.e. only net changes in gases are readily quantified Limits on measurable rates set by the concentrations of O2 and TCO2 in water and precision of methods for determining O2 and TCO2 in water.

7. Phytoplankton Production: Oxygen • solubility determined by water temperature and salt concentration • O2 in equilibrium with atmosphere varies by factor of 3. • variation from thermodynamically defined equilibrium O2 concentration can result from biological (and chemical) O2 consumption or production processes. Can express variations as % saturation % saturation = [O2]/[O2]eq or Apparent oxygen utilization (AOU) AOU = [O2]-[O2]eq where [O2] is measured O2 concentration and [O2]eq is thermodynamically defined. In upper water column with low chlorophyll/algal biomass O2 satuation is 90-110% In eutrophic waters <10% to >200%

8. Phytoplankton Production: Oxygen Light & Dark Incubation method Gross photosynthesis = [O2] light bottle- [O2] dark bottle Net photosynthesis = [O2] light bottle- [O2] zerotime bottle Respiration = [O2] zerotime bottle- [O2] dark bottle Determine [O2] using Winkler Method: 2Mn2++4OH- -> 2Mn(OH)2 2Mn(OH)2 + O2 -> 2MnO(OH)2 2MnO(OH)2 + 4I- + 8H+ -> 2I2 + 2Mn2+ 2I2 + 4S2O32- -> 4I- + 2S4O62- Thus O2= 4S2O32-

9. Oxygen Light & Dark Incubation method Disadvantages/Problems • Lacks sensitivity • PQ must be known • bubble formation affects precision of method • temperature changes must be kept to minimum and similar in light and dark - respiration is quite temperature sensitive. • Photorespiration NOT measured by the O2 light and dark bottle method - assumes no light effect on respiration • storage of samples not straightforward Advantages of Method • simple approach • provides information on photosynthesis and respiration • can be used in situ to measure bulk changes in oxygen and in vitro

10. Determine 14C in all samples in scintilation counter 14C ampoule 14C method Water sample Add 14C to 100 ml water samples and incubate Add to scintilant in vials Filter 100 ml of sample Measure pH and alkalinity to determine TCO2 Take 10 ml of filtrate and bubble with N2 to remove 14CO2

11. 14C method • most commonly used method • add a small tracer amount of 14C labelled CO2 • uptake of 14CO2 is proportional to 12CO2 uptake Rate of carbon fixation (photosynthesis)= 14CO2taken up (PO14C+DO14C) x 12CO2 available(TCO2) 14CO2 available What does the 14C method measure? Net or Gross Primary Production? (i)Problems of interpretation a) extent of respiration of photosynthetically fixed 14CO2 b) recycling of respired CO2 and its preferential use over external CO2.

12. (ii) Methodological problems • ampoule contamination with silicate • preservation of sample prior to filtration not reccommended filter immediately • cell damage on filter causes loss of 14C • 14CO2 organic contamination of stock 14CO2 • scintillation counter efficiency • organic excretion of 14C during incubation • dark bottle fixation of 14CO2 Other common problems of enclosing samples during incubations BiologicalChemical sedimentation bottle volume population changes bottle type respiration rate changes incubation time grazing metals

13. Phytoplankton Production: NOTE: Oxygen difference method measures “total energy flux” 14C method measures “carbon flux” 14C method does NOT measure respiration