Geoscience Reference
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saturating it with a rapid series of light pulses. The electron transport rate within
the photosystem can be calculated knowing the fluorescence characteristics,
leading to a measure of photosynthesis in terms of oxygen evolution which can
then yield a rate of carbon fixation (Kolber and Falkowski, 1993 ). The great
attraction of this technique is that it has the potential to provide instantaneous,
in situ measurements; the instrument can be fitted alongside a profiling CTD and
could yield productivity data on scales compatible with our knowledge of the
physical structure and short-term temporal variability of the water column. How-
ever, while comparisons with 14 C incubations do show good agreement (Kolber
and Falkowski, 1993 ; Moore et al., 2006 ) , the technique is not yet ready to be
employed as an alternative to 14 C. So far the real strength of the FRR fluores-
cence technique is in laboratory analyses of water samples collected by the CTD,
allowing determination of the photo-physiological state of the phytoplankton
community (Suggett et al., 2009 ).
5.1.4
Measuring water column production and the photosynthesis-radiation curve
There are two basic aims of collecting measurements of primary production at
different depths. The first is to be able to calculate a depth-integrated estimate of
primary production (in units of g C m 2 d 1 ), where the integration goes from the sea
surface to the base of the photic zone (generally taken to be where PAR drops to
0.1% or 1% of its surface value). Second, by plotting how the primary production
rate varies under different levels of light we can measure several parameters that
provide us with information on the physiology of the phytoplankton, which are vital
inputs to numerical models.
At sea, collecting the samples for experiments on primary production begins with
the biogeochemists carrying out a CTD profile before dawn. In our work in the
northwest European shelf seas, this means typically at 0300. The reason for this very
early start is that the water samples need to be stored in bottles without being
exposed to daylight; taking samples from deep in the surface layer and exposing
them to daylight as they are taken from the CTD rosette bottles could damage the
phytoplankton photosystems. Once the samples are collected there are two proced-
ures commonly used. The first is mainly carried out in order to estimate net rates of
water column photosynthesis, while the second allows the calculation of a curve
describing the variation of photosynthesis with light. We will look at them both in
some detail.
Simulated in situ productivity measurements
In the simulated in situ 14 C method, samples are taken from depths set by the light
distribution, e.g. close to the surface, and at, say, 0.1%, 1%, 14%, 33%, 55% and
97% of surface PAR; the choice tends to be driven by the capacity for making the
measurements and the filters available to mimic the light gradient. Determining the
corresponding sampling depths requires a measure of K PAR , which can be based on a
light profile collected the previous evening. The bottles containing the water to be
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