Primary Productivity Calculator
Gross and net primary productivity and community respiration of a water sample from the light-and-dark-bottle method — in oxygen and in carbon, per hour and per day.
Fill three bottles with the same water: measure oxygen in one now, hang one in the light and one wrapped in foil, and measure both after a few hours.
How the primary productivity calculator works
Fill three bottles with the same water: measure oxygen in one now, hang one in the light and one wrapped in foil, and measure both after a few hours. The dark bottle only respires, so its oxygen loss is respiration; the light bottle photosynthesises and respires, so its change is net productivity; adding respiration back gives gross productivity. Oxygen converts to carbon at 0.375 g C per g O₂ for a photosynthetic quotient of 1.
Formula: NPP = light − initial; R = initial − dark; GPP = NPP + R (per unit time); C = O₂ × 0.375
Worked examples
| Inputs | Gross primary productivity | Note |
|---|---|---|
| 8.0 → 9.2 light, 7.4 dark, 4 hours | 0.45 mg O₂/L/h | GPP 0.45, NPP 0.3, R 0.15 |
| A productive pond over 3 hours | 1.2 mg O₂/L/h | GPP 1.2 |
| A night-time-dominated system | 0.24 mg O₂/L/h | P/R below 1 |
FAQFrequently asked questions
Why does the dark bottle measure respiration?
Without light there is no photosynthesis, so any oxygen lost went to respiration by everything in the bottle — algae, bacteria and zooplankton alike. That is why it is community respiration rather than the plants’ alone.
How long should I incubate?
Long enough for a measurable change (0.5 mg/L or more) but short enough that oxygen does not saturate in the light bottle or run out in the dark one — typically two to six hours around midday.
Why convert to carbon?
Carbon is the currency of productivity and food-web studies. With a photosynthetic quotient of 1 (one O₂ per CO₂), 32 g of oxygen corresponds to 12 g of carbon, the 0.375 factor; PQ values of 1.1–1.4 are common for nitrate-fed growth.
What is a typical value?
Oligotrophic lakes and open ocean under 0.1 mg O₂/L/h; productive lakes and estuaries 0.5–2; algal blooms several mg/L/h. A P/R ratio above 1 over a day means the system is a net producer of organic matter.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- CSIRO — Australia's national science agency
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.