Fróðskaparrit - 01.01.2005, Page 88

86 THE relative importance of protozooplankton and copepods as grazers
ON PHYTOPLANKTON DURING THE 1999 SPRING BLOOM ON THE FAROE SHELF
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P. pouchetii
Hf Nitzschia
Thalassiosira
■ Chaetoceros
Pre-bloom
Mid-bloom
19-Apr 3-May 17-May 31-May 14-Jun
19-Apr 3-May 17-May31-May 14-Jun
Figure 2. Left: Average chlorophyll a concentration (mg nr3) l’-axis, and phytoplankton biomass (mgC
m'3) 2’ axis. Right: Phytoplankton abundance (ml'1) and speeies composition. Vertical bars indicate standard
error of the mean of the 2 m and 20 m depth samples (n = 2). The shaded area indicates the transition
period between the pre-bloom and the mid-bloom.
efficiency of 33% (Peterson, 1988) assum-
ing juvenile somatic growth is equal to the
specific egg production rate (Berggreen et
al., 1988), and that all copepods followed
the P/B for C. finmarchicus.
As a second estimate of copepod inges-
tion rate we used the temperature de-
pendent production method from Hunt-
ley and Lopez (1992), where growth G
= 0.0445eo mT, where T is the ambient
temperature of the water column. The co-
pepod ingestion was calculated as I =
3 x G x B, where B = the biomass of
copepods and assuming a gross growth ef-
ficiency of 33% (Peterson, 1988).
Results
Hydrography and phytoplankton
The water column on the Faroe Shelf is
well mixed with usually no summer strati-
fication. The temperature measured at sta-
tion S (Fig. 1) is thus considered to be
representative for the temperature in the
whole water column in the central part of
the shelf. There was a steady rise in tem-
perature throughout the investigation frorn
6.3°C on 19 April to 8.4°C in late June
(data not shown).
The experimental period was divided
into two different scenarios based on
standing stock of phytoplankton: the pre-
bloom and the mid-bloom period, with a
transition period of 2 weeks. The chl a
concentration started to increase in mid
May, and reached a maximum in early
June (Fig. 2, left). The chl a concentration
at 2 and 20 m depth were almost identical
as would be expected since the water col-
umn usually is well mixed. The standard
error in Fig 2 is taken between the mean
chl a concentration (duplicates) at the two
depths, n = 4 per date. The phytoplank-