Aquatic ecology and Biodiversity

Practical 7- Measurement of primary production

Experiment No. 7
The term primary production refers to the rate of formation of organic matter by the phytoplankton. Whereas standing crop refers to the phytoplankton present at any one time in a unit volume of water. In other words, it is the rate at which new organic matter is added to the existing phytoplankton standing crop. It is expressed as grams of carbon per square meter or cubic meter/hour/day/year.
Methods
• Light and dark bottle method
• Carbon ¹⁴C method
Light and dark bottle method (Garden and Gran, 1930)
The amount of oxygen liberated by phytoplankton during photosynthesis is considered as a measure of primary production.
Procedure
Collect water sample in 300 ml BOD round stoppered bottles (1light bottle, 1dark bottle and 1conrol bottle). Remove zooplankton by filtering through plankton net (300µm).Water sample in the control bottle is immediately fixed by using Winkler’s fixatives. The dark bottle is wrapped with aluminum foil and kept in a black bag to protect from light. The light and dark bottles are then suspended on to a raft and anchored. The bottles are normally incubated for a period of 4-6hrs between dawn to midday or between midday and sunset in the respective depths from where these samples have been collected. After incubation period, the bottles are taken out and fixed with oxygen fixatives. The oxygen content in the sample is determined by using Winkler’s method.
Calculation
Gross Production = LB=-DB ………………………..A
Net Production=LB-IB ………………………..B
Respiration=IB – DB ………………………..C

Gross production (mgC/l/h)=
A x 0.536/PQ x 4 or A x 0.375 / PQ x 4………X

Net production (mgC/l/h)=
Bx0.536/PQx4 or B x 3.75 / PQ x 4………..Y

Gross or net production mgC/l/day
X/Y x 8 (as sunlight only for 8hr a day)……………….Z

Gross or net production gC/m³/day
=Z x 1000 x 1000
Carbon ¹⁴c method (Steemann-Nielsen, 1952):The principle of this method is based on the fact that the phytoplankton cells entrapped in the bottle containing water sample absorb 14C from the added radio-isotope labeled compound (NaH ¹⁴Co₃ + NaH ¹⁴Co₃) as the corresponding nonlabelled compound present naturally in the water. The uptake of radioactive carbonate, as a fraction of the whole, is assumed to be the measure of total carbonate, (as a fraction of the whole), and hence the rate of photosynthesis may be evaluated.
Appraratus
BOD bottle (300ml) with screw capped, filter paper, black cloth, filter paper, scintillation counter, syringe (2ml), scintillation liquid, etc.
Procedure
Collect the sample in dark and light bottles (100ml) after screening through 300µm meshsize nylon cloth to remove zooplankton and keep in a dark box. Add 1ml of carbon amuple into both dark and light bottle with a syringe (5µCi=1.11x107 dpm). Mix well, incubate the bottles for a known period by suspending in their respective depths from which samples have been taken. After incubation period, bottles are removed and kept in a dark box. Filter the water samples on membrane filters of about 0.45µm porocity by using vacuum filtration apparatus. Remove the filter from the filtration apparatus and place onto plachets which are then kept in a desiccator containing silica gel. Remove the filter from the holder and place in a scintillation vial containing 10ml of scientillation fluid. Allow it for overnight after shaking. Radioactive counts are measured in liquid scintillation counter.
Calculation

Where CPM : Counts per minute
Tot.CO₂ : Assumed to be constant in oceanic waters = 90mgCo₂/l
1.06 : Correlation factor for the isotope determination effect and to be used as the ¹⁴C incorporation will be slow compared to ¹²C.
1000 : To convert the value for m³
12/14 : To get the value of V from CO₂ as the mol.Wt.of CO₂ is 44 and At.wt of C is 12.