Defining and measuring production Flashcards
Gross Primary Production
GPP/ Pg
Rate of gross photosynthesis over some specified period, uncorrected for respiration
Net Primary Production
NPP/ Pnp
Gross PP minus the losses due to (photo)autoptrophic respiration (Rphyto)
Net Community Production
Gross PP minus all losses in C due to respiration
Potential measurement methods
Biomass changes, organic C or chlorophyll
Gas flux measurements. Oxygen light-dark method
Isotopes, C13 and 14, H2O(18), N15 of Ammonium15
No method is perfect
Biomass change over time
Directly measures how much organic C is present.
Quantitative data processed through exponentials and log data.
Growth rate (µ, d-1)
Fitting a straight line to the exponential phase
Multiplying by biomass (Chl) gives Net Production (Chl l-1 d-1))
Multiplying by biomass (C) gives net production (C I-1 d-1)
Not very successful in an open experiment:
- Communitty uses of carbon
Movement of phytoplankton
Gas Flux Measurements
Rates of photosynthesis from changes in O or CO2
Measurements complicated by simultaneous oxygen consumption and carbon dioxide evolution by respiration
Only net changes are quantified.
Limits on measurable rates set by the concs of O and DIC in water and precision of methods for determining concs in water
Oxygen light dark method
Pg = (O in light bottle - O in dark bottle) / time
Pn = (O light bottle - O in zero-time bottle) / time
Respiration = (O zero-time bottle - O in dark bottle) / time
Oxygen light dark method advs and disadvs
Advs:
- simple
- provides info on photosynthesis and respiration
Disadvs:
- Lacks sensitivity
- Technical issues with oxygen measurement, bubble formation affects precision.
- Temp changes must be kept to minimum
- Requires incubation of samples
In situ method
In situ over time should relate to NCP
However physical processes might decouple oxygen concentration from the saturated value.
Can correct using parallel measurements of Ar, only being influenced by the physics so can compare with O to see biology.
In situ advs and disadvs
Advs:
- no bottle incubations
- longer integration time (1-2 weeks)
- measured continuously underway (O / Ar)
Disadvs:
- Biases, primarily due to mixing/entrainment and non steady state conditions
- Substantial uncertainties
- requires specialised equipment
C14 isotope measurements
Add a small tracer amount of C14 labelled carbon dioxide.
Uptake of 14 proportional to 12 uptake.
V sensitive method, good precision.
Pn or Pg (g on short and n on long timescales)
- extent of respiration of photosynthetically fixed 14.
- recycling of respired carbon dioxide is preferred overuse of external carbon dioxide.
C 14 method
- collect sample
- add tracer
- incubate
- filter
- measure tracer (liquid scintillation counting for 14)
C14 advs and disadvs
Advs:
- relatively simple
- very sensitive
- small volumes can be used, short timescales and many samples can be measured.
disadvs:
- tracer dynamics complicate interpretation
- radioactive tracer requires special precautions (dedicated equipment, separate labs)
Other tracers
C13:
- Use of stable isotope bicarbonate enriched in C13 added to sea water.
- changes in 13C:12C in particles measured by Isotope Radio Mass Spectrometry (IRMS)
O18:
- water is enriched in 18O, small vol added to incubated seawater.
- measurement of production of 18O labelled oxygen from water enables Pg to be measured.
- Detection by Membrane Inlet Mass Spectrometry
- Sensitive but not commonly used.
Photosynthesis measurements
Often when measuring O and C fixation levels O is higher
Because the light reaction that produces O produces reductants (NADPH) and energy (ATP)
These have other uses within the cell, so they don’t all go towards the light independent reaction.
