Fróðskaparrit - 01.01.1996, Page 93

PHYTOPLANKTON COMMUNITY STRUCTURE ON THE FAROE SHELF
97
June-July 1995, the nitrate samples were
analysed on board the research vessel. On
the monitoring station S the samples from
May and June were frozen immediately af-
ter sampling. The other samples were pre-
served in chloroform, stored in a refrigera-
tor and analysed 5-30 days after sampling.
Nitrate and nitrite were mc asured automati-
cally and silicate was analysed manually
using the methods described by Grasshoff
et al. (1983).
In situ fluorescence was measured with a
Sea Tech fluorometer interfaced to the
CTD and fluorescence was calibrated from
selected samples which were analysed for
chlorophyll a. The chlorophyll a measure-
ments were carried out using the methods
described by Baltic Marine Biologists
(1979) with a difference in that the ho-
mogenisation was carried out with a
Soniprep 140 ultrasound homogeniser.
When computing the results, the equation
of Jeffrey and Humphrey (1975) was ap-
plied.
The algae were preserved in 0.4% neu-
tralised formaldehyde and were counted in
2, 5 or 10 ml of seawater after overnight
settlement using an inverted microscope.
Results
The primary production in the Faroe Shelf
water started earlier in spring and also de-
clined earlier in autumn than outside the
front (Fig. 2). The phytoplankton biomass
during spring and summer was significant-
ly higher in the Faroe Shelf water and in the
frontal region than outside the front and at
the same time the nutrient concentrations
decreased much in this water and were
much lower in the shelf water than in the
open area outside the front.
The highest phytoplankton biomass in
spring was found in the northern region of
the shelf water (Fig. 3a) and the spring
bloom, apparently, has started in this re-
gion. The primary production clearly af-
fected the nutrient concentrations and they
decreased very much in this area (Fig. 3.b).
The results indicate that the water masses
in the northern region were more isolated
than in the southern region. This is also
supported by the nutrient distribution in
mid summer (Fig. 3c). The nitrate concen-
trations in the shallowest part in the
northern region of the Faroe Shelf were be-
tween 2 and 3 /rmol F'. The concentrations
in the southern region were more than
twice as high.
Both the nutrient gradients and the distri-
bution pattern of the phytoplankton bio-
mass were related to the hydrographic fea-
tures (Fig. 4). The north-westem transect
showed strong horizontal density gradients
in the frontal area and very high phyto-
plankton biomass above the pycnocline.
However, in the eastern region, there was
weaker thermal stratification, stronger ver-
tical gradients, and low phytoplankton bio-
mass outside the front.
The concentrations of nutrients in the
central region of the shelf water (station S)
decreased dramatically during spring and
early summer 1995 (Fig. 5). The primary
production in particular affected the nitrate
concentrations, which decreased rapidly
during early May and reached zero around
mid July in the shallow areas. Only at the
very beginning of the sampling period (ear-