Fróðskaparrit - 01.01.1996, Side 99

PHYTOPLANKTON COMMUNITY STRUCTURE ON THE FAROE SHELF
103
Fig. 6. Algal concentra-
tions on 20 meters depth
on transect D-E on May
29, 1995. (Seefig. 4).
| Chaetoceros
Pennate diatoms
Thalassiosira
Other
(SSSSl Rhizosolenia
The concentration gradients shown in
Figs. 3. and 4 indicate that advection into
the shelf water was most pronounced in the
eastern part of the shelf. This fits well with
current measurements which have shown
import to the eastern part of the shelf
(Hansen, 1992).
Both in May and late June-early July
1995, the nitrate concentrations were sig-
nificantly higher in the southern region of
the Faroe Shelf than in the central and
northern region. Part of the explanation for
this may be the narrow shelf in the southern
region, facilitating transport of nutrient-
rich water to the shelf. This is also suppor-
ted by measurements of zooplankton which
have shown high advection of copepods to
the southern region of the shelf (Gaard and
Reinert, 1996).
Succession ofthe algae
It is well known that hydrographic condi-
tions highly influence the phytoplankon
composition. Normally, large chain-for-
ming diatoms are more abundant in turbu-
lent water, while dinoflagellates and small-
er flagellates are more common in stratified
water. Most diatoms are large and unable to
swim. They tend to sink and turbulence or
upwelling may therefore be a necessary re-
quirement for large diatoms to remain in
the euphotic zone. Furthermore, turbulence
may increase the advective transport of nu-
trients to the cell surface and hence, in-
crease the nutrient availability to the phyto-
plankton. This may be important for large
non-swimming diatoms (Kiørboe, 1993).
However, in stratified water turbulence is
reduced and diatoms not favoured. Di-
noflagellates and smaller flagellates, on the
other hand, normally are able to swim and