Fróðskaparrit - 01.01.1996, Side 97

PHYTOPLANKTON COMMUNITY STRUCTURE ON THE FAROE SHELF
101
the genus Rhizosolenia and Chaetoceros.
1-2 weeks later, the diatoms decreased
rapidly and were replaced by the colony-
forming prymnesiophyte Phaeocystis
pouchetii (Fig. 5). This species then totally
dominated in the shelf water until around
late July-early August, when they too de-
creased much in concentrations. This coin-
cided with the very low nitrate concentra-
tions what were found in the water at that
time.
The diatom spring bloom on the Faroe
Shelf in 1995 thus peaked between late
April and early May. It is more difficult to
identify the timing of the spring bloom out-
side the front but it was significantly later
than in the shelf region. Fig. 4 shows that
the thermal stratification outside the front
(section F-G) was weak in late May and the
phytoplankton biomass was low. On May
29-30 (well after the collapse of the diatom
bloom in the shelf water) measurements
along a section going towards north-west
off the shelf edge showed that the concen-
trations of diatoms were still high in this
frontal area (Fig. 6). Outside this frontal re-
gion, only very low concentrations of phy-
toplankton were found at this time.
Discussion
Environmental influences on the phyto-
plankton
The well-mixed water column on the shelf
provides quite different hydrographic con-
ditions for the phytoplankton than in the
area outside the tidal front. Furthermore,
the isolation of the water mass inside the
front may cause limitation of the potential
new primary production (Dugdale and Go-
ering, 1967) due to the limited amount of
nutrients on the shelf. The shelf water, thus,
has an isolated plankton ecosystem with
the phytoplankton community separated
from the surrounding water and with quite
different environmental conditions than the
open area outside the tidal front.
The differences between the neritic
ecosystem in the shelf water and the sur-
rounding ocean were seen already in spring
as the phytoplankton spring bloom started
earlier in the shelf water than outside the
tidal front. This was probably the result of
a shallower mixed layer on the shelf than
outside the front. The spring bloom can
only start when the depth of the upper
mixed layer is less than the critical depth
(Sverdrup, 1953). Inside the tidal front, the
mixed layer is the total water column, and
because of the relatively shallow depth on
the Faroe Shelf, the critical depth during
spring may very well be deeper than the
bottom depth. Therefore, the spring bloom
may start in the Faroe Shelf water before
development of a summer thermocline
makes it possible in the surrounding area
outside the tidal front. This earlier develop-
ment of the phytoplankton spring bloom in
the Faroe Shelf water than outside the front
has also been observed earlier (Gaard,
1994) but the timing of the bloom may vary
significantly between years.
The bloom started in the northern and
central region of the shelf (Fig. 3). A possi-
ble explanation for this is that this water
may have been more isolated than the
southern part of the shelf, thus allowing the
phytoplankton stocks to develop and in-
crease in concentration during spring.