Fróðskaparrit - 01.01.2005, Page 94

92 THE relative importance of protozooplankton and copepods as grazers
ON PHYTOPLANKTON DURING THE 1999 SPRING BLOOM ON THE FAROE SHELF
Based on C. fmmarchicus egg production
the average ingestion rate of the copepods
was 0.4 mgC m'3 d'1 during pre-bloom
(Table 2 and Fig. 5A) corresponding to
13% of the phytopiankton standing stock.
Here we have made the assumption that
juvenile somatic growth is equal to the
C. fmmarchicus specific egg production
rate, and that all copepods follow the P/B
for C. fmmarchicus. This does, however,
probably result in an underestimation of
the total copepod ingestion since several
recent studies have indicated that smaller
copepods may often play a more impor-
tant role than the larger animals, not only
in terms of abundance but also in terms
of biomass and grazing pressure on the
phytoplankton (e.g. Morales et al., 1991;
Dam and Peterson, 1993). Calculating the
ingestion rates per C. finmarchicus female
gives an average ingestion rate of 2531 ngC
f'1 d"1 during pre-bloom. Comparing this
result with the pre-bloom ingestion rate
of C. finmarchicus females (1433 ngC f"1
d'1) at the weather station M (Mike) in
the middle of the Norwegian Sea (Irigo-
ien et al., 1998) shows a slightly higher
ingestion rate during our pre-bloom situ-
ation. Our results are calculated using the
egg production results and length-weight
regressions for weight of the females and
eggs. The results in Irigoien et al. (1998)
are pure herbivory based gut-fluorescence
measurements without correction for pos-
sible pigment destruction. Their results
are thus possibly slightly underestimated.
The degree of pigment destruction during
gut passage in copepods varies greatly but
is generally believed to be <20% (Har-
ris, 1996). Taking this into account our
ingestion rates based on pre-bloom C.
finmarchicus egg production seem to be
in the same range as the ingestion rates
measured at station M.
Application of the temperature depend-
ent production method from Huntley and
Lopez (1992) to our data gives an average
ingestion rate of 1.0 mgC m'3 d'1 during
pre-bloom (Table 2 and Fig. 5B), which
corresponds to 37% of the phytoplankton
standing stock. This gives an unrealisti-
cally high ingestion rate of 8443 ngC f'1
d"1 per female.
To estimate the growth rate of the whole
protozooplankton community during pre-
bloom we used the estimated copepod in-
gestion rates from both the egg-production
and from the temperature dependent pro-
duction method. In addition we assumed
that the entire loss rate of the whole pro-
tozooplankton community during this pe-
riod was due to copepod grazing, and no
prey selectivity by the copepod commu-
nity. This yields protozooplankton growth
rates of 0.08 d'1 and 0.28 d'1, for the egg-
production and the temperature depend-
ent production method, respectively. The
latter of these two is unrealistically high,
and using a gross growth efficiency of
40% (Hansen et al., 1997) yields an esti-
mated ingestion rate of 1.0 mgC m'3 d'1,
and a grazing impact of 37% on the phy-
toplankton standing stock. Based on egg-
production the estimated ingestion rate of
protozooplankton was 0.3 mgC m'3 d'1,
corresponding to 10% of the phytoplank-
ton standing stock during pre-bloom.
However, the protozooplankton growth