Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 13

Chapter 5 . The development of the fluorescence technique opened the door
to in situ chlorophyll fluorescence measurements alongside those of temperature
and salinity. Practical in situ instruments first became available in the 1970s, and
are now commonly used on lowered and towed/undulating CTDs, and, with the
addition of batteries and some further control electronics, on moorings. By making
changes to the optics (i.e. the wavelengths of the emitted and received light),
fluorometers can also be used to observe dye releases in mixing experiments
(e.g. Houghton and Ho, 2001 ), and can be made to measure pigments other than
chlorophyll a (for instance phycoerythrin, a pigment associated with a cyanobac-
terium). Multi-spectral fluorometers measure several plant pigment concentrations
simultaneously to build up a picture of the relative proportions of phytoplankton
groups via knowledge of what pigments dominate in different phytoplankton.
A more recent development in fluorometry is fast repetition rate fluorescence,
which uses very rapid flashes of light to gradually (over hundreds of milliseconds)
saturate the light-gathering capacity of phytoplankton cells and so measure the
photosynthetic status of a population of phytoplankton as well as their biomass.
We will look at this technique in more detail in Chapter 5 . Measuring fluorescence
is a convenient, but far from perfect, estimator of phytoplankton biomass. The
fluorescence characteristics of chlorophyll vary depending on ambient light and
also between species of phytoplankton, so calibrations will change during the
day and between different phytoplankton communities. We also need to remember
that a measure of chlorophyll, no matter how accurately you feel you have
calibrated your fluorometer, is still only a proxy for phytoplankton biomass.
A phytoplankton cell's carbon:chlorophyll ratio is not fixed. Ideally we really want
to be able to measure phytoplankton carbon, but as yet there is no straightforward
way to do that.
Chemistry has also seen technology developments that have led to some conver-
gence with the way we make physical measurements. Two key parameters that
can be measured routinely from a CTD package are dissolved oxygen (DO) and
nitrate (NO 3 ). Measurement of dissolved oxygen involves the DO reacting with a
cathode to set up a current in an electric circuit, with the seawater separated from
the circuit by an oxygen-permeable membrane. DO sensors can be attached to
moorings or interfaced with a CTD. With the latter it is important to note that DO
sensors tend to have relatively slow response times (a few seconds) compared to
conductivity and temperature sensors (both 0.1s in a good CTD) and so careful

processing of the data is required to match the DO with the physical structure of
the water column.
Instruments measuring all four macro nutrients (nitrate, phosphate, silicate and
ammonium) in autonomous packages that can operate on moorings began to be
commercially available in the 1990s; you will see an example of such an instru-
ment's use in Chapter 6 . Developing a technique with a response time suitable for
use on a profiling CTD has been more challenging, but ultra-violet fluorescence-
based instruments are now available for nitrate determination (e.g. Johnson and
Coletti, 2002 ). Sensors are also available commercially for pH and for redox

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