Geoscience Reference
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Figure 5.3 The interior of the radioisotope laboratory aboard RRS Discovery, Celtic Sea,
June 2010. The laboratory, part of the UK National Marine Facilities, is installed within a
standard shipping container so that it can be installed on different research vessels as
required. (Photo by J. Sharples.)
disadvantage is practical; taking even the tiniest amounts of radioactive material to sea
requires conforming with a host of regulations, and dedicated, expensive laboratory
space is usually needed (an example is shown in Fig. 5.3 ). A recent alternative is to use
the stable isotope 13 Cinsteadof 14 C(Hamaet al., 1983 ), which avoids the regulatory
constraints, but in this method the analysis of the filters requires a mass spectrometer
and a larger amount of carbon biomass per sample.
Phytoplankton or community?
When seawater is incubated we observe the combined effects of all of the components
of the plankton community trapped in the bottle; i.e. the photo-autotrophs (produ-
cing oxygen when photosynthesising, and also consuming oxygen in respiration) and
the heterotrophs (largely consuming oxygen as they oxidise organic carbon). The
simple oxygen incubation technique measures the impacts of oxygen production and
consumption by the whole community (autotrophs and heterotrophs). Using the
18 Oor 14 C technique allows measurement of production by the autotrophs, as only
the photosynthesisers will be fixing 14 C or releasing the 18 O. Whether we are measur-
ing gross or net photosynthetic production depends on how long the incubations are
carried out. When using 14 C, the incubation time could be anywhere from 1 hour to 1
day. Short 14 C incubation times are usually viewed as providing something close to a
gross photosynthetic production rate, while longer times (particularly where a day-
night cycle is included) will measure net photosynthetic production.
At first glance none of the above techniques appears to allow us to separate the
autotrophic and heterotrophic contributions to the community respiration. Often
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