Fróðskaparrit - 01.01.2005, Page 98

96 THE relative importance of protozooplankton and copepods as grazers
ON PHYTOPLANKTON DURING THE 1999 SPRING BLOOM ON THE FAROE SHELF
pepod and protozooplankton community,
respectively.
Mid-bloom
Based on C. fmmarchicus egg production,
the average ingestion rate for the copepod
community during mid-bloom, was 1.6
mgC m'3 d'1 (Table 2 and Fig. 5A). This
corresponds to a daily grazing impact of
less than 1 % on the phyto- and protozoo-
plankton standing stock during this pe-
riod. Here we also made the assumption
that juvenile somatic growth is equal to the
C. fmmarchicus specific egg production
rate, and that all copepods have the same
P/B as C. finmarchicus. If we calculate
the ingestion per C. finmarchicus female
and compare it to corresponding results
from weather station M in the Norwegian
Sea (Irigoien et al., 1998), we see that our
results are slightly higher (19637 ngC f'1
d'1 in our research and 14295 ngC f'1 d'1
at station M). However, considering that
the chl a concentration during our mid-
bloom situation was twice as high as at
station M and the fact that the gut fluores-
cence measurements at station M were not
corrected for possible pigment destruction
(see above), our ingestion rate based on
egg production measurements during mid-
bloom seems to be in the same range as
the ingestion rate measured by gut pig-
ment analysis at station M.
Application of the temperature depend-
ent production method by Huntley and
Lopez (1992), yields an average ingestion
rate of 1.2 mgC m'3 d'1 during mid-bloom
(Table 2 and Fig. 5B), and corresponds
to an ingestion rate of 15098 ngC f"1 d'1.
This is concordant with the ingestion rate
from station M (Irigoien et al., 1998), and
in the same range as the ingestion rate
calculated from our own egg production
measurements during mid-bloom. The
ingestion rate corresponds to an average
and negligible grazing impact of 0.6% on
the phyto- and protozooplankton standing
stock.
The copepod grazing pressure on the
protozooplankton during mid-bloom was
probably low, due to the relatively low
protozooplankton biomass. The average
biomass of protozooplankton during mid-
bloom was approximately 3.5 mgC m'3.
This is much less than reported at other
locations (Kattegat: Hansen, 1991; Dog-
ger Bank: Nielsen et al., 1993; Disko
Bay: Nielsen and Hansen, 1995). How-
ever, at station M in the Norwegian Sea,
Irigoin et al. (1998) found biomass values
of protozooplankton in the same range as
in this study, with bloom values of 4 mgC
m"3. The physical environment is, howev-
er, different since station M is an oceanic
locality, and thus not directly comparable
to the Faroe Shelf.
Of the protozooplankton only the thecate
dinoflagellates increased significantly in
numbers during mid-bloom with a calcu-
lated growth rate of 0.15 d"1. This is simi-
lar to values reported elsewhere. Hansen
(1992b) found a maximum growth rate for
Protoperidinium pellucidum of 0.15 d"1,
(temperature corrected to 7°C, Q)0=2.8)
and other measurements of growth rates
of Protoperidinium spp. are in the same
range (Hansen et al., 1997). Using a
growth rate of 0.15'1 for the thecate hetero-