Aquatic ecology and Biodiversity

Unit 5- Productivity

5.3.3. Methods of measurement of primary production
1. Light and dark bottle method
2. Carbon ¹⁴C method

The both bottles are filled with water collected from a particular place and depth. The bottles are closed with stoppers then they are suspended at the same depth from which the water was originally taken. Inside the bottles CO₂ is being consumed, and O₂ is being produced, and we can measure the change over time in either one of these gases.
The amount of oxygen dissolved in water can be measured by titration. Before suspending the bottles, the initial O₂ concentration is determined and expressed as mg of O₂ per Liter of water (mg/L). Then, the final value is measured in both the light and dark bottles after incubation.
• Light bottle: In the light bottle there is photosynthesis or gross primary production (GPP) and there is respiration (R). The difference between these two processes is net primary production ( NPP) = (GPP - R)
• Dark bottle: In the dark bottle there is no photosynthesis and only respiration.
For example if the measured oxygen concentrations of Initial bottle = 8 mg O₂ /L ; Light bottle = 10 mg O₂ /L and Dark bottle = 5 mg O₂ /L, the oxygen increased in the light bottle compared to the initial due to photosynthesis and the oxygen decreased in the dark bottle due to respiration. With this information we can calculate the Respiration, NPP, and GPP for the given system:
(Light - Initial) = (10 - 8) = 2 mg/L/hr = (GPP - R) = NPP
(Initial - Dark) = (8 - 5) = 3 mg/L/hr = Respiration
(Light - Dark) = (10 - 5) = 5 mg/L/hr = (NPP + R) = GPP