Primary production is measured in situ as part of the monthly Bermuda Atlantic Time Series (BATS) cruises.
Scope and field of application
Primary production is a fundamental ecological variable for understanding the flow of energy into an ecosystem as it supports the availability of organic material as building blocks for higher trophic levels. This method uses a radiocarbon 14C spike and liquid scintillation counter (LSC) techniques to quantify the rate of primary production. This procedure describes the method for the determination of primary production in seawater, expressed as milligrams of carbon per cubic meter per day (mg C m−3 day−1) . This method is suitable for the assay of all levels of primary production found in the ocean.
Primary production is defined as the rate of uptake of inorganic carbon (DIC) into particulate organic carbon (POC),
DICn * POC14C * 1.05
C uptake = ----------------------------- , where
DIC14C
C uptake = rate of carbon fixation (mg Carbon m−3 day−1)
DICn = naturally occurring dissolved inorganic carbon
POC14C = 14C spiked particulate organic carbon
DIC14C = 14C spiked dissolved inorganic carbon
1.05 = metabolic discrimination factor due to biological isotopic fractionation (preferable uptake of lighter isotopes)
Principle of analysis
The rate of carbon fixation by autotrophs in seawater is measured by tracing the uptake of radioactive 14C from the inorganic form to the particulate organic form. Radiocarbon is added at an assumed ratio to the total inorganic carbon content of the seawater sample. The uptake of radiocarbon by the particulate phytoplankton is converted to total carbon uptake by the application of this radiocarbon: total carbon ratio. Inorganic carbon is not measured because samples are acidified before analysis. The seawater is collected using the CTD at discrete depths every 20 meters from the surface to 140 meters. The radioactive 14C spike is added and samples are incubated in situ at their respective depths using a free-floating array. The array is deployed prior to first light and recovered after sunset to capture the dawn to dusk light cycle.
A liquid scintillation counter (LSC) is used to calculate the level of radioactivity in the sample and therefore the amount of 14C particulate organic carbon. The LSC measures the conversion of radioactive decay events into photons of light, which are detected by photomultiplier tubes and converted into electrical pulses. In order to aid the detection of radioactivity, a liquid scintillation cocktail is added (Ultima gold for this method). The cocktail contains both solvent and scintillator molecules. The radioactive decay from the 14C excites the solvent molecule, and the energy is transferred to the scintillator which re-emits the energy in the form of light. Often more than one type of scintillator is present in the cocktail to allow for the emission of light at a suitable wavelength to be detected by the photomultiplier tubes. The resulting electrical signal that is generated is recorded as counts per minute (CPM).
Field sampling
Samples for primary production are collected two hours before dawn (pre-dawn production cast) and no other samples are taken during this cast. Nitrile gloves are used during the handling of samples. The polycarbonate incubation bottles are filled directly from the Niskins under low light conditions. Each bottle is rinsed 3 times before filling. Five bottles are filled for each sample depth. 250 µl of the 14C working solution is added to each of the five bottles in the shared use radioisotope lab container. Low light levels are maintained by using red lights in the lab. One of the five bottles is wrapped in electrical tape; this bottle is then wrapped in aluminium foil to ensure it is kept in dark conditions. One of the five productivity bottles is used as the time-zero (T-0) sample. The spike is added, the sample is then thoroughly shaken before 50 ml is filtered. A 250 µl aliquot -- to be used for counting total added 14C activity -- is removed from each of the T-0 bottles and is placed in a 20 ml glass scintillation vial containing 250 µl ethanolamine.
Approximately one hour before sunrise the productivity array is deployed. The incubation occurs throughout the day and the array is recovered approximately half an hour after sunset. Upon recovery and under low light conditions, a 50 ml aliquot is withdrawn from each productivity bottle and filtered onto a 25 mm Whatman® Glass Fibre Filter, maintaining vacuum levels of 70 mm Hg or less. Neither the filter nor the syringe is rinsed. The filter is placed into a 20 ml glass scintillation vial. Under a fume hood, excess radioactive carbon is driven off by adding 250 µl 0.5 N hydrochloric acid. A 250 µl aliquot for counting total added 14C activity (Time End Specific Activity) is removed from one of the light productivity bottles. This is placed in a 20 ml glass scintillation vial containing 250 µl ethanolamine (Sigma), similar to the T-0 described in Time Zero Specific Activity Sample. The samples are then stored at room temperature until analysis.
For additional details, please see Protocols for the Bermuda Atlantic Time-series Study Core Measurements.
