Fróðskaparrit - 01.01.2005, Side 96

94 THE relative importance of protozooplankton and copepods as grazers
ON PHYTOPLANKTON DURING THE 1999 SPRING BLOOM ON THE FAROE SHELF
Ingestion
_____________Copepod______Copepod*________Protozooplankton_____Protozooplankton *
Pre-bloom 0.4 ±0.16 1.0 ± 0.26 0.3 ± 0.05 1.0 ±0.17
Mid-bloom 1.6 ± 0.42 1.2 ± 0.21 0.6 ± 0.09 0.6 ± 0.09
Table II. Average ingestion (mgC m'3 d'1) ± SE (standard error of the mean), for copepods and protozooplankton
during pre- and mid-bloom. * = based on the temperature dependent production method (Huntley and
Lopez, 1992). Average protozooplankton ingestion rates during mid-bloom are calculated based on thecate
heterotrophic dinoflagellates only (see Fig. 5).
Sea (Irigoien et al., 1998), where stored
lipid reserves may be used to fuel, or sup-
plement reproduction in C. ftnmarchicus.
Thus, to use egg production as an es-
timate for copepod community ingestion
during pre-bloom may be erroneous. At
this time of the year the amount of im-
mature females is relatively high leading
to an underestimation of the actual inges-
tion rate, as only spawning females are
assumed to be actively feeding. Using
only active spawning females may there-
fore give a more reliable indication of the
ingestion rate for the copepod community.
Also considering that egg production dur-
ing pre-bloom situations may partially be
fueled by stored lipid reserves makes it
complicated to interpret egg production
results from these periods and may overes-
timate the actual grazing impact. The tem-
perature dependent production method,
on the other hand, takes only the ambient
temperature and the actual copepod bio-
mass into account. This approach yields
a very high potential ingestion rate dur-
ing pre-bloom in our research, which then
again is not reflected in the copepod egg
production. Using this method thus prob-
ably also overestimates the actual grazing
impact of the grazer community during
pre-bloom.
Another way to estimate ingestion rates
for copepods is to measure clearance rates
for copepods. Levinsen et al. (2000) mea-
sured clearance rates by adult females of
several different copepods on chl a, dino-
flagellates and ciliates in Disko Bay, West
Greenland, and Young Sound, NE Green-
land during a post-bloom situation. Their
results using C. finmarchicus females gave
clearance rates of 312, 465 and 200 ml
female"1 day"1 for phytoplankton, ciliates
and dinoflagellates, respectively. Multiply-
ing these clearance rates with the biomass
per cubic meter of phytoplankton, ciliates
and dinoflagellates and the amount of C.
finmarchicus females per cubic meter, and
using a Q10 of 2.8 for temperature correc-
tion (Hansen et al., 1997), yields a fernale
ingestion rate of 0.083 mgC m'3 d"1. Cor-
recting this to total copepod community
ingestion yields an ingestion rate of 0.18
mgC m"3 d'1 on the Faroe Shelf during
pre-bloom. This is probably a more realis-
tic value, and leads to a protozooplankton
growth rate of 0.06 d"1, corresponding to
a grazing impact of 4% and 7% on the
phytoplankton standing stock, for the co-