Fróðskaparrit - 01.01.2005, Blaðsíða 92

90 THE relative importance of protozooplankton and copepods as grazers
ON PHYTOPLANKTON DURING THE 1999 SPRING BLOOM ON THE FAROE SHELF
Egg production Eggs x f1 x d'1 n Eggs X m'2 X d'1
Pre-bloom 4.5 ±1.9 42 9781 ± 4799

Mid-bloom 25 ± 6.2 47 42108 ± 18156
Table I. Average egg production ± SE (standard error of the mean) of Calanus finmarchicus during pre- and
mid-bloom; n = number of replicates during the period.
Egg production
Fecundity of Calanus finmarchicus fol-
lowed the pattern of chl a concentration
(Fig. 4D). During pre-bloom it was low
with an average of 4.5 eggs female"1 d"1,
corresponding to an average of 9781 eggs
m'2 d"' (Table 1). In late May, the egg
production increased, reaching an average
of 25 eggs female"1 d'1, corresponding to
42108 eggs m"2 d"1 during mid-bloom. On
the last two sampling dates the abundance
of healthy C. fmmarchicus females was too
low for egg production measurements.
The percentage of spawning females in-
creased during the investigation period
from 75% during pre-bloom to more than
95% during mid-bloom (Fig. 4D).
Discussion
Response of protozooplankton and
copepods to the spring bloom
The thecate heterotrophic dinoflagellates
showed a clear response to the increase
in phytoplankton standing stock from pre-
to mid-bloom. Protoperidinium spp. first
appeared during the transition period and
increased in numbers during the rest of the
investigation. This response in thecate het-
erotrophic dinoflagellates is consistent with
other results from coastal areas showing a
dominance of thecate dinoflagellates in as-
sociation with diatom blooms (Jacobson,
1987; Lessard 1991). The naked hetero-
trophic dinoflagellates Gyrodinium spp. on
the other hand, did not show any biomass
response to the increased phytoplankton
biomass. This is in contrast to other in-
vestigations where the abundance of this
species was closely related to the phyto-
plankton spring bloom (Hansen, 1991;
Smetacek, 1981). One possible reason for
a lack of response in the naked hetero-
trophic dinoflagellates could be due to the
nature of their feeding mechanism. Naked
heterotrophic dinoflagellates usually ingest
intact prey of their own size by direct en-
gulfment (Hansen and Calado, 1999); a
phytoplankton bloom totally dominated
by large spiny Chaetoceros species would
therefore not seem to be their ideal prey
item. Thecate dinoflagellates of the genus
Protoperidinium, on the other hand, usu-
ally ingest their prey with a pseudopodium
that extends through the flagellar pore and
envelopes the prey (Jacobson and Ander-
son, 1986; Gaines and Elbráchter, 1987;
Hansen and Calado, 1999). This feeding
mechanism makes it possible for them to
ingest relatively large prey organisms, like