An estimation of gross primary productivity by monitoring rates of production of oxygen, the by-product of photosynthesis. In field practice this principle is most often used for productivity measurement in aquatic ecosystems. Paired ‘light and dark’ bottles are filled with water from the required sample depth and suspended at this level for several hours. The dark bottle records oxygen uptake by respiration of phytoplankton and other micro-organisms during the measurement period. Changes in oxygen levels in the light bottle reflect production in photosynthesis as well as loss in respiration. Assuming that respiration rates in the paired bottles are similar, gross primary productivity may be estimated by adding oxygen loss in the dark bottle to the change recorded in the light bottle. The light bottle alone records net (ecosystem) productivity. Net primary productivity estimates are only possible in dense phytoplanktonic communities (e.g. algal blooms in which the oxygen loss caused by respiration of other micro-organisms is proportionately very small). An important limitation and source of error in the application of the oxygen method to macrophytes arises from the internal storage and utilization of oxygen produced by photosynthesis.
Subjects: Plant Sciences and Forestry — Ecology and Conservation.