Fróðskaparrit - 01.01.2005, Side 85

LUTFALSLIGA AVIRKANIN AV PROTOZOOPLANKTON OG KOPEPODUM A VARBLOMING g3
AV PLANTUPLANKTON A LANDGRUNNINUM í 1999
Introduction.
The structure of the grazer community
during spring bloom situations in temper-
ate and arctic waters has been intensively
studied in the past. Most attention has
been paid to the mesozooplankton, espe-
cially the copepods. However, two decades
ago it became evident that protozoa poten-
tially play an important role in the linkage
between primary production and higher
trophic levels (e.g. Smetacek, 1981; Azam
al., 1983; Fenchel, 1988). More effort
has therefore been allocated in research
concerning the protozooplankton commu-
nity and its role in carbon flow (Hansen,
1991; Nielsen et al., 1993; Ohman and
Runge, 1994; Nielsen and Hansen, 1995;
Levinsen et al., 1999; Jensen and Hansen,
2000).
The Faroe Shelf is basically a neritic
ecosystem relatively isolated from its
oceanic surroundings by a persistent tid-
al front surrounding the islands at about
100-130m bottom depth contour (Hansen,
1992a). There is an anticyclonic circula-
tion of these shelf water masses, and the
average residence time has been estimated
to be about 3 months (Gaard and Hansen,
2000). Because of very strong tidal cur-
rents the water column in the shallow parts
°1 the shelf is well mixed during summer,
usually without any stratification. Theo-
retically, sufficient light conditions for
spring bloom development therefore are
when the critical depth has exceeded the
bottom depth in this region.
Due to these hydrographic conditions
the Faroe Shelf hosts a unique phytoplank-
t°n (Gaard, 1994; 1996a; Gaard et al.,
1998) and mesozooplankton (Gaard, 1994;
1996b; 1999) composition compared to
the oceanic surroundings. Although fairly
isolated the Faroe Shelf is, however, also
affected by the surrounding oceanic en-
vironment, including import of Calanus
fmmarchicus. Once on the shelf, this large
copepod seems to have a great influence
on the shelf ecosystem. This oceanic in-
fluence is, however, highly variable, and
the amount of C. finmarchicus that are
advected onto the shelf, varies very much
between years (Gaard, 1999; 2000; Gaard
and Hansen, 2000).
The importance of the protozooplank-
ton as phytoplankton grazers on the Faroe
Shelf ecosystem is still not investigated
despite the recent years acknowledgment
of its importance for grazing and carbon
flow elsewhere (e.g. Paranjape, 1987;
1990; Levinsen et al., 1999).
The objective of this work was:
1. to study the structure of the grazer
community on the Faroe Shelf during
two different periods in the spring-
summer succession: a low productive
pre-bloom period, and a high produc-
tive mid-bloom period.
2. to investigate the relative importance
of copepods and protozooplankton as
grazers on the phytoplankton standing
stock during these two periods.
Materials and methods
This investigation was carried out on the
Faroe Shelf. Seawater temperature was
monitored at a permanent station (station
S in Fig. 1), while biological measure-
ments were carried out at station H north-