Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 154

also providing the basis for a paleoceanographic proxy of past sea surface conditions
(e.g. Colmenero-Hidalgo et al., 2004 ). Coccolithophores do have a pair of flagella,
similar to the dinoflagellates, so may be able to position themselves depending on
resource needs in low turbulence environments.

Diatoms
The diatoms tend to be among the largest of the phytoplankton, with single cells
between 10 and 100

m and with some species forming large aggregations or colonies
up to 1 mm or so in size. Diatoms frequently appear as the first and dominating
species in phytoplankton blooms, just as conditions become favourable for phyto-
plankton growth, and are, therefore, regarded as the 'opportunists' within the
phytoplankton. A defining characteristic of a diatom is that the cell is enclosed in a
silica shell (a frustule), and as a result the diatoms are the only phytoplankton group
that requires silicate as a nutrient. The reasons for the silica frustules are not entirely
clear. The energy required to synthesise a frustule is less than that needed to make an
organic cell wall by about 8% (Raven, 1983 ), while the silica may act as a chemical
buffer aiding the absorption of carbon from seawater (Milligan and Morel, 2002 ).
The mechanical strength of the silica walls, compared to an organic membrane, could
also provide protection against grazing (Hamm et al., 2003 ).
Diatoms do not have any ability to swim, but cell buoyancy has been linked to
physiology. Actively growing diatoms are often found to be neutrally or positively
buoyant, while physiological stress (e.g. nutrient deficiency) leads to cells sinking
(Richardson and Cullen, 1995 ). This physiological switching of cell buoyancy is
similar in outcome to the dinoflagellate swimming capability described earlier, with
downward motion taking the cells to higher nutrient concentrations. As with the
dinoflagellates, we might consider motion through the fluid as an aid to nutrient
uptake by steepening the nutrient gradient across which molecular diffusivity acts to
transport nutrient molecules to the cell wall. Analysis does show that for large
diatoms (cells or colonies

m

m) there could be an increase in nutrient supply
by a factor of 10-100 (Kiørboe and Saiz, 1995 ). However, sinking to deeper water
when in need of nutrients means that diatoms are dependent on vertical turbulent
mixing to bring them back towards the sea surface. There are also observations of
some species using positive buoyancy to return to the photic zone (Villareal, 1988 ;
Villareal, 1992 ).

1000

∼ m

Contrasts in community structure
There is no simple rule to define contrasts between the relative abundances of
phytoplankton groups in shelf and open ocean phytoplankton communities. Repre-
sentatives from all the groups are found in the open ocean and on the shelf. Contrasts
in the size-structure of phytoplankton communities are instead more correlated with
contrasts in turbulence: larger-celled species tend to be found in more turbulent
environments, as turbulence is necessary to transport sinking cells back towards
the photic zone and nutrient fluxes are typically greater in more turbulent environ-
ments. Thus shelf phytoplankton communities, while including cyanobacteria, will