Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 136

this is not necessary. For instance, a key question in marine biogeochemistry which
can be answered without separating the respiration contributions is whether a whole
community is net autotrophic or heterotrophic; the former will result in a net
drawdown of CO 2 from the atmosphere to the ocean while the latter will lead to a
net release of inorganic carbon into the seawater and potentially back to the atmos-
phere. If, however, we want to parameterise phytoplankton growth in a numerical
model, we will require a value for the autotrophic respiration rate. Careful consider-
ation of the information yielded by dawn-to-dusk and day-night 14 C incubations has
suggested how autotrophic respiration can be separated from that of the whole
community (Marra and Barber, 2004 ).
Incubations in a turbulent layer
Incubations are typically done over a fixed time interval at a fixed irradiance. In the
real ocean, turbulence will mix phytoplankton vertically, presenting them with a
rapidly changing light environment. Imagine a water sample collected at a depth of
5 metres in a surface layer of thickness 20 metres. Clearly if we were to incubate the
sample at a fixed irradiance as measured at 5 metres, then the phytoplankton in our
sample would experience a very different light regime compared to when they were
being mixed continuously between the sea surface and the base of the surface mixed
layer. This can lead to an incorrect estimate of primary production in the incubated
sample by as much as 40%, though it is possible to compensate for the effect
(Barkmann and Woods, 1996 ).
Other techniques for measuring production
In Chapter 1 we showed a global map of net primary production based on a satellite
technique (Behrenfeld and Falkowski, 1997 ). As we noted, an important drawback of
satellite techniques is that they only view conditions at the sea surface, and so miss
any activity deeper than a few metres (about an optical depth, 1/K PAR ). We will see in
Chapter 7 that an important component of a shelf sea's annual primary production
occurs in the thermocline, which is several optical depths below the sea surface, so
satellite imagery is likely to under-estimate shelf sea productivity without applying
some technique to account for sub-surface production (e.g. Longhurst et al., 1995 ) .
Satellite techniques will also be undermined by the difficulty in accurately determin-
ing chlorophyll concentrations against the signals caused by dissolved organic sub-
stances and suspended sediments, a problem particularly associated with coastal seas.
Coastal waters are categorised optically as 'case 2' waters, in contrast to the clear,
oceanic 'case 1' water. Accurate chlorophyll detection using satellite sensors in case 2
waters still remains a key challenge in satellite oceanography (e.g. Blondeau-Patissier
et al., 2004 ).
A technique with particular promise is the use of active fluorescence measured with
fast repetition rate (FRR) fluorometers. The fluorometers that we normally use to
estimate chlorophyll biomass flash a strong light source into the water which satur-
ates that phytoplanktons' photosystems, resulting in the maximum fluorescence. Fast
repetition fluorometry interrogates the photosystem more subtly by gradually