Fróðskaparrit - 01.01.1996, Side 101

PHYTOPLANKTON COMMUNITY STRUCTURE ON THE FAROE SHELF
105
tial new production of about lgC m-3 or
about 80 gC m 2. In addition to this, there is
a contribution of nutrients to the area
through advection from outside the shelf
water. The magnitude of this advection is
not well known and may vary between
years. However, it is assumed to be in the
same order of magnitude during the sum-
mer period as the nutrient pool in the shelf
water prior to the onset of the primary pro-
duction in spring. The potential new prima-
ry production can therefore be roughly esti-
mated to be about 150 gC nr2, from April
to September this year. By far the most of
the new production was in spring and early
summer, and measurements of nitrate dur-
ing 1995 showed that the nitrate pool in the
shelf water was emptied after about 70-80
days. This gives a mean potential new pri-
mary production of about 1.5 gC m 2 d1
during the spring and early summer period.
Based on Fig. 5, about half of the new
production was based on the diatom spring
bloom (mainly Thalassiosira nordenskio-
eldii and Chaetoceros sp.) during the first
3-4 weeks of the spring bloom period. The
daily new production of diatoms in this
period therefore has been as high as 2-3 gC
m 2 d1. The rest of the new production was
mainly by the colony-forming Prymnesio-
phycea Phaeocystis pouchetii.
Large phytoplankton, such as diatoms,
are more effectively grazed by the meso-
zooplankton than small. Thus, there is a
greater potential for the classical food chain
to develop when the large species are pre-
sent (Fenchel, 1988). Smaller species, such
as Phaeocystis are thought to primarily be
indicative of a microbial loop food chain
structure, with much waste of energy (e.g.
Azam et al., 1983; Kiørboe et al1990,
Kiørboe and Nielsen, 1994). Therefore,
even if the production time of diatoms was
short and apparently was limited to only 3-
4 weeks in spring, it may have been of ma-
jor importance as food supply to the higher
trophic level on the Faroe Shelf.
The primary production and nutrient
concentrations in the Shelf water may vary
significantly between years. Some years
may be highly productive, causing distinct
nutrient depletion, but other years may
show lower productivity, resulting in high-
er nutrient concentrations and a more di-
atom based flora (Gaard and Hansen, un-
publ.). The year discussed in this work has
to be considered as one of the most produc-
tive for the shelf, since the nitrate concen-
trations in the shallow parts of the shelf
reached approximately zero.
Acknowledgements
I thank Drs. Bogi Hansen, Pehr H. Enckell and Kather-
ine Richardson for their helpful comments on the manu-
script.
References
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