This technique was used widely during the first part of this century. Samples of seawater with their natural phytoplankton populations, are collected from several depths within the photosynthetic zone and their oxygen contents measured. Pairs of bottles are then filled from each sample and sealed. The bottles are identical except that one of each pair is transparent - the light bottle - and the other is covered with black opaque material - the dark bottle.
The pairs of bottles are next suspended in the sea at the series of depths from which their contents were obtained and left for a measured period. Alternatively, if temperature and illumination at the sampling depths are known, the bottles can be immersed in tanks at corresponding temperatures and provided with artificial illumination at the correct intensity. This is advantageous at sea because the vessel does not need to remain hove-to at each station for the duration of the experiment. With bottles in tanks, it is necessary to keep them in sufficient motion to prevent settlement of the plant cells. Whichever method is used, the amount of oxygen in each bottle is measured again after the set time interval. The reduction in oxygen in the dark bottles with respect to the original measurements is due to the respiration of the plant, animal and bacterial cells contained. On the assumption that respiration is not influenced by light, the difference in oxygen content between the light and dark bottles of each pair is regarded as being due to the production of oxygen by photosynthesis. This assumption is probably not justified but the method has been widely used and can give tolerably consistent results. One complication which applies to all experiments in which seawater is enclosed in bottles is the rapidity of bacterial growth in these conditions, and this may vary with the intensity of illumination.
Measurement of carbon dioxide uptake: the Steeman Nielsen 14C method
This is a method of measuring carbon fixation by using the radioactive isotope of carbon, carbon-14 (14C), as a tracer. The experimental technique is very similar to the oxygen bottle method described above. Samples of seawater are collected from a series of depths and the carbon dioxide content in each is measured. Bottles are filled from these samples and a small measured quantity of bicarbonate containing 14C is added to each. The bottles are then sealed and suspended in the sea at appropriate depths for a measured period or incubated in tanks as described above.
When the bottles are hauled in or removed from the tanks, the water is filtered to collect the phytoplankton. The cells are washed and their 14C-content estimated by measurement of the beta-radiation. The total carbon fixation is calculated from the known amounts of 14CO2 and total CO2 originally present in the water, making due allowances for the slight differences in rates of assimilation of 14CO2 and 12CO2.
There are several problems in interpreting the results of these measurements associated largely with the difficulties of allowing for losses of the organic products of photosynthesis in solution. This method measures the amount of 14CO2 retained on particulate matter filtered from the water. However, there is evidence that part of the compounds formed by primary production pass into the water in soluble form (see pages 120 and 194). This method gives results of the same order as are obtained by oxygen-bottle methods, and is generally thought to give the most accurate measurements of net primary production in particulate form. It is necessary that some estimate is made of losses of soluble organic material.
Another source of inaccuracy inherent in all methods of estimating production for the complete photosynthetic zone from discrete water samples is the extremely patchy distribution of phytoplankton which occurs in some conditions (see Section 5.3.4). At times the phytoplankton is concentrated in narrow lines or layers at particular depths, which may easily be missed in taking water samples.
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