Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawaii
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SUMMARY: The 14C-radiotracer method is used to measure the assimilation of dissolved inorganic carbon (DIC) by phytoplankton as an estimate of the rate of photosynthetic production of organic matter in the euphotic zone.
The 14C method, originally proposed by Steeman-Nielsen (1952), is used to estimate the uptake of dissolved inorganic carbon (DIC) by planktonic algae in the water column. The method is based on the fact that the biological uptake of 14C-labeled DIC is proportional to the biological uptake of 12C-DIC. If one knows the initial concentration of DIC in a water sample, the amount of 14C-DIC added, the 14C retained in particulate organic matter (14C-POC) at the end of the incubation and the metabolic discrimination between the two isotopes of carbon (i.e., 5% discrimination against the heavier 14C isotope), then it is possible to estimate the total uptake of carbon from the following relationship:
DIC * 14C-POC * 1.05 C uptake = -------------------- 14C-DIC added
Due to the potentially toxic effects of trace metals on phytoplankton metabolism in oligotrophic waters, the following procedure is used to minimize the contact between water samples and possible sources of contamination.
3. Isotope Stock
4. Incubation Systems
Typically we measure primary production using in situ incubation techniques.
6. Isotope Addition and Sample Incubation
8. 14C Sample Processing
From the data derived above we can estimate several properties of the phytoplankton populations at Station ALOHA. Total daylight organic carbon production is calculated from the 12-hour uptake data (after corrections for 12-hour dark activities). Net daily organic carbon production is calculated from the 24-hour light/dark samples (corrected for the time-zero blank activities). Phytoplankton population respiration is taken as the difference between the 12-hour light and the 24-hour light/dark incubations. Net primary production is used as the estimate of phytoplankton carbon production for the purposes of comparison to other ecosystem-level processes (e.g., standing stock assessments, vertical C-flux, etc.).