GLOSSARY - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 370

GLOSSARY

accessory pigments The main
photosynthetic pigment used by
photoautotrophs is chlorophyll a.
Accessory pigments have different light
absorption spectra compared to
chlorophyll a, and so are useful for
capturing photons not accessible to
chlorophyll a and transferring energy to
the photosystem. Pigments are often
specific to particular phytoplankton
groups, as detection of the accessory
pigments provides a means to estimate the
proportions of different phytoplankton
groups within a sampled community.
Acoustic Doppler Current Profiler A device
for measuring profiles of water velocity
using the Doppler shift of sound reflected
back by particles drifting freely with the
water.
active fluorescence The active fluorescence
technique uses sequences of very short
flashes of light to gradually saturate the
light-gathering capacity of a
phytoplankton sample. It allows
assessment of the state of the
phytoplankton photophysiology (e.g.
number and size of light-gathering units in
the cells), and shows some promise in
estimating rates of primary production that
are compatible with the more standard 14 C
incubation method.
ageostrophic

time of high water and other phases of the
tide due to the constituent rotate around
the amphidrome. See also degenerate
amphidrome.
autotrophs (autotrophic) Organisms that
synthesise organic compounds such as
carbohydrates from inorganic matter.
In this topic we deal exclusively with
the photoautotrophs, or the autotrophic
phytoplankton, that fix inorganic
carbon using sunlight as an energy
source.
baroclinic Flow in which surfaces of equal

density (isopycnals) are not parallel to
isobars (surfaces of equal pressure). In
baroclinic regions the horizontal pressure
gradient varies with height. A baroclinic
pressure gradient might be set up by having
a source of freshwater, or by the passage of
an internal wave. The flows driven by the
pressure gradient will vary through the
water column. (See also barotropic.)
baroclinic instability An instability in flows
where there are horizontal density
gradients; the motion grows through the
conversion of potential energy in the
density field to kinetic energy of the mean
flow leading to the formation of meanders
and eddies.
barotropic Flow in which surfaces of equal

density (isopycnals) are parallel to isobars,
so that the pressure gradient is independent
of depth. Barotropic flows tend, therefore,
to be depth uniform and the term is often
used (rather loosely) to signify depth
independence. (See also baroclinic.)
brine rejection In freezing conditions, ice is
formed mainly from freshwater. The
rejected salt enhances the salinity of the
unfrozen seawater and increases its density.

Not in geostrophic balance.
Ageostrophic flows are the result of an
imbalance between pressure and Coriolis
forces which occurs, for example, due to
frictional effects in the surface and bottom
boundary layers.
amphidrome A point of zero tidal
displacement in the response of a shelf
region to forcing by a tidal constituent. The