Fróðskaparrit - 01.01.1996, Side 100

104
PHYTOPLANKTON COMMUNITY STRUCTURE ON THE FAROE SHELF
therefore these algae are favoured in strati-
fied waters (Margalef, 1978; Holligan,
1987; Fogg, 1991). Such favorisation of the
diatoms in turbulent environment is also
seen on the Faroe Bank, which is a similar
ecosystem to the Faroe Shelf (Gaard and
Mortensen, 1993).
The strong turbulence on the Faroe Shelf
should therefore favour diatoms. Neverthe-
less, diatoms were only abundant on the
shelf during the spring bloom period. Al-
ready in mid-late May they declined in con-
centrations and instead Phaeocystis domi-
nated the phytoplankton flora on the shelf
during most of the summer.
Egge and Aksnes (1992) have shown
that silicate concentrations may be a regu-
lating nutrient in the phytoplankton compe-
tition. They found that diatoms, as a group,
were outcompeted by flagellates at silicate
concentrations below a threshold of about 2
/íM. Our measurements showed that the di-
atoms in the shallow part of the shelf (sta-
tion S) collapsed around mid May at the
same time as the silicate concentrations
reached this value. However, even if the
summer concentrations, which were
around 2-3 piM, may have slowed down the
production rate of the diatoms, this factor
alone is not a sufficient explanation for the
diatom collapse and the almost total domi-
nance of Phaeocystis.
The nutrient uptake by the algae is much
affected by the cell size and is most effi-
cient in small cells due to their higher spe-
cific surface area (Kiørboe, 1993). Thus,
large celled phytoplankters such as most di-
atoms that are common in spring blooms
may be outcompeted by smaller algal
species when the nutrient concentrations
are low.
Other possible advantages which may be
important for the colony-forming Phaeo-
cystis compared to diatoms may be lower
predation pressure by mesozooplankton
(Estep et al., 1990; Lancelot, 1995).
Potential new primary productivity
The primary production occurrs almost en-
tirely in the summer period. Measurements
of the primary production during 1985
(Vandkvalitetsinstituttet, 1987) showed
that about 80% of the total primary produc-
tion was within the four months period
from May to August. Outside this period
the primary production was light limited.
Some fraction of the primary production
is derived from new nitrogen (primarily ni-
trate) and another fraction is derived from
recycled nitrogen (primarily ammonia and
urea) (Dugdale and Goering, 1967). The
harvestability is related to the size of the
new primary production, i.e. the fraction of
the primary production that is based on new
nutrients.
Figure 5 shows that the nitrate concen-
trations the shallow area of the Faroe Shelf
reached approximately zero in mid and late
summer, 1995. The potential new primary
production is, therefore, limited to the nu-
trient pool in this water in spring plus ad-
vection of nutrients into the shelf water
during the summer.
The winter concentration of nitrate in the
Faroe Shelf water is about 12-12.5 //mol l1.
Using the Refield ratio of C:N = 106:16 and
assuming a mean bottom depth on the shelf
to about 80m, this corresponds to a poten-