OC211(OA211) Phytoplankton & Primary Production Dr Purdie SOC (566/18) LECTURE 5 Week 5 (i) Measuring Primary Production: The oxygen light and dark method and the 14 C method

Scales of Measurement of Primary Production

Phytoplankton Production: Possible Methods..Any method must be both: accurate & precise (or sensitive) Photosynthesis: CO 2 + H 2 O > CH 2 O + O 2 In principle photosynthesis can be measured by measuring a) increase in organic carbon b) decrease in TCO 2 c) increase in O 2

Phytoplankton Production: Possible Methods 1)Gas Flux Measurements a)CO 2 changes: TCO 2, pCO 2, pH b)O 2 Changes 2)Isotopes 14 CO 2 3 H 2 O 18 O 2 15 NO 3 or 15 NH 4 33 PO 4 or 32 PO 4 3) Biomass changes: organic carbon or chlorophyll 4) Fluorescence Kinetics

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

Phytoplankton Production: Gas Flux Measurements: Rates of photosynthesis from changes in O 2 or CO 2. Measurements complicated by simultaneous O 2 consumption and CO 2 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.

Phytoplankton Production: Oxygen solubility determined by water temperature and salt concentration O 2 in equilibrium with atmosphere varies by factor of 3. variation from thermodynamically defined equilibrium O 2 concentration can result from biological (and chemical) O 2 consumption or production processes. Can express variations as % saturation % saturation = [O 2 ]/[O 2 ]eq or Apparent oxygen utilization (AOU) AOU = [O 2 ]-[O 2 ]eq where [O 2 ] is measured O 2 concentration and [O 2 ]eq is thermodynamically defined. In upper water column with low chlorophyll/algal biomass O 2 satuation is % In eutrophic waters 200%

Phytoplankton Production: Oxygen Light & Dark Incubation method Gross photosynthesis = [O 2 ] light bottle- [O 2 ] dark bottle Net photosynthesis = [O 2 ] light bottle- [O 2 ] zerotime bottle Respiration = [O 2 ] zerotime bottle- [O 2 ] dark bottle Determine [O 2 ] using Winkler Method: 2Mn 2+ +4OH - -> 2Mn(OH) 2 2Mn(OH) 2 + O 2 -> 2MnO(OH) 2 2MnO(OH) 2 + 4I - + 8H + -> 2I 2 + 2Mn 2+ 2I 2 + 4S 2 O > 4I - + 2S 4 O 6 2- Thus O 2 = 4S 2 O 3 2-

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

14 C method Water sample Measure pH and alkalinity to determine TCO 2 14 C ampoule Add 14 C to 100 ml water samples and incubate Filter 100 ml of sample Take 10 ml of filtrate and bubble with N2 to remove 14CO2 Add to scintilant in vials Determine 14 C in all samples in scintilation counter

14 C method most commonly used method add a small tracer amount of 14 C labelled CO2 uptake of 14 CO2 is proportional to 12 CO2 uptake Rate of carbon fixation (photosynthesis)= 14 CO 2 taken up (PO 14 C+DO 14 C)x 12 CO 2 available(TCO 2 ) 14 CO 2 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.

(ii) Methodological problems ampoule contamination with silicate preservation of sample prior to filtration not reccommended filter immediately cell damage on filter causes loss of 14 C 14 CO 2 organic contamination of stock 14 CO 2 scintillation counter efficiency organic excretion of 14 C during incubation dark bottle fixation of 14 CO 2 Other common problems of enclosing samples during incubations BiologicalChemical sedimentationbottle volume population changesbottle type respiration rate changesincubation time grazingmetals

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