Fróðskaparrit - 01.01.1996, Page 92

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PHYTOPLANKTON COMMUNITY STRUCTURE ON THE FAROE SHELF
Fig. 1. Bottom topography and main features ofthe
flow field in the upper layers around the Faroes and
on the Faroe Shelf. (Based on Hansen, 1992 and
Hansen etal., 1994)
med the Modified North Atlantic Water
(MNAW) (Fig. 1).
These hydrographical circumstances
cause quite different environmental condi-
tions for the phytoplankton on the shelf
compared to the outer areas. Inside the tidal
front, the phytoplankton is relatively well
isolated from the outer area with the water
column well-mixed throughout the sum-
mer. Outside the tidal front, the hydrograpi-
cal conditions are. typically oceanic with
stratification of the water column during
summer due to the development of a ther-
mocline.
The Faroe Shelf water, therefore, has its
own plankton ecosystem, separated from
the surrounding area. It has its own phyto-
plankton community which is affected by
quite different hydrographical, chemical
and light conditions than the phytoplankton
in the area outside the tidal front.
While the hydrography on the Faroe
Shelf is relatively well described (Hansen,
1992; Hansen et al., 1994), no description
of the phytoplankton community has been
given. The aim of this paper is to describe
the community on the Faroe Shelf in 1995
and to relate it to its hydrographical and
chemical environment.
Material and methods
The study was conducted on 5 cruises from
April to November 1995, with the Faroese
Research vessel „Magnus Heinason”. Sam-
ple collection and measurements were car-
ried out of hydrography, nutrients, fluores-
cence, chlorophyll a and phytoplankton. In
addition, samples for nutrient analysis and
phytoplankton were collected from a moni-
toring station marked »S« on Fig. 5. This
water was pumped through an underwater
tunnel (0.8 m in diameter) from about 18
meters depth in an area with strong tidal
currents and the samples were collected
with 3 or 4 days intervals during the period
May-December 1995.
On the research vessel the temperature
and salinity (and hence the density) was in
April 1995 measured with a EG&G Mark
III CTD and from May 1995 and the rest of
1995 they were measured with a Seabird
Electronics SBE911 + CTD. Both instru-
ments were equipped with a rosette samp-
ler.
Water samples were collected with
Niskin bottles from the CTD rosette. The
samples for analysis of nitrate and silicate
were preserved with 3 drops of chloroform
per 25 ml of sample immediately after
sampling. Normally, they were stored in a
refrigerator and analysed in the laboratory
3-15 days after sampling. However, in